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.
Please find below some points which should help you understand the BGP Synchronization in much easier terms:-
1. Its a approach against potential network Loop
2. It simply means that, if a Route is advertised by IBGP to another IBGP (ofcourse with in the same AS) then, DO NOT include in the Routing Table (of the reciever) unless the same Route has been recieved from an IGP running in the same AS or there is a Static Route for the same destination.
I hope I have tried to make it easier for you.
Please let us know if there is anything more in the direction.
It is basically BGP's method of avoiding routing loops or blackholes. It is the only protocol where the next hop does not have to be directly connected synchronization was implemented to make sure that the next hop is reachable. It does this by refusing to publish IBGP routes that aren't known to the IGP.
Say you have the topology below:
Routers 1 and 2 are eBGP speakers used to connect to the internet. They also run ospf with R2 and advertise a default route to it, with router3's default being given a higher metric to cause it to be the backup. Routers 1 and 3 are also iBGP peers and are advertising the entire (for illustation only) internet routing table to each other. You are a PC behind router 2 and you try to ping a verizon customer at 152.x.x.x. Technically router 3 will have a better route since it is peered with Verizon, but R2 will send traffic to router1 because it is advertising a better default route and Router1 will use the ATT peer because the Verizon route is not synchronized. If you turn synchronization off (and configure next-hop self at R3) router 1 will install the iBGP route for the verion customer in it's routing table and send the traffic towards R3 when it receives it. This will cause a routing loop. Router2 will send the ping packets to router1 because of it's default route. Router1 will lookup the route for the verizon customer and find a next hop of router3. Rotuer1 will then lookup the next hop for router3 and find a next-hop of router2. It will then send the packet back to router2. Router2 having no knowledge of the BGP route will obey it's default route and send the packet back to router1, repeat ad infinitum (or at least until the TTL's expire).
If your autonomous system is passing traffic from another AS to a third AS, BGP should not advertise a route before all routers in your AS have learned about the route via IGP.
BGP will wait until IGP has propagated the route within the AS and then will advertise it to external peers. This is called synchronization.
In some cases you do not need synchronization. If you will not be passing
traffic from a different autonomous system through your AS, or if all routers in your AS will be running BGP, you can disable synchronization.
Disabling this feature can allow you to carry fewer routes in your IGP and allow BGP to converge more quickly.
Disabling synchronization is not automatic, if you have all your routers in the AS running BGP and you are not running any IGP, the router has no way of knowing that, and your router will be waiting forever for an IGP update about a certain route before sending it to external peers. You have to disable synchronization manually in this case for routing to work correctly.
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