OSPF Lab: FULL Adjacency but the External Route Is Missing

Every neighbor shows FULL, yet one router has no route to a prefix that another router is redistributing. This is a classic OSPF interview scenario, and in this lab you will build it in Cisco Packet Tracer, find the cause, and fix it.

The root cause is an unreachable forwarding address on a Type 5 LSA that was translated from a Type 7 LSA in an NSSA.

Topology

R1 ---- R2 (ABR) ---- R3 (ASBR) ---- R4 (customer)
Area 0     |   Area 1 (NSSA)   |   192.168.99.0/24

Use four Cisco 2911 routers. Connect them with crossover cables (or use the automatic cable option in Packet Tracer).

Link	Subnet	Addresses
R1 Gi0/0 to R2 Gi0/0	10.0.12.0/30	R1 .1, R2 .2
R2 Gi0/1 to R3 Gi0/0	10.0.23.0/30	R2 .1, R3 .2
R3 Gi0/1 to R4 Gi0/0	192.168.99.0/24	R3 .2, R4 .1
R1 Gi0/1 (branch LAN, optional)	10.1.1.0/24	R1 .1
R4 Loopback0 (customer network)	172.16.50.0/24	R4 .1

Configuration

R1

hostname R1
!
interface GigabitEthernet0/0
 ip address 10.0.12.1 255.255.255.252
 no shutdown
!
interface GigabitEthernet0/1
 ip address 10.1.1.1 255.255.255.0
 no shutdown
!
router ospf 1
 router-id 1.1.1.1
 network 10.0.12.0 0.0.0.3 area 0
 network 10.1.1.0 0.0.0.255 area 0

R2 (ABR)

hostname R2
!
interface GigabitEthernet0/0
 ip address 10.0.12.2 255.255.255.252
 no shutdown
!
interface GigabitEthernet0/1
 ip address 10.0.23.1 255.255.255.252
 no shutdown
!
router ospf 1
 router-id 2.2.2.2
 area 1 nssa
 network 10.0.12.0 0.0.0.3 area 0
 network 10.0.23.0 0.0.0.3 area 1
 area 1 filter-list prefix NO-FA out
!
ip prefix-list NO-FA seq 5 deny 192.168.99.0/24
ip prefix-list NO-FA seq 10 permit 0.0.0.0/0 le 32

R3 (ASBR)

hostname R3
!
interface GigabitEthernet0/0
 ip address 10.0.23.2 255.255.255.252
 no shutdown
!
interface GigabitEthernet0/1
 ip address 192.168.99.2 255.255.255.0
 no shutdown
!
router ospf 1
 router-id 3.3.3.3
 area 1 nssa
 network 10.0.23.0 0.0.0.3 area 1
 network 192.168.99.0 0.0.0.255 area 1
 redistribute static subnets
!
ip route 172.16.50.0 255.255.255.0 192.168.99.1

R4 (customer)

hostname R4
!
interface GigabitEthernet0/0
 ip address 192.168.99.1 255.255.255.0
 no shutdown
!
interface Loopback0
 ip address 172.16.50.1 255.255.255.0
!
ip route 0.0.0.0 0.0.0.0 192.168.99.2

What you should see

• show ip ospf neighbor shows FULL on R1, R2, and R3.
• On R3, show ip ospf database nssa-external lists 172.16.50.0 with forwarding address 192.168.99.1.
• On R1, show ip ospf database external lists the translated Type 5 LSA for 172.16.50.0, still carrying forwarding address 192.168.99.1.
• On R1, show ip route 172.16.50.0 returns % Network not in table.
• On R1, show ip route 192.168.99.1 also returns no route.

Questions to answer before reading the solution

1. All adjacencies are FULL and the Type 5 LSA is in the R1 database. Why is the route not in the routing table?
2. What is the role of the forwarding address?
3. Which commands confirm your theory?
4. Give at least two ways to fix it.

---

Solution

Try the lab yourself before reading on.

Root cause

R3 has a next hop (192.168.99.1) on an OSPF-enabled, non-passive interface, so it sets that address as the forwarding address in the Type 7 LSA. R2 translates the Type 7 to a Type 5 and keeps the forwarding address. R2 also has a prefix-list on the Area 1 outbound filter that blocks 192.168.99.0/24 from reaching Area 0.

As a result, R1 receives the Type 5 LSA but has no OSPF route to the forwarding address 192.168.99.1. A router does not install an external route when the forwarding address is unreachable through an intra-area or inter-area route.

Confirm it

R1# show ip ospf database external 172.16.50.0
R1# show ip route 192.168.99.1
R2# show ip ospf database nssa-external 172.16.50.0
R2# show ip prefix-list NO-FA
R2# show run | section router ospf

Fix options

1. Remove the filter on R2:

   R2(config)# router ospf 1
   R2(config-router)# no area 1 filter-list prefix NO-FA out

2. Or permit the forwarding address subnet in the prefix-list:

   R2(config)# ip prefix-list NO-FA seq 3 permit 192.168.99.0/24

3. Or avoid the non-zero forwarding address by making the R3 link to R4 passive. The network statement stays, so the subnet is still advertised, but the forwarding address becomes 0.0.0.0 and traffic is sent to the ASBR (R3):

   R3(config)# router ospf 1
   R3(config-router)# passive-interface GigabitEthernet0/1

   You may need to clear the Type 7 LSA (clear ip ospf process on R3) so it is re-originated with the new forwarding address.

Verify

R1# show ip route 172.16.50.0
R1# ping 172.16.50.1

R1 should now show an O E2 route for 172.16.50.0/24.

Interview takeaway

FULL only means the link-state databases are synchronized. It says nothing about whether a route gets installed. When an external route is in the LSDB but missing from the routing table, check these in order:

1. Is the forwarding address reachable through an OSPF route?
2. Is the path to the ASBR (Type 4) available?
3. Is another protocol offering the same prefix with a lower administrative distance?
4. Is a distribute-list in blocking installation?

For NSSA designs, always check that the Type 7 is being translated and look at the forwarding address first.

How to Study for the CCNA: A Practical 12-Week Plan

 

The CCNA is one of the most popular entry points into networking, and it’s easy to study badly for. People watch videos for weeks, feel productive, then freeze when they face a CLI simulation. This plan is built around the opposite idea: learn a little, then configure it yourself.

First, know what you’re aiming at

The CCNA 200-301 is a single exam. It runs 120 minutes, is taken at a Pearson VUE test center or online with proctoring, and costs 300 USD plus local taxes. Its content is split into six domains, and the weights tell you where your hours should go.

Domain	Exam weight
IP Connectivity	25%
Network Fundamentals	20%
Network Access	20%
Security Fundamentals	15%
IP Services	10%
Automation and Programmability	10%

Timing matters. Version 1.1 of the exam retires on February 2, 2027, and CCNA v2.0 goes live on February 3, 2027, keeping the same 200-301 exam number. The v2.0 blueprint reportedly puts much more weight on troubleshooting. Decide early which version you’re targeting:

• Sitting v1.1: book your exam before February 2, 2027, and work backwards.
• Sitting v2.0: download Cisco’s v2.0 topic list as your map and add extra troubleshooting labs.

Set up your toolkit (Week 0)

1. One main video course or book. Pick a primary source, such as Jeremy’s IT Lab’s free course or the Cisco Official Cert Guide, and stick with it. Switching resources every week wastes time.
2. A lab tool. Cisco Packet Tracer is free and covers most of the exam. Use it for every topic.
3. A notebook or flashcard app for commands, port numbers, and protocol facts.

Plan for 1.5 to 2 hours on weekdays and 3 to 4 hours on one weekend day. Consistency beats marathon sessions.

The 12-week plan

Weeks 1–2: Network Fundamentals

Cover the OSI and TCP/IP models, Ethernet, cables, and the basics of switches and routers. Then spend real time on IPv4 addressing and subnetting. It shows up everywhere, so drill it daily until you can do it quickly without a calculator. Add IPv6 basics at the end.

Lab: Build a small network, assign IPs, and ping between devices.

Weeks 3–4: Network Access

Cover VLANs, trunking (802.1Q), Spanning Tree Protocol, EtherChannel, and wireless architecture basics.

Lab: Configure VLANs and trunks across two switches, then deliberately break STP and see what happens.

Weeks 5–7: IP Connectivity

This is the biggest domain, so give it the most time. Cover the routing table, static and default routes, and OSPFv2 (neighbor adjacency, DR/BDR, cost). Add first-hop redundancy concepts.

Lab: Build multi-router topologies, configure OSPF, and read show ip route until it feels natural.

Week 8: IP Services

Cover NAT/PAT, DHCP, DNS, NTP, SNMP, syslog, and QoS basics.

Lab: Configure DHCP on a router and PAT for internet access.

Week 9: Security Fundamentals

Cover ACLs, port security, DHCP snooping, dynamic ARP inspection, VPN concepts, and wireless security (WPA2 and WPA3). Know common threats and the controls that counter them.

Lab: Write standard and extended ACLs and test what they block.

Week 10: Automation and Programmability

Only 10% of the exam, but often the easiest place to pick up points. Learn REST APIs, JSON, controller-based networking, and how SDN differs from traditional networking. If you’re targeting v2.0, review the new AI-related topics in Cisco’s official outline.

Weeks 11–12: Review, mock exams, weak spots

Take full-length timed practice exams and review every wrong answer. Redo labs on your weakest domains. Spend the final days on commands, subnetting speed, and quick lab drills rather than new material.

A simple daily routine

Time	Activity
30–40 min	Learn: video or reading for one topic
40–60 min	Lab: configure what you just learned
15–20 min	Review: flashcards, subnetting drills, yesterday’s notes

Habits that separate people who pass

• Type commands from memory. Watching someone configure OSPF is not the same as doing it.
• Break things on purpose. Misconfigure a VLAN or an ACL, then fix it. This builds troubleshooting skill, which matters even more under v2.0.
• Practice subnetting daily. Ten minutes a day beats a two-hour weekend cram.
• Mix up your practice questions. The real exam shuffles topics, so one question might be subnetting and the next OSPF.
• Time yourself. Simulations eat the clock. Know when to move on from a hard question.

Common mistakes to avoid

1. Only watching videos and never labbing.
2. Memorizing practice-test answers instead of understanding the concepts.
3. Ignoring the lower-weight domains. Automation and IP Services are very passable points.
4. Using outdated study material. Check that your resources match the exam version you’re sitting.
5. Booking the exam too late. A fixed date gives your study a deadline.

The week before your exam

Keep it light. Do one or two timed mock exams, skim your command notes, and sleep well. If you’re testing online, check your ID, room setup, and internet in advance. Don’t learn new topics in the last 48 hours.

Final thoughts

The CCNA rewards steady, hands-on practice far more than raw memorization. Follow the weekly structure, lab every topic, and review your mistakes honestly, and 12 weeks is plenty of time. Pick your exam version, book your date, and open Packet Tracer today.

Exam details checked against published CCNA 200-301 information as of November 2026. Always confirm dates, fees, and topics on Cisco’s official site before booking.

5 Tips to Crack a Network Engineer Interview Without Any Experience

Breaking into network engineering without hands-on job experience is very doable. Interviewers for entry-level roles mostly want to see that you can think, learn, and troubleshoot. Here are five tips to help you get there.

1. Build a Home Lab and Talk About It Like Real Experience

Use Cisco Packet Tracer, GNS3, EVE-NG, or CML to build real topologies: VLANs, inter-VLAN routing, OSPF, EIGRP, BGP basics, ACLs, NAT, and STP.

In the interview, don't just say "I know OSPF." Say:

"I built a three-router OSPF lab, broke the adjacency by mismatching the hello timers, and fixed it."

Lab stories count as experience when you can explain what broke and why.

2. Master the Fundamentals, Not Just the Buzzwords

Most entry-level interviews go deep on the basics. Be ready to explain these without hesitation:

• The OSI model and what happens when you type a URL in a browser (DNS, ARP, TCP handshake, routing, switching)
• Subnetting, which you should be able to do fast and on paper
• TCP vs UDP, how a switch builds its MAC table, and how a router picks a path
• Static vs dynamic routing, and link-state vs distance-vector

If you've studied for the CCNA, you already have the right foundation. Certification also signals commitment when you lack work history.

3. Use a Structured Troubleshooting Method

Scenario questions like "A user can't reach the internet, what do you do?" test your thinking more than your knowledge. Walk through a layered approach out loud:

1. Check the physical link
2. Check IP configuration and the default gateway
3. Ping the gateway
4. Check DNS
5. Check routing and ACLs

Interviewers love candidates who stay calm and methodical, even if they don't reach the exact answer.

4. Turn Non-Job Experience Into Relevant Experience

Anything counts if you frame it well:

• Helpdesk work
• College lab projects or internships
• Volunteering to fix a friend's or family member's network
• A home network you segmented with VLANs
• A YouTube channel or blog where you teach networking

Teaching a concept publicly shows you understand it deeply. Prepare two or three short stories using this format: Situation, Action, Result.

5. Prepare for Behavioral Questions and Ask Smart Ones

Entry-level hires are often chosen on attitude. Prepare answers for:

• "Tell me about yourself"
• "Why networking?"
• "Tell me about a time you solved a hard problem"

Be honest about what you haven't done yet, then show how you learn:

"I haven't configured that in production, but in my lab I did X, and here's how I'd approach it."

At the end, ask questions about the team's tools, how they handle change management, or what a new hire's first 90 days look like.

Bonus: Learn a Little Automation

Learn some Python basics, Netmiko, or Ansible. Even light familiarity makes you stand out from other freshers.

Final Thoughts

No experience doesn't mean no proof. A solid lab, strong fundamentals, a calm troubleshooting method, and honest storytelling can put you ahead of candidates who only list buzzwords on a resume. Start building today, and good luck with your interviews!

10 Advanced BGP Interview QnA

Preparing for a networking interview? Here are 10 advanced BGP interview questions and answers, with the CLI commands and sample outputs you should know. Each answer is short, practical and easy to explain in an interview.

Quick index

1. BGP timers, and why is the Hold timer high?
2. update-source vs next-hop-self
3. BGP session stuck in "Active" state
4. Route in BGP table but not in routing table
5. Why use a loopback for BGP peering?
6. eBGP neighbors using loopbacks: extra configuration
7. How does eBGP prevent routing loops?
8. Soft reset vs hard reset
9. BGP best path selection order
10. Why is iBGP full mesh needed, and how do route reflectors help?

Question 1: What are the BGP timers, and why is the BGP Hold timer high compared to other routing protocols?

Default BGP timers:

• Keepalive: 60 seconds
• Hold timer: 180 seconds

Even if a router receives no message from its neighbor for up to 3 minutes, the BGP session stays up. Unlike IGPs such as OSPF or EIGRP, BGP does not react quickly to small network changes.

Why is it so high?

• BGP is designed for stability, not speed. It is a deliberately "slow" protocol.
• BGP is the protocol of the Internet. It connects thousands of ISPs, enterprises and data centers, whereas OSPF and EIGRP operate inside a single organization (intra-AS).
• BGP exchanges millions of routes and carries critical Internet traffic, so any instability has global consequences.
• A long Hold timer prevents sessions from flapping on short link blips, which avoids constant route withdrawals and re-advertisements.

R1(config-router)# neighbor 2.2.2.2 timers 10 30
! keepalive 10s, hold 30s (the lower value of the two peers is negotiated)

Question 2: What is the difference between update-source and next-hop-self in BGP?

update-source controls the source IP of the BGP session. It makes the peer use a loopback instead of the physical interface.

neighbor x.x.x.x update-source Loopback0

next-hop-self changes the NEXT_HOP attribute of the routes you advertise to your own IP.

neighbor x.x.x.x next-hop-self

Default next-hop behavior in BGP:

• Advertising a route to an eBGP neighbor: the next hop becomes the router itself.
• Advertising a route to an iBGP neighbor: the next hop is not changed.

Remember: update-source affects the session (TCP source address). next-hop-self affects the routes (NEXT_HOP attribute).

Question 3: A BGP session is stuck in the "Active" state. What could be the issue?

"Active" means the router is actively trying to open a TCP connection to the neighbor, but it is not succeeding.

R1# show ip bgp summary

BGP router identifier 1.1.1.1, local AS number 65001
BGP table version is 3, main routing table version 3

Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
2.2.2.2         4 65002       0       0        0    0    0 00:01:34 Active

1) Neighbor reachability issue

Check connectivity to the neighbor with ping <neighbor IP>.

R1# ping 2.2.2.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2.2.2.2, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)

2) BGP peer not configured correctly

Wrong neighbor IP or wrong remote-as. Verify with show running-config | section bgp. In the example below, R1 is configured with neighbor 3.3.3.3, but the real peer is 2.2.2.2.

R1# show running-config | section bgp

router bgp 65001
 bgp log-neighbor-changes
 neighbor 3.3.3.3 remote-as 65002

3) Incorrect update-source interface

The peer expects the session from a specific address, but R1 sources it from a different interface. Here R1 uses Loopback1 while the peer is configured for Loopback0.

R1# show running-config | section bgp

router bgp 65001
 bgp log-neighbor-changes
 neighbor 2.2.2.2 remote-as 65002
 neighbor 2.2.2.2 update-source Loopback1

Fix: neighbor <IP> update-source Loopback0

4) Firewall or ACL blocking TCP 179

BGP runs over TCP port 179. Check with show access-lists and debug ip tcp transactions.

R1# show access-lists

Extended IP access list BLOCK_BGP
    10 deny tcp host 1.1.1.1 host 2.2.2.2 eq bgp
    20 permit ip any any

R1# debug ip tcp transactions
*Jul 19 10:11:32.223: TCP: sending SYN to 2.2.2.2 port 179
*Jul 19 10:11:34.227: TCP: SYN retry timeout
*Jul 19 10:11:36.231: TCP: SYN retry timeout
*Jul 19 10:11:38.235: TCP: SYN retry timeout

Also check: eBGP peers that are not directly connected need ebgp-multihop (see Question 6), and MD5 password mismatches can also keep a session from establishing.

Question 4: You are receiving a BGP route in the BGP table, but it is not installed in the routing table. Why?

• The BGP table holds the valid BGP routes.
• The RIB (routing table) installs only the best route from all protocols (OSPF, BGP, static, etc.).

1) Administrative Distance (AD) preference

• eBGP AD = 20, iBGP AD = 200
• If another protocol has the same prefix with a lower AD (for example OSPF at 110, or a static route at 1), that route wins and the BGP route is not installed.

Check with show ip route <prefix>. In the output below, the prefix 100.1.1.1/32 is learned from BGP (marked with * only, not *>), but OSPF owns the routing table entry.

R1# show ip bgp

     Network          Next Hop            Metric LocPrf Weight Path
 *>i 192.168.20.0     2.2.2.2                  0    100      0 65002 i
  *   100.1.1.1/32     2.2.2.2                  0    100      0 65002 i

R1# show ip route 100.1.1.1

Routing entry for 100.1.1.1/32
  Known via "ospf 1", distance 110, metric 20, type intra area
  Last update from 10.0.0.2 on GigabitEthernet0/1, 00:01:12 ago
  Routing Descriptor Blocks:
  * 10.0.0.2, from 10.0.0.2, via GigabitEthernet0/1
      Route metric is 20, traffic share count is 1

Tip: * only means the route was received by BGP but not selected as best. *> means it is the best path and is a candidate for the RIB.

2) Next-hop unreachable

BGP will not install a route if its next hop is not reachable.

show ip bgp <prefix>        ! shows the next hop
show ip route <next-hop>    ! is the next hop reachable?

R1# show ip bgp
     Network          Next Hop            Metric LocPrf Weight Path
  *   100.1.1.1/32     192.168.100.2            0    100      0 65002 i

R1# show ip route 192.168.100.2
% Network not in table

Fix: make sure the next hop is known via an IGP or a static route.

3) iBGP route without next-hop-self

Between iBGP peers the next hop is not modified. If the original eBGP next hop (a link outside your AS) is not in your IGP, downstream routers cannot reach it.

Fix: use neighbor x.x.x.x next-hop-self on the sending iBGP router.

4) Prefix affected by a policy or route-map

A route-map, prefix-list or filter in the BGP path can leave a prefix in the BGP table without it being used. Check with show run | section route-map and show ip bgp <prefix>.

R1# show ip bgp 100.1.1.1/32
BGP routing table entry for 100.1.1.1/32
Paths: (1 available, best #1, not installed in IP routing table)
  Not advertised to any peer
  65002
    2.2.2.2 (metric 0) from 2.2.2.2 (2.2.2.2)
      Origin IGP, metric 0, localpref 100, valid, internal
      rx pathid: 0, tx pathid: 0

R1# show ip route 100.1.1.1
% Network not in table

R1# show run | section route-map
route-map FILTER-BLOCK deny 10
 match ip address prefix-list DENY-PFX
route-map FILTER-BLOCK permit 20
!
ip prefix-list DENY-PFX seq 5 deny 100.1.1.1/32
ip prefix-list DENY-PFX seq 10 permit 0.0.0.0/0 le 32

The key line is "best #1, not installed in IP routing table". That tells you the route won best-path selection inside BGP but was rejected when handed to the RIB.

Question 5: Why is a loopback interface preferred over physical interfaces when configuring a BGP neighbor?

• Stability: a loopback never goes down unless it is manually shut. If a physical link fails, a session bound to that interface drops.
• Redundancy: if one link fails, the loopbacks stay reachable over an alternate path, so the BGP session survives. This requires the IGP (or static routes) to provide reachability to the peer's loopback.
• Load balancing / ECMP: with multiple equal-cost IGP paths to the loopback, traffic towards it can be load-balanced. A session tied to one physical interface cannot do this.
• Fewer sessions, simpler configuration: one session per router pair instead of one per link.
• Easier identification and troubleshooting: the router is always known by one predictable address.

neighbor 4.4.4.4 remote-as 100
neighbor 4.4.4.4 update-source Loopback0

Question 6: In the scenario below, if the eBGP neighbors are configured using loopback interfaces, what additional configuration is required?

Scenario: R1 (AS 100, Loopback 1.1.1.1) is connected to R2 (AS 200, Loopback 2.2.2.2) over a WAN link (R1 s0: 12.1.1.1/24, plus a second link s1: 21.1.1.1/24).

R1
neighbor 2.2.2.2 remote-as 200
neighbor 2.2.2.2 update-source Loopback0

R2
neighbor 1.1.1.1 remote-as 100
neighbor 1.1.1.1 update-source Loopback0

Answer: ebgp-multihop

• eBGP neighbors are expected to be directly connected (same subnet).
• Loopbacks are in different subnets and are not directly connected to each other.
• By default, eBGP packets are sent with TTL = 1, so they die after one hop and never reach the loopback.
• Increase the TTL with the ebgp-multihop command (range 1 to 255, default 1). For loopbacks, use 2 or more.

R1(config-router)# neighbor 2.2.2.2 ebgp-multihop 2
R2(config-router)# neighbor 1.1.1.1 ebgp-multihop 2

Don't forget: there is no IGP between two different ASes, so each router also needs a route to the other's loopback, usually a static route, for the session to come up.

R1(config)# ip route 2.2.2.2 255.255.255.255 12.1.1.2
R2(config)# ip route 1.1.1.1 255.255.255.255 12.1.1.1

Question 7: How does eBGP prevent routing loops?

eBGP prevents loops primarily using the AS_PATH attribute.

• Every time a route passes through an Autonomous System, that AS number is prepended to the AS_PATH.
• If a BGP router sees its own AS number in the AS_PATH of a received route, it rejects the route.

R1# debug ip bgp

BGP: 10.0.0.0/8 received via eBGP from 192.0.2.2
BGP: AS_PATH = 65003 65002 65001
BGP: route ignored due to AS path loop, local AS 65001 in path

Bonus: inside an AS, iBGP uses a different rule, iBGP split horizon: a route learned from one iBGP peer is not advertised to another iBGP peer (see Question 10).

Question 8: What is the difference between a soft reset and a hard reset in BGP?

	Hard reset	Soft reset
What it does	Tears down the BGP session and rebuilds it from scratch	Re-sends BGP updates without tearing down the session
Command	clear ip bgp [neighbor]	clear ip bgp [neighbor] soft in clear ip bgp [neighbor] soft out
Impact	Session drops, routes are withdrawn and re-advertised, so there is a temporary disruption	No session disruption; routes are refreshed in place
Use case	Session problems (stuck session), or you want to fully rebuild it	Applying new route-maps, filters or policies and reprocessing routes

• Soft in: asks the neighbor to resend (or re-evaluates stored) received updates so inbound policy is reapplied.
• Soft out: resends your local BGP updates to the neighbor so outbound policy is reapplied.

Note: inbound soft reset works through the BGP route refresh capability, which modern IOS supports by default. Older setups relied on neighbor x.x.x.x soft-reconfiguration inbound, which stores an extra copy of received routes in memory.

Question 9: What is the BGP best path selection order?

When BGP has multiple paths to the same prefix, it picks one best path by comparing attributes in this order (Cisco IOS). It stops at the first step where a path wins:

1. Weight: highest wins (Cisco-proprietary, local to the router)
2. Local Preference: highest wins (default 100, shared across the AS)
3. Locally originated: prefer routes originated by this router (network, aggregate, redistribute)
4. AS_PATH length: shortest wins
5. Origin code: IGP (i) over EGP (e) over incomplete (?)
6. MED (metric): lowest wins (compared between paths from the same neighboring AS by default)
7. eBGP over iBGP
8. Lowest IGP metric to the BGP next hop
9. Oldest eBGP path (for stability)
10. Lowest router ID
11. Lowest neighbor IP address

R1# show ip bgp 10.10.10.0/24
! look for the "best" flag and compare weight, localpref, AS path, origin, metric

Interview tip: a quick way to answer is "Weight, Local Pref, Originated, AS-path, Origin, MED, eBGP over iBGP, IGP metric, oldest, router-ID". Also remember that a path must have a reachable next hop and be valid to even enter the comparison.

Question 10: Why is an iBGP full mesh required, and how do route reflectors solve it?

The problem: iBGP split horizon

• Inside an AS, the AS_PATH does not change between iBGP peers, so it cannot be used to detect loops.
• Instead, a router will not advertise a route learned from one iBGP peer to another iBGP peer.
• So every iBGP router must peer directly with every other iBGP router: a full mesh of n(n-1)/2 sessions. With 10 routers that is 45 sessions, and with 100 routers it is 4,950.

The solution: Route Reflector (RR)

• An RR is allowed to re-advertise iBGP-learned routes to its iBGP clients.
• Clients peer only with the RR, not with each other, which cuts the session count dramatically.
• Loops are prevented with the ORIGINATOR_ID and CLUSTER_LIST attributes.

! On the Route Reflector
router bgp 65001
 neighbor 10.0.0.2 remote-as 65001
 neighbor 10.0.0.2 update-source Loopback0
 neighbor 10.0.0.2 route-reflector-client
 neighbor 10.0.0.3 remote-as 65001
 neighbor 10.0.0.3 update-source Loopback0
 neighbor 10.0.0.3 route-reflector-client

Other option: BGP confederations split a large AS into smaller sub-ASes to reduce the full-mesh requirement.

---

Found this useful? Share it with someone preparing for a networking interview, and subscribe to Network Engineer Stuff for more interview questions.

Write to us: networkengineerstuff@gmail.com | WhatsApp: +91-8828002605

Best of Luck!

MTU on Cisco Routers: The Layer 2 vs Layer 3 Difference Most Engineers Miss

 

If you’ve ever run show ip interface and show interface on a Cisco router, you may have noticed that both commands report an MTU. In most lab outputs the two numbers match, which makes it easy to assume they are the same thing. They are not. This post explains what each value means, how they relate, and why the distinction matters in real networks.

The Output That Raises the Question

Take a typical Ethernet interface on a Cisco router:

R1#show ip interface eth1/0
Ethernet1/0 is up, line protocol is up
  Internet address is 12.1.1.1/30
  MTU is 1500 bytes
  ...

R1#show interface eth1/0
Ethernet1/0 is up, line protocol is up
  Hardware is AmdP2, address is ca01.518f.001c
  Internet address is 12.1.1.1/30
  MTU 1500 bytes, BW 10000 Kbit/sec, DLY 1000 usec
  ...

Both say 1500 bytes. So why does the router show it twice?

What Is MTU?

MTU (Maximum Transmission Unit) is the largest unit of data an interface will transmit in a single piece. Anything bigger has to be fragmented or dropped. The catch is that “unit” depends on which layer you are talking about, and that is exactly where these two commands differ.

IP MTU (Layer 3)

The MTU shown by show ip interface is the IP MTU. It is the largest IP packet, header plus payload, that the interface will send without fragmenting it.

You configure it under the interface:

interface Ethernet1/0
 ip mtu 1400

After this, any IP packet larger than 1400 bytes leaving that interface is fragmented. If the packet has the DF (Don’t Fragment) bit set, the router drops it and sends back an ICMP “fragmentation needed” message instead.

Interface MTU (Layer 2)

The MTU shown by show interface is the interface MTU, also called the hardware or Layer 2 MTU. It is the largest payload a frame on that link can carry. On Ethernet the default is 1500 bytes, and this figure does not include the 14-byte Ethernet header or the 4-byte FCS.

You configure it like this:

interface Ethernet1/0
 mtu 1600

This is the ceiling for everything riding on the link: IPv4, IPv6, MPLS and so on.

How the Two Values Relate

The relationship is simple but has a few rules worth remembering:

• By default they track each other. If you never configure ip mtu, the IP MTU follows the interface MTU. Change mtu and the IP MTU changes with it.
• IP MTU cannot exceed the interface MTU. The router will not let you set ip mtu larger than what the link can carry.
• Once you set ip mtu explicitly, it becomes independent. From then on, the two numbers can differ.
• Other protocols have their own knobs. ipv6 mtu and mpls mtu are also capped by the interface MTU.

A Quick Example

Suppose you configure:

interface Ethernet1/0
 ip mtu 1400

Then:

• show ip interface reports MTU is 1400 bytes
• show interface reports MTU 1500 bytes

The link can physically carry 1500-byte payloads, but the router will only build IP packets up to 1400 bytes on it.

Why This Matters in Practice

Tunnels and VPNs

GRE and IPsec add extra headers to every packet. If the encapsulated packet grows beyond what the underlying link can carry, you get fragmentation or silent drops. Lowering the IP MTU on the tunnel interface (and often adjusting TCP MSS as well) leaves room for that overhead.

Jumbo Frames

Raising the interface MTU, for example to 9000, enables jumbo frames in data center or storage networks. Every device on the path, including both ends of the link, must support the larger size, or large frames will be dropped.

OSPF Adjacencies

This one catches many engineers during troubleshooting. OSPF routers include their MTU in Database Description (DBD) packets, and a mismatch between neighbors can leave the adjacency stuck in the ExStart or Exchange state. On Cisco IOS this check uses the interface’s IP MTU. You can bypass it with:

interface Ethernet1/0
 ip ospf mtu-ignore

That command works, but fixing the underlying mismatch is the better answer. Ignoring the check can hide a real problem and lead to dropped large packets later.

Quick Reference

	IP MTU	Interface MTU
Layer	3 (IP packet)	2 (frame payload)
Show command	show ip interface	show interface
Configure with	ip mtu	mtu
Default on Ethernet	Follows interface MTU	1500 bytes
Upper limit	Interface MTU	Hardware dependent

Key Takeaways

• Both commands show MTU, but at different layers.
• Interface MTU describes what the link can carry; IP MTU describes what the router will send as an IP packet.
• They match by default, and diverge as soon as ip mtu is configured explicitly.
• When troubleshooting fragmentation, tunnel overhead, or OSPF neighbors stuck in ExStart, check both values on both ends of the link.

The next time you see two identical MTU numbers on a router, you’ll know they are two different settings that simply happen to agree.

 

OSPF Troubleshooting Techniques: A Complete Guide for Network Engineers

By Network Engineer Stuff

Introduction

Open Shortest Path First (OSPF) is one of the most widely used Interior Gateway Protocols (IGPs) in enterprise networks. It is a link-state routing protocol that uses the SPF (Shortest Path First) algorithm to calculate the best path to reach a destination.

Although OSPF is considered a reliable and scalable routing protocol, network engineers frequently encounter issues such as:

• OSPF neighbor adjacency stuck in INIT, EXSTART, or EXCHANGE states.

• OSPF neighbors not forming.

• Routes missing from the routing table.

• OSPF routes not getting advertised.

• Incorrect router IDs.

• Area ID mismatches.

• MTU mismatches.

• Authentication failures.

As a Network Engineer, understanding how to troubleshoot these problems systematically is essential, especially when working in production environments.

In this blog, we will explore practical OSPF troubleshooting techniques using Cisco IOS commands and real-world scenarios.

---

1. Understand the OSPF Neighbor States

Before troubleshooting OSPF, it is important to understand its neighbor adjacency states.

OSPF goes through different states while establishing a neighbor relationship.

State	Description
Down	No Hello packets received from the neighbor.
Attempt	Router is attempting to contact a manually configured neighbor (NBMA networks).
Init	Hello packet received, but bidirectional communication is not established.
2-Way	Bidirectional communication established.
ExStart	Routers negotiate master/slave roles and initial database sequence numbers.
Exchange	Routers exchange Database Description (DBD) packets.
Loading	Routers request missing LSAs.
Full	OSPF adjacency is successfully established.

Important: On broadcast networks, two DROTHER routers can remain in the 2-Way state. This is normal and does not necessarily indicate a problem.

2. Essential OSPF Troubleshooting Commands

When troubleshooting OSPF on Cisco routers, start with these commands.

Command 1: show ip ospf neighbor

This command displays the OSPF neighbor table.

Router# show ip ospf neighbor

Check:

• Neighbor router ID.

• Neighbor state.

• Dead time.

• Local interface.

• Neighbor IP address.

Expected output should show the neighbor in the FULL state, where a full adjacency is required.

Command 2: show ip ospf interface brief

Router# show ip ospf interface brief

This command provides a quick overview of OSPF-enabled interfaces.

Verify:

• Correct interface participation.

• Area assignment.

• OSPF process ID.

• Interface IP address.

Command 3: show ip protocols

Router# show ip protocols

This command helps verify:

• OSPF process information.

• Router ID.

• Networks configured for OSPF.

• Passive interfaces.

• Routing information sources.

Command 4: show ip route ospf

Router# show ip route ospf

This command displays routes learned through OSPF.

OSPF routes are generally identified by:

• O – Intra-area route.

• O IA – Inter-area route.

• O E1 / O E2 – External Type 1 / Type 2 routes.

• O N1 / O N2 – NSSA external routes.

Command 5: show ip ospf database

Router# show ip ospf database

This command displays the OSPF Link-State Database (LSDB).

Use it to verify whether the router has received the expected LSAs.

---

3. Common OSPF Troubleshooting Scenarios

Let's understand some real-world issues that network engineers encounter.

Scenario 1: OSPF Neighbor Stuck in INIT State

Problem:

OSPF neighbor adjacency is not progressing beyond the INIT state.

Possible reasons:

1. Hello packets are being received, but the router does not see its own Router ID in the neighbor's Hello packet.

2. One-way communication.

3. Multicast filtering or ACL restrictions.

4. Layer 2 connectivity problems.

Troubleshooting commands:

show ip ospf neighbor
show ip ospf interface
show ip interface

Solution:

• Verify bidirectional IP connectivity.

• Check ACLs and multicast filtering.

• Verify that OSPF Hello packets are exchanged in both directions.

Scenario 2: OSPF Neighbor Stuck in EXSTART / EXCHANGE

Problem:

OSPF neighbors are stuck in EXSTART or EXCHANGE states.

This is one of the commonly encountered OSPF troubleshooting scenarios.

Possible reasons:

1. MTU mismatch between neighboring routers.

2. Duplicate Router IDs.

3. DBD packet negotiation issues.

4. Packet loss or interface problems.

Troubleshooting commands:

show ip ospf neighbor
show ip ospf interface
show interface

Solution:

Check the interface MTU on both routers.

For example:

interface GigabitEthernet0/0
 ip mtu 1500

Ensure that both routers have compatible MTU settings.

Avoid using ip ospf mtu-ignore as the first solution. It bypasses the MTU check but does not resolve the underlying MTU inconsistency.

Scenario 3: OSPF Neighbor Not Forming

Problem:

Two directly connected routers are unable to establish an OSPF neighbor relationship.

Possible reasons:

• Area ID mismatch.

• Hello or Dead interval mismatch.

• Authentication mismatch.

• Subnet mask mismatch on broadcast networks.

• Area type mismatch.

• OSPF not enabled on the interface.

• Passive interface configuration.

Troubleshooting commands:

show ip ospf interface
show ip ospf neighbor
show running-config | section router ospf

Solution:

Verify the following parameters on both routers:

Parameter	Router 1	Router 2
Area ID	0	0
Hello Timer	10 sec	10 sec
Dead Timer	40 sec	40 sec
Authentication	Matching	Matching
Network Type	Matching	Matching

Correct any configuration mismatch.

Scenario 4: OSPF Routes Missing from Routing Table

Problem:

OSPF neighbor adjacency is FULL, but expected routes are missing from the routing table.

Possible reasons:

1. The destination network is not advertised.

2. The required LSA is missing.

3. Route filtering is configured.

4. The route is not eligible for installation because another route is preferred.

5. Area configuration or summarization issues.

Troubleshooting commands:

show ip route
show ip route ospf
show ip ospf database
show ip protocols

Solution:

• Verify network advertisements.

• Check the LSDB for the expected LSA.

• Verify route filtering and summarization.

• Check the administrative distance and competing routes.

Remember: OSPF adjacency being FULL does not guarantee that every expected route is installed in the routing table.

Scenario 5: OSPF Router ID Issues

Problem:

OSPF is not identifying the router correctly, or the expected Router ID is not being used.

Possible reasons:

• Incorrect router ID configuration.

• Duplicate Router ID.

• Router ID selected from an unexpected interface.

Troubleshooting command:

show ip ospf

Solution:

Configure a unique Router ID:

router ospf 1
 router-id 1.1.1.1

If OSPF is already running, a process restart may be required for the new Router ID to take effect:

clear ip ospf process

Use this command carefully because it resets OSPF adjacencies and can temporarily interrupt routing.

---

4. OSPF Troubleshooting Using Debug Commands

When normal show commands are insufficient, debug commands can provide deeper visibility into OSPF operations.

Useful commands:

debug ip ospf hello
debug ip ospf adj
debug ip ospf events

These commands help identify:

• Hello packet exchange issues.

• Neighbor state transitions.

• Adjacency negotiation failures.

• OSPF events and protocol errors.

To stop debugging:

undebug all

Production environment warning: Debug commands can generate significant CPU load and large volumes of output. Use them cautiously, preferably during an approved troubleshooting window.

---

5. A Systematic OSPF Troubleshooting Approach

Instead of randomly changing configurations, follow a structured troubleshooting methodology.

Step 1: Verify physical connectivity

show ip interface brief

Check whether the interfaces are up/up.

Step 2: Verify IP connectivity

ping <neighbor-ip>

Confirm basic IP reachability.

Step 3: Verify OSPF configuration

show running-config | section router ospf

Check the OSPF process and network configuration.

Step 4: Verify OSPF interfaces

show ip ospf interface brief

Confirm that the correct interfaces are participating in OSPF.

Step 5: Check neighbor adjacency

show ip ospf neighbor

Identify the neighbor state.

Step 6: Verify LSDB

show ip ospf database

Confirm the expected LSAs are present.

Step 7: Verify routing table

show ip route ospf

Check whether the expected routes are installed.

Step 8: Test end-to-end connectivity

ping <destination-ip>
traceroute <destination-ip>

Validate actual traffic forwarding.

---

6. Real-World OSPF Troubleshooting Example

Consider a simple network with two Cisco routers connected directly.

Network topology:

• R1 interface: 192.168.1.1/24

• R2 interface: 192.168.1.2/24

• OSPF Process ID: 1

• Area: 0

R1 Configuration:

router ospf 1
 router-id 1.1.1.1
 network 192.168.1.0 0.0.0.255 area 0

R2 Configuration:

router ospf 1
 router-id 2.2.2.2
 network 192.168.1.0 0.0.0.255 area 0

Issue:

The OSPF neighbor adjacency is not forming.

Troubleshooting process:

1. Check interface status using show ip interface brief.

2. Verify IP connectivity using ping.

3. Check OSPF interface configuration.

4. Verify area ID and Hello/Dead timers.

5. Check authentication and network type.

6. Check neighbor state using show ip ospf neighbor.

Once the mismatch is identified and corrected, the routers should establish the expected adjacency.

Expected neighbor state:

Neighbor ID     State
2.2.2.2         FULL/-

---

7. Important OSPF Troubleshooting Tips for Network Engineers

Here are some practical tips to remember when troubleshooting OSPF in an enterprise environment:

1. Always check physical and Layer 3 connectivity first.

2. Never assume that FULL adjacency means routing is working correctly.

3. Understand the difference between 2-Way and FULL states.

4. Check MTU whenever adjacency gets stuck in EXSTART or EXCHANGE.

5. Verify Hello and Dead intervals.

6. Check for duplicate Router IDs.

7. Verify the correct OSPF area assignment.

8. Check ACLs and firewall policies for OSPF protocol 89.

9. Verify that OSPF is enabled on the intended interfaces.

10. Use debug commands only when necessary.

---

Conclusion

OSPF troubleshooting is not just about remembering commands. It is about understanding how the protocol operates, identifying where communication is failing, and systematically isolating the root cause.

As a Network Engineer, you should develop the habit of troubleshooting layer by layer:

Physical Connectivity → IP Connectivity → OSPF Configuration → Neighbor Adjacency → LSDB → Routing Table → End-to-End Connectivity

This structured approach can help you resolve OSPF issues faster and minimize network downtime.

Keep Learning. Keep Configuring. Keep Troubleshooting.

About Network Engineer Stuff

Network Engineer Stuff is a learning platform dedicated to networking professionals, aspiring network engineers, and IT enthusiasts.

We share practical networking tutorials, Cisco configurations, troubleshooting scenarios, interview questions, and real-world networking concepts.

Subscribe to Network Engineer Stuff on YouTube and take your networking skills to the next level!

About EIGRP

 

EIGRP Explained: Enhanced Interior Gateway Routing Protocol – Configuration, Working, Metrics and Interview Questions

Introduction to EIGRP

EIGRP (Enhanced Interior Gateway Routing Protocol) is an advanced distance-vector routing protocol developed by Cisco. It is designed to help routers dynamically exchange routing information and identify the best paths to destination networks.

EIGRP is widely used in enterprise networking because of its fast convergence, efficient routing updates, and flexible metric calculation.

If you are preparing for CCNA, CCNP, or a Network Engineer interview, understanding EIGRP is an important part of learning dynamic routing protocols.

In this blog, we will explore EIGRP fundamentals, how it works, neighbor adjacency requirements, metric calculation, basic configuration, verification commands, and troubleshooting.

1. What is EIGRP?

EIGRP stands for Enhanced Interior Gateway Routing Protocol.

It is an Interior Gateway Protocol (IGP), which means it is used to exchange routing information within an autonomous system.

Important characteristics:

• Advanced distance-vector routing protocol.

• Uses DUAL (Diffusing Update Algorithm).

• Supports IPv4 and IPv6.

• Supports unequal-cost load balancing.

• Provides fast convergence.

• Uses incremental routing updates.

• Supports route summarization.

• Uses IP Protocol Number 88.

2. How Does EIGRP Work?

EIGRP routers exchange Hello packets to discover neighboring routers.

Once neighbors are discovered, they establish adjacency and exchange routing information.

EIGRP uses three important tables:

A. Neighbor Table

Contains information about directly connected EIGRP neighbors.

B. Topology Table

Contains routes learned from EIGRP neighbors, including successor and feasible successor information.

C. Routing Table

Contains the best routes selected by EIGRP and other routing protocols.

3. EIGRP Neighbor Adjacency Requirements

For two routers to establish an EIGRP neighbor relationship, the following conditions must be satisfied:

1. Both routers must use the same EIGRP Autonomous System (AS) number.

2. K-values must match.

3. IP connectivity must exist on the common link.

4. EIGRP must be enabled on the connecting interfaces.

5. Authentication parameters must match, if authentication is configured.

6. EIGRP must not be configured as passive on the connecting interface.

7. The routers must exchange compatible EIGRP Hello packets.

Important: The network statements do not need to be identical on both routers. They must enable EIGRP on the common interface.

4. EIGRP Packet Types

EIGRP uses five packet types:

Packet Type	Purpose
Hello	Discovers and maintains neighbors
Update	Exchanges routing information
Query	Requests alternative routing information
Reply	Responds to Query packets
Acknowledgment	Acknowledges reliable EIGRP packets

EIGRP uses RTP (Reliable Transport Protocol) to provide reliable delivery of certain EIGRP packets.

5. EIGRP Metric Calculation

EIGRP uses a composite metric to select the best path.

By default, the metric calculation considers:

• Bandwidth

• Delay

EIGRP also supports additional metric components:

• Reliability

• Load

The default K-values are:

K-value	Value	Metric Component
K1	1	Bandwidth
K2	0	Load
K3	1	Delay
K4	0	Reliability
K5	0	Additional reliability-related factor

The default metric formula is based on minimum bandwidth and cumulative delay.

6. EIGRP Administrative Distance

Administrative Distance (AD) determines the trustworthiness of a routing source.

Route Type	Administrative Distance
Internal EIGRP	90
External EIGRP	170
EIGRP Summary Route	5

Lower Administrative Distance is preferred when multiple routing sources advertise the same destination prefix.

7. Basic EIGRP Configuration Between Two Routers

Let's understand EIGRP with a practical Cisco Packet Tracer example.

Network Topology

Two routers are connected through an inter-router network.

IP Addressing Scheme

Device	Interface	IP Address
R1	G0/0	192.168.1.1/24
R1	G0/1	10.0.0.1/30
R2	G0/0	192.168.2.1/24
R2	G0/1	10.0.0.2/30

Router R1 Configuration

enable
configure terminal

hostname R1

interface gigabitEthernet 0/0
ip address 192.168.1.1 255.255.255.0
no shutdown
exit

interface gigabitEthernet 0/1
ip address 10.0.0.1 255.255.255.252
no shutdown
exit

router eigrp 100
network 192.168.1.0
network 10.0.0.0
no auto-summary
exit

end
write memory

Router R2 Configuration

enable
configure terminal

hostname R2

interface gigabitEthernet 0/0
ip address 192.168.2.1 255.255.255.0
no shutdown
exit

interface gigabitEthernet 0/1
ip address 10.0.0.2 255.255.255.252
no shutdown
exit

router eigrp 100
network 192.168.2.0
network 10.0.0.0
no auto-summary
exit

end
write memory

Understanding the EIGRP Commands

router eigrp 100

Enables EIGRP using Autonomous System number 100.

network 192.168.2.0

Enables EIGRP on interfaces matching the specified network and advertises the connected network.

network 10.0.0.0

Enables EIGRP on the interface connecting the two routers.

no auto-summary

Disables automatic classful route summarization, preserving more specific network prefixes.

exit

Exits EIGRP configuration mode.

8. EIGRP Verification Commands

After configuring EIGRP, use these commands to verify the configuration.

Command	Purpose
show ip eigrp neighbors	Displays EIGRP neighbors
show ip eigrp topology	Displays EIGRP topology
show ip route eigrp	Displays EIGRP-learned routes
show ip protocols	Displays routing protocol information
show ip eigrp interfaces	Displays EIGRP-enabled interfaces
show ip interface brief	Displays interface status

In the routing table, EIGRP-learned internal routes are identified by the letter D.

9. What is DUAL in EIGRP?

DUAL stands for Diffusing Update Algorithm.

It is the algorithm used by EIGRP to calculate loop-free paths and provide fast convergence.

Two important EIGRP route concepts are:

Successor: The best path to a destination, installed in the routing table.

Feasible Successor: A loop-free backup path that satisfies the Feasibility Condition.

The Feasibility Condition is satisfied when the reported distance from a neighbor is less than the current feasible distance.

This allows EIGRP to use an eligible backup path without performing a new route calculation.

10. EIGRP vs OSPF

Feature	EIGRP	OSPF
Protocol Type	Advanced Distance Vector	Link State
Algorithm	DUAL	SPF
Metric	Composite Metric	Cost
AD	90 Internal	110
IP Protocol	88	89
Multicast IPv4	224.0.0.10	224.0.0.5 and 224.0.0.6
Load Balancing	Equal and Unequal Cost	Equal Cost
Convergence	Generally Fast	Generally Fast

11. Common EIGRP Interview Questions

Q1. What is the full form of EIGRP?

Enhanced Interior Gateway Routing Protocol.

Q2. What is the default Administrative Distance of internal EIGRP?

90. 
    

Q3. What is the default Administrative Distance of external EIGRP?

170. 
     

Q4. Which algorithm does EIGRP use?

DUAL (Diffusing Update Algorithm).

Q5. What is the IP protocol number of EIGRP?

88. 
    

Q6. What is the multicast address used by EIGRP for IPv4?

224.0.0.10.

Q7. What are the default EIGRP K-values?

K1=1, K2=0, K3=1, K4=0, K5=0.

Q8. What is a successor route?

The best path to a destination selected by EIGRP.

Q9. What is a feasible successor?

A loop-free backup path that satisfies the Feasibility Condition.

Q10. What is the purpose of no auto-summary?

It disables automatic classful route summarization.

Conclusion

EIGRP is an important dynamic routing protocol for network engineers. Its DUAL algorithm, fast convergence, efficient routing updates, and support for unequal-cost load balancing make it an interesting protocol to understand.

For practical learning, configure EIGRP in Cisco Packet Tracer, verify neighbor adjacency, inspect the topology table, and test end-to-end connectivity.

Keep Learning. Keep Configuring. Keep Growing!

---

About Network Engineer Stuff

Network Engineer Stuff is a learning platform for networking professionals, students, and aspiring network engineers.

Subscribe to our YouTube channel for practical Cisco configurations, networking tutorials, real-world troubleshooting, and technical interview preparation.

NETWORK ENGINEER STUFF

OSPF Configuration | Same Area | Cisco Packet Tracer

 Topology 

Step 1 - Basic configuration

R1

hostname R1

int Se0/1/0

ip address 12.1.1.1 255.255.255.252

description **WAN_to_R2**

no shut

int gi0/0/0

ip address 192.168.1.1 255.255.255.0

description **LAN**

no shut

int loopback 1

ip address 1.1.1.1 255.255.255.255

R2

hostname R2

int Se0/1/0

ip address 12.1.1.2 255.255.255.252

description **WAN_to_R1**

no shut

int gi0/0/0

ip address 192.168.2.1 255.255.255.0

description **LAN**

no shut

int loopback 1

ip address 2.2.2.2 255.255.255.255

Step 2 - Configuration of OSPF 

<Verifying no OSPF configuration>

R1#

R1#

R1#show run | section ospf 

R1#

R1#show ip ospf

R1#

R2#show run | section ospf 

R2#

R2#show ip ospf

R2#

<Configuring OSPF>

R1

router ospf 1

router-id 1.1.1.1

network 12.1.1.0 0.0.0.3 area 0

network 192.168.1.0 0.0.0.255 area 0

R2

router ospf 1

router-id 2.2.2.2

network 12.1.1.0 0.0.0.3 area 0

network 192.168.2.0 0.0.0.255 area 0

<Verification Commands>

<Verification 1 # OSPF is running and OSPF RID>

R1#show ip ospf 

 Routing Process "ospf 1" with ID 1.1.1.1

 Supports only single TOS(TOS0) routes

 Supports opaque LSA

 SPF schedule delay 5 secs, Hold time between two SPFs 10 secs

 Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs

 Number of external LSA 0. Checksum Sum 0x000000

 Number of opaque AS LSA 0. Checksum Sum 0x000000

 Number of DCbitless external and opaque AS LSA 0

 Number of DoNotAge external and opaque AS LSA 0

 Number of areas in this router is 1. 1 normal 0 stub 0 nssa

 External flood list length 0

    Area BACKBONE(0)

        Number of interfaces in this area is 2

        Area has no authentication

        SPF algorithm executed 3 times

        Area ranges are

        Number of LSA 2. Checksum Sum 0x01a195

        Number of opaque link LSA 0. Checksum Sum 0x000000

        Number of DCbitless LSA 0

        Number of indication LSA 0

        Number of DoNotAge LSA 0

        Flood list length 0

<Verification 2 #Confirm OSPF is enabled on the correct interfaces>

R1#show ip ospf interface brief

Interface     PID   Area                     IP Address/Mask          Cost  State  Nbrs F/C

Se0/1/0         1   0                       12.1.1.1/255.255.255.252   64   POINT  0/0

Gig0/0/0        1   0                      192.168.1.1/255.255.255.0   1       DR  0/0

<Verification 3 #Confirm OSPF neighbor adjacency is formed>

R1#show ip ospf neighbor 

Neighbor ID     Pri   State           Dead Time   Address         Interface

2.2.2.2           0   FULL/  -        00:00:30    12.1.1.2        Serial0/1/0

R1#

<Verification 4 #Confirm OSPF routes are installed>

R1#

R1#show ip route ospf

O    192.168.2.0 [110/65] via 12.1.1.2, 00:04:28, Serial0/1/0

<Verification 5 # End to End PC-PC Ping>

Ping from PC0 to PC2

C:\>ping 192.168.2.200

Pinging 192.168.2.200 with 32 bytes of data:

Reply from 192.168.2.200: bytes=32 time=5ms TTL=126

Reply from 192.168.2.200: bytes=32 time=16ms TTL=126

Reply from 192.168.2.200: bytes=32 time=13ms TTL=126

Reply from 192.168.2.200: bytes=32 time=1ms TTL=126

Ping statistics for 192.168.2.200:

    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),

Approximate round trip times in milli-seconds:

    Minimum = 1ms, Maximum = 16ms, Average = 8ms

Download the lab 

https://drive.google.com/file/d/13x7pHJj-CQPFbby3WmwhEVgFgjogOHtK/view?usp=drive_link

Thanks for Reading !

Network Engineer Stuff

TOP 10 Basic NOC Network Engineer Interview Questions & Answers | 2026

Question 1 – What is a NOC?

Ans- 

NOC  - Network Operations Center

It is a centralized location where IT/network team/s are responsible for 24×7 monitoring and troubleshooting of network infrastructure.

This is to ensure maximum uptime, 

Performance and security.

Types of Networks Have NOCs – 

ISPs 

Banks 

Data Center

Cloud Services Providers 

Etc.

Question 2 – What is your understanding about the role of L1 NOC Engineer ?

Ans-

A NOC Engineer- 

Monitors alerts

Handles incidents

Performs basic troubleshooting

Opens tickets

Escalates issues.

Question 3 – What is difference between alert and incident?

Ans-

Question 4 – What is ticket?

Ans- 

A ticket is a record used to track issues, actions taken, and resolution.

Ticketing Tool Examples:

ServiceNow

BMC Remedy

Jira Service Management

Question 5 – What is Escalation?

Ans-

Escalation is the process of passing an issue to a higher support level or specialized team when it cannot be resolved within defined time or capability limits. 

Transferring an issue to a higher technical team (L1 → L2 → L3) because advanced skills are required.

  Example:L1 cannot resolve a routing issue → escalated to L2 Network Engineer

Question 6 – What is SLA?

Ans-

SLA - Service Level Agreement

An SLA is a formal agreement that defines the expected service levels, including response time, resolution time, and availability.

Example 

It will define what will be the time to response or resolution the failure. 

The percentage of time a network or a service should remain operational 

The time to escalate the issue to a higher support level for resolution . 

Etc….

Question 7 – What is MTTR?

Ans-

MTTR (Mean Time to Repair) is the average time taken to restore a service or resolve an incident after it occurs.

In simple terms:It measures how quickly an issue is fixed.

Formula:MTTR = Total downtime ÷ Number of incidents

Example:If 3 link failures took a total of 90 minutes to resolve:MTTR = 90 ÷ 3 = 30 minutes

Why MTTR is important (NOC perspective):

Measures incident response efficiency

Helps meet SLA commitments

Used to evaluate NOC and Network Team performance.

Question 8 – What is difference between packet loss and latency ?

Ans-

Packet Loss - Packets that fail to reach the destination. 

It is measured in % 

Latency - Time taken for a packet to travel from source to destination.

It is measured in ms

Question 9 – What is a maintenance window?

Ans-

A maintenance window is a pre-approved, scheduled time period during which planned work (such as upgrades, patches, or configuration changes) is performed on network or IT systems.

Planned Downtime.

Example

A router upgrade is scheduled from 01:00 AM – 03:00 AM IST.Any link-down alerts during this time fall under the maintenance window.

Question 10 – What tools are used in NOC?

Ans-

NOC teams use different tools to 

Monitor

Detect

Troubleshoot

Document

Communicate network issues.

Ticketing / Incident Management Tools

Used to log, track, and close incidents.

Examples:

ServiceNow

BMC Remedy

Jira Service Management

Purpose:

Incident creation

SLA tracking

Escalation management

Network Diagnostic Tools

Used for basic and advanced troubleshooting.

Examples:

ping

traceroute / tracert

Purpose:

Connectivity testing

Latency & packet loss analysis

Configuration & Device Access Tools 

Used to access and manage network devices.

Examples:

SecureCRT

PuTTY

MobaXterm

Many more tools 

Log & Event Management Tools

Performance & Flow Analysis Tools

Documentation & Knowledge Base Tools

etc.

Thanks for reading 

Network Engineer Stuff

 

Interview Questions & Answers for L1 Support Engineer

Section 1 - Desktop & Windows Troubleshooting

Q: How would you troubleshoot a slow or unresponsive Windows system?

✅Answer:

Check Task Manager for high CPU/RAM/disk usage.

Verify if unnecessary startup apps are running.

Run disk cleanup, remove temp files.

Check for Windows updates or driver issues.

Scan for malware/viruses.

If unresolved, consider restoring to a previous restore point or reimaging.

Q: What steps will you take to troubleshoot a Blue Screen of Death (BSOD)?

✅ Answer:

Note the error code (e.g., 0x0000007B).

Boot in Safe Mode to check recent driver/software changes.

Update or roll back drivers.

Check hardware (RAM/Hard disk) via diagnostics.

Run sfc /scannow and DISM commands.

Check Windows Event Viewer for logs.

Section 2 - Networking & IP

Q: How do you troubleshoot if a user cannot access the internet?

✅ Answer:

Verify physical connectivity (LAN/WiFi).

Check IP address via ipconfig.

Ping gateway & DNS.

Check DNS resolution by pinging websites (e.g., ping google.com).

Flush DNS (ipconfig /flushdns).

Restart NIC or renew IP (ipconfig /release & ipconfig /renew).

Verify firewall/proxy settings.

Q: Difference between DNS and DHCP?

✅ Answer:

DNS resolves hostnames to IP addresses.

DHCP automatically assigns IP addresses to devices in a network.

Q: How do you troubleshoot printer not working?

✅ Answer:

Check physical connections (USB/Network).

Verify if printer is online & default.

Restart spooler service (services.msc).

Update/reinstall printer drivers.

Test with another device.

Section 3 - Active Directory & O365

Q: What is Active Directory and why is it used?

✅ Answer:

A centralized directory service for managing users, computers, and permissions in a domain.

Used for authentication, authorization, and policy enforcement (via Group Policy).

Q: A user is unable to log into O365 – how would you troubleshoot?

✅ Answer:

Verify correct credentials & account status in Azure AD/Admin Portal.

Reset password if required.

Check internet and firewall/proxy settings.

Verify license assignment in O365 Admin.

Section 4 - SQL & WordPress (Basic)

Q: How do you connect to a SQL database and check connectivity?

✅ Answer:

Use SQL Server Management Studio (SSMS).

Test connection string (hostname/IP, username, password).

Ping database server or test via telnet <ip> 1433.

Q: What basic troubleshooting steps would you do for WordPress not loading?

✅ Answer:

Check if the web server (Apache/Nginx) is running.

Verify database connection in wp-config.php.

Clear browser cache and disable plugins.

Check domain DNS propagation.

Section 5 Behavioral & SLA-based Questions

Q: How do you handle an issue that you cannot resolve immediately?

✅ Answer:

Document steps tried.

Escalate to L2 team with logs/screenshots.

Keep user updated on SLA and resolution timeline.

Q: How do you prioritize multiple tickets?

✅ Answer:

Based on impact and urgency.

Critical business-impacting issues (server outage) first, then medium/low issues.

Follow ITIL guidelines if applicable.

Summary of L1 Support Engineer Role

Core Focus: First-level technical support (desktops, laptops, network basics, AD, O365).

Tasks: Troubleshoot user issues, maintain system health, escalate unresolved cases to L2/L3.

Tools Knowledge: AD, O365 admin portal, basic SQL, ticketing tools (e.g., ServiceNow), antivirus console.

Soft Skills: Client communication, vendor coordination, SLA adherence.

Tips to Ace the Interview

Brush up Basics:

IP addressing (private/public ranges), DNS/DHCP.

Windows troubleshooting commands (ping, ipconfig, netstat, nslookup).

Active Directory basics (adding/removing users).

Hands-On Practice:

Simulate printer setup, BSOD fixes in VM, create test O365 accounts (trial).

ITIL Awareness:

Know ticket priorities, escalation flow, SLA handling.

Communication Skills:

Explain technical fixes clearly to non-technical users.

Real Scenarios Prep:

Be ready for scenario-based Qs (e.g., "User can't access shared folder", "WiFi drops frequently").

Extra Edge:

Learn basic PowerShell commands, server patching, antivirus console basics.

 YOUTUBE CHANNEL - NETWORK ENGINEER STUFF

https://www.youtube.com/@NetworkEngineerStuff

INSTAGRAM ACCOUNT - networkengineerstuff

EMAIL ID - networkengineerstuff@gmail.com