Today we will discuss regarding BGP Synchronization Rule which states-
"Don't use or advertise the route/s learned via an iBGP neighbor to an eBGP neighbor unless & until the same is/are learned via some other IGP like RIP , OSPF ,EIGRP etc . "
This feature is off by default in Cisco IOS Software release 12.2(8)T and later.
To disable BGP Synchronization
R1(config)#router bgp 100 -------> AS is 100
R1(config-router)#no synchronization
To enable BGP Synchronization
R1(config-router)#synchronization
Lets check the interfaces of all the Routers
R1#sh int des
Interface Status Protocol Description
Fa0 up up ***CONNECTION_TO_R2***
Se0 up up ***CONNECTION_TO_R3***
R1#sh ip int brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0 12.1.1.1 YES manual up up
Serial0 13.1.1.1 YES manual up up
R2#sh int des
Interface Status Protocol Description
Fa0 up up ***CONNECTION_TO_R1***
Lo0 up up
R2#sh ip int brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0 12.1.1.2 YES manual up up
Loopback0 100.0.0.1 YES manual up up
R3#sh int des
Interface Status Protocol Description
Se0 up up ***CONNECTION_TO_R1***
R3#sh ip int brief
Interface IP-Address OK? Method Status Protocol
Serial0 13.1.1.3 YES manual up up
Lets configure BGP on all Routers by keeping synchronization off .
R1(config)#router bgp 100
R1(config-router)#neighbor 12.1.1.2 remote-as 100
R1(config-router)#neighbor 13.1.1.3 remote-as 200
R1(config-router)#no synchronization
R2(config)#router bgp 100
R2(config-router)#neighbor 12.1.1.1 remote-as 100
R2(config-router)#no synchronization
R3(config)#router bgp 200
R3(config-router)#neighbor 13.1.1.1 remote-as 100
Lets advertise 100.0.0.0/24 network on R2 which is iBGP neighbor of R1
R2(config)#router bgp 100
R2(config-router)#network 100.0.0.0 mask 255.255.255.0
Since BGP Synchronization is off on R1 , R1 is using the route i.e installed in its routing and also advertising to R3 which is its eBGP neighbor
R1#sh ip bgp
BGP table version is 2, local router ID is 13.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*>i100.0.0.0/24 12.1.1.2 0 100 0 i
R1#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
100.0.0.0/24 is subnetted, 1 subnets
B 100.0.0.0 [200/0] via 12.1.1.2, 00:39:07
12.0.0.0/24 is subnetted, 1 subnets
C 12.1.1.0 is directly connected, FastEthernet0
13.0.0.0/24 is subnetted, 1 subnets
C 13.1.1.0 is directly connected, Serial0
R3#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
100.0.0.0/24 is subnetted, 1 subnets
B 100.0.0.0 [20/0] via 13.1.1.1, 00:38:31
13.0.0.0/24 is subnetted, 1 subnets
C 13.1.1.0 is directly connected, Serial0
Now lets ON the Synchronization on R1 !!!!!
R1(config)#router bgp 100
R1(config-router)#synchronization
and check the routing table
R1#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
100.0.0.0/24 is subnetted, 1 subnets
B 100.0.0.0 [200/0] via 12.1.1.2, 00:41:03
12.0.0.0/24 is subnetted, 1 subnets
C 12.1.1.0 is directly connected, FastEthernet0
13.0.0.0/24 is subnetted, 1 subnets
C 13.1.1.0 is directly connected, Serial0
Still we are able to view the route .
Lets clear the bgp neighborship once
R1#clear ip bgp *
R1#
R1#
Apr 2 15:54:13.055: %BGP-5-ADJCHANGE: neighbor 12.1.1.2 Down User reset
Apr 2 15:54:13.059: %BGP-5-ADJCHANGE: neighbor 13.1.1.3 Down User reset
Apr 2 15:54:13.839: %BGP-5-ADJCHANGE: neighbor 12.1.1.2 Up
R1#
Apr 2 15:54:14.195: %BGP-5-ADJCHANGE: neighbor 13.1.1.3 Up
!! Now you can see the route is present in bgp table but not in routing table
R1#sh ip bgp
BGP table version is 2, local router ID is 13.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
* i100.0.0.0/24 12.1.1.2 0 100 0 i
R1#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
12.0.0.0/24 is subnetted, 1 subnets
C 12.1.1.0 is directly connected, FastEthernet0
13.0.0.0/24 is subnetted, 1 subnets
C 13.1.1.0 is directly connected, Serial0
Lets check the routing table of R3 as well
R3#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
13.0.0.0/24 is subnetted, 1 subnets
C 13.1.1.0 is directly connected, Serial0
You can see the 100.0.0.0/24 route is not present in the routing table.
Hope you all find the blog useful !!!!
Thanks for reading !!!!
Very good article 😊
ReplyDelete