BGP LAB DAY 6

 

 

Task:- Understanding BGP Synchronization Rule

Dont use or advertise the routes to eBGP learned by iBGP neighbor unless and until
same are learned via other IGP .

By default BGP Synchronization Rule is not enabled.

R3 have eBGP neighbor as R1 and iBGP neighbor as R4

!!On R4 lets create an loopback and advertise to R3 via iBGP
and then check on R3 and R4 what is happening !!

Remember one thing we wont advertise the same network via any IGP

R4(config)#int loopback 400
R4(config-if)#ip address 140.0.0.1 255.255.255.0

R4(config)#router bgp 300
R4(config-router)#network 140.0.0.0 mask 255.255.255.0

R4#sh ip bgp 140.0.0.0
BGP routing table entry for 140.0.0.0/24, version 39
Paths: (1 available, best #1, table default)
  Advertised to update-groups:
     2          4
  Refresh Epoch 1
  Local
    0.0.0.0 from 0.0.0.0 (40.1.4.1)
      Origin IGP, metric 0, localpref 100, weight 32768, valid, sourced, local, best
      rx pathid: 0, tx pathid: 0x0

Lets check on R3

R3#sh ip bgp 140.0.0.0
BGP routing table entry for 140.0.0.0/24, version 39
Paths: (1 available, best #1, table default)
  Advertised to update-groups:
     3
  Refresh Epoch 1
  Local
    4.4.4.4 (metric 435200) from 4.4.4.4 (40.1.4.1)
      Origin IGP, metric 0, localpref 100, valid, internal, best
      rx pathid: 0, tx pathid: 0x0

The route is present in Bgp table of R3

But lets check whether it is using it or not

For that lets have a look at the routing table

R3#sh ip route 140.0.0.0
Routing entry for 140.0.0.0/24, 1 known subnets
B        140.0.0.0 [200/0] via 4.4.4.4, 00:03:16

It is present in Routing table also

Now lets check on R1

R1#sh ip route 140.0.0.0
Routing entry for 140.0.0.0/24, 1 known subnets
B        140.0.0.0 [20/0] via 13.1.1.3, 00:04:34  ------------> Getting learned via R3

It is present in Routing table of R1 also

That means BGP Synchronization is disable by default

Now lets enable BGP Synchronization on R3

R3#sh run | section bgp
router bgp 300
 bgp log-neighbor-changes
 network 30.1.1.0 mask 255.255.255.0
 network 30.1.2.0 mask 255.255.255.0
 network 30.1.3.0 mask 255.255.255.0
 network 30.1.4.0 mask 255.255.255.0
 neighbor 4.4.4.4 remote-as 300
 neighbor 4.4.4.4 description **iBGP with R4**
 neighbor 4.4.4.4 update-source Loopback0
 neighbor 13.1.1.1 remote-as 100
 neighbor 13.1.1.1 description **eBP with R1**

R3(config)#router bgp 300
R3(config-router)#synchronization
R3(config-router)#^Z
R3#

R3#sh run | section bgp
router bgp 300
 synchronization ==================================> enabled
 bgp log-neighbor-changes
 network 30.1.1.0 mask 255.255.255.0
 network 30.1.2.0 mask 255.255.255.0
 network 30.1.3.0 mask 255.255.255.0
 network 30.1.4.0 mask 255.255.255.0
 neighbor 4.4.4.4 remote-as 300
 neighbor 4.4.4.4 description **iBGP with R4**
 neighbor 4.4.4.4 update-source Loopback0
 neighbor 13.1.1.1 remote-as 100
 neighbor 13.1.1.1 description **eBP with R1**

R3#clear ip bgp * soft in
R3#clear ip bgp * soft out

R3#sh ip route 140.0.0.0
% Network not in table
R3#
R3#sh ip bgp 140.0.0.0
BGP routing table entry for 140.0.0.0/24, version 0
Paths: (1 available, no best path)
  Not advertised to any peer
  Refresh Epoch 2
  Local
    4.4.4.4 (inaccessible) from 4.4.4.4 (40.1.4.1)
      Origin IGP, metric 0, localpref 100, valid, internal, not synchronized
      rx pathid: 0, tx pathid: 0


Now you can see the route is present in BGP but not in the routing table

Lets check on R1

R1#sh ip route 140.0.0.0              
Routing entry for 140.0.0.0/24, 1 known subnets
B        140.0.0.0 [20/0] via 2.2.2.2, 00:03:51  =========> it is stop learning via R3 and now its learning via R2

Thanks for Reading !!!!!

amartechstuff