12-1 synchronization

Syntax Description:

This command has no arguments.

Purpose: IBGP learned routes are not installed in the IP routing table if the route has not been learned by an IGP or if the advertised next hop is inaccessible. This is called synchronization.

IOS Release: 10.0

Configuration Example: BGP Synchronization

In Figure 12-1, BGP synchronization is enabled by default. Router A is advertising network 199.172.1.0/24 to Router B via EBGP, and Router B is advertising 199.172.1.0/24 to Router C via IBGP.

Figure 12-1. Synchronization Applies to IBGP Routes

 Router A  interface Loopback0   ip address 199.172.1.1 255.255.255.0   !   router bgp 1   neighbor 172.17.1.1 remote-as 2   !   ___________________________________________________________________________  Router B  router bgp 2   neighbor 172.17.1.2 remote-as 1   neighbor 10.1.1.2 remote-as 2   ___________________________________________________________________________  Router C  router bgp 2   neighbor 10.1.1.1 remote-as 2  

Routers A and B have an EBGP connection, so there is no need for the BGP learned routes to be synchronized with an IGP. The 199.172.1.0/24 route will be installed in the IP routing table on Router B:

 rtrB#  show ip route  p2#  show ip route  Codes: C - connected, S - static, I - IGRP, 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, E - EGP        i - IS-IS, 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      172.17.0.0/24 is subnetted, 1 subnets C       172.17.1.0 is directly connected, Ethernet0/1      10.0.0.0/30 is subnetted, 1 subnets C       10.1.1.0 is directly connected, Serial2/1  B    199.172.1.0/24 [20/0] via 172.17.1.2, 00:22:24  

The 199.172.1.0/24 route will not be installed in the IP routing table on Router C, because this route was not learned by an IGP. Also, Router C does not have an entry in the IP routing table for the next hop. IBGP preserves the EBGP next-hop information.

 rtrC#  show ip bgp 199.172.1.0  BGP routing table entry for 199.172.1.0/24, version 0 Paths: (1 available, no best path)   Not advertised to any peer   1  172.17.1.2 (inaccessible) from 10.1.1.1 (132.1.1.1)   Origin IGP, metric 0, localpref 100, valid, internal, not synchronized, ref 2  rtrC#  show ip route  Codes: C - connected, S - static, I - IGRP, 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, E - EGP        i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default        U - per-user static route, o - ODR Gateway of last resort is not set      10.0.0.0/30 is subnetted, 1 subnets C       10.1.1.0 is directly connected, Serial1 

Disable synchronization on router C and determine if the 199.172.1.0 route will be installed in the IP routing table.

 Router C router bgp 2  no synchronization  neighbor 10.1.1.1 remote-as 2 rtrC#  show ip route  Codes: C - connected, S - static, I - IGRP, 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, E - EGP        i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default        U - per-user static route, o - ODR Gateway of last resort is not set      10.0.0.0/30 is subnetted, 1 subnets C       10.1.1.0 is directly connected, Serial1 rtrC#  show ip bgp 199.172.1.0  BGP routing table entry for 199.172.1.0/24, version 0 Paths: (1 available, no best path)   1  172.17.1.2 (inaccessible) from 10.1.1.1 (132.1.1.1)  Origin IGP, metric 0, localpref 100, valid, internal 

Even with synchronization disabled, the route will not be installed in the IP routing table, because the next hop is inaccessible. There are three ways to correct this problem:

• Advertise the next hop via an IGP.

• Have router B advertise itself as the next hop for all advertised BGP routes.

• Advertise the BGP routes via an IGP.

The third method involves redistributing the BGP routes into your IGP. Redistribution is not recommended due to the possible size of the BGP routing table. Also, when you redistribute BGP into an IGP, the BGP attributes such as AS path, metric, weight, and local preference are lost. If an autonomous system is being used as a transit AS, it is important to preserve the BGP attributes. Modify the configurations on Routers B and C so that the EBGP next hop received from Router A is advertised to router C via an IGP:

 Router B  router eigrp 1   network 172.17.0.0   network 10.0.0.0   ___________________________________________________________________________  Router C  router eigrp 1   network 172.17.0.0  

Verification

Verify that the BGP route 199.172.1.0/24 has been installed in the IP routing table on Router C:

 rtrC#  show ip route  Codes: C - connected, S - static, I - IGRP, 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, E - EGP        i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default        U - per-user static route, o - ODR Gateway of last resort is not set      10.0.0.0/30 is subnetted, 1 subnets C       10.1.1.0 is directly connected, Serial1  B    199.172.1.0/24 [200/0] via 172.17.1.2, 00:00:38   D    172.17.0.0/16 [90/2195456] via 10.1.1.1, 00:01:37, Serial1  

Although we have managed to get the 199.172.1.0/24 network in the IP routing table, we do not have IP connectivity to this network. Router A does not have a route back to Router C. Router B would need to advertise the 10.0.0.0 network to Router A via EBGP. If we ping the 199.172.1.0 network from Router C, we will be unsuccessful . Advertise the 10.1.1.0 network to Router A.

 Router B  router bgp 2   network 10.1.1.0 mask 255.255.255.252  

Verify IP connectivity to 199.172.1.0/24 by pinging the loopback on Router A:

 rtrC#  ping 199.172.1.1  Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 199.172.1.1, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 28/31/32 ms 

Troubleshooting

Step 1. Verify that the BGP neighbors are in the Established state using the show ip bgp neighbors command.

If the neighbor relationship is not in the Established state, see section 8-23.

Step 2. If synchronization is enabled, routes learned via IBGP will not be installed in the IP routing table unless the next hop is accessible and the prefixes have been learned via an IGP.