Because the routes learned by means of EBGP are extensive, redistributing those routes into your IGP consumes processor and memory resources. You can disable synchronization if your AS does not pass traffic from one AS to another or if all the transit routers in your AS run BGP. Figure 37 shows the same configuration as in the previous example, except that all the routers in AS 100 now run IBGP. As a result, all the routers receive updates learned by the area border routers from external BGP speakers.
Figure 37: Disabling Synchronization
If synchronization is disabled, a BGP speaker propagates a BGP route learned from a peer only if it is the best route to the prefix in the IP routing table. However, the speaker does advertise the routes that it originates.
The following commands show how to configure routers Boston, NY, and Chicago (shown in Figure 37) with synchronization disabled for routers NY and Boston. The no synchronization command enables router NY to put the route to 192.56.0.0/16 in its IP routing table and advertise it to router Chicago without learning about 192.56.00/16 from router Albany. The command also enables router Boston to put the route to 192.30.0.0/16 in its IP routing table and advertise it to router LA without learning about 192.30.00/16 from router Albany.
To configure router Boston:
- host1((config)#router bgp 100
- host1(config-router)#neighbor 2.2.2.2 remote-as
100
- host1(config-router)#neighbor 4.4.4.1 remote-as
100
- host1(config-router)#neighbor 1.1.1.2 remote-as
300
- host1(config-router)#no synchronization
To configure router NY:
- host2(config)#router bgp 100
- host2(config-router)#neighbor 3.3.3.2 remote-as
200
- host2(config-router)#neighbor 2.2.2.1 remote-as
100
- host2(config-router)#neighbor 4.4.4.3 remote-as
100
- host2(config-router)#no synchronization
To configure router Albany:
- host3(config)#router bgp 100
- host3(config-router)#neighbor 4.4.4.4 remote-as
100
- host3(config-router)#neighbor 4.4.4.2 remote-as
100
- host3(config-router)#no synchronization
To configure router Chicago:
- host4(config)#router bgp 200
- host4(config-router)#neighbor 3.3.3.1 remote-as
100
To configure router LA:
- host5(config)#router bgp 300
- host5(config-router)#neighbor 1.1.1.1 remote-as
100
- host5(config-router)#network 192.56.0.0
synchronization
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