Before the discussion of synchronization, look at this scenario. RTC in AS300 sends updates about 170.10.0.0. RTA and RTB run iBGP, so RTB gets the update and is able to reach 170.10.0.0 via next hop 2.2.2.1. Remember that the next hop is carried via iBGP. In order to reach the next hop, RTB must send the traffic to RTE.
Assume that RTA has not redistributed network 170.10.0.0 into IGP. At this point, RTE has no idea that 170.10.0.0 even exists.
If RTB starts to advertise to AS400 that RTB can reach 170.10.0.0, traffic that comes from RTD to RTB with destination 170.10.0.0 flows in and drops at RTE.
Synchronization states that, if your AS passes traffic from another AS to a third AS, BGP should not advertise a route before all the routers in your AS have learned about the route via IGP. BGP waits until IGP has propagated the route within the AS. Then, BGP advertises the route to external peers.
In the example in this section, RTB waits to hear about 170.10.0.0 via IGP. Then, RTB starts to send the update to RTD. You can make RTB think that IGP has propagated the information if you add a static route in RTB that points to 170.10.0.0. Make sure that other routers can reach 170.10.0.0.
In some cases, you do not need synchronization. If you do not pass traffic from a different AS through your AS, you can disable synchronization. You can also disable synchronization if all routers in your AS run BGP. The disablement of this feature can allow you to carry fewer routes in your IGP and allow BGP to converge more quickly.
The disablement of synchronization is not automatic. If all your routers in the AS run BGP and you do not run IGP at all, the router has no way to know. Your router waits indefinitely for an IGP update about a certain route before the router sends the route to external peers. You have to disable synchronization manually in this case so that routing can work correctly:
router bgp 100
no synchronization
Note: Make sure that you issue the clear ip bgp address command to reset the session.
RTB#
router bgp 100
network 150.10.0.0
neighbor 1.1.1.2 remote-as 400
neighbor 3.3.3.3 remote-as 100
no synchronization
!--- RTB puts 170.10.0.0 in its IP routing table and advertises the network
!--- to RTD, even if RTB does not have an IGP path to 170.10.0.0.
RTD#
router bgp 400
neighbor 1.1.1.1 remote-as 100
network 175.10.0.0
RTA#
router bgp 100
network 150.10.0.0
neighbor 3.3.3.4 remote-as 100
