Tuesday, May 4, 2010

BGP Syncronization

We all know the BGP rule of syncronization states that BGP speaking routers will not advertise a route within the AS unless all other routers know about the route via an IGP such as EIGRP, RIP, IGRP, etc... This is basically a black hole prevention mechanism.  Take the following diagram:

Worry about AS 6501 here.  Imagine R0 is hosting a route to  The only routers running BGP in AS 6501 are R3 and R4.  R3, R4, and R7 run EIGRP AS 6501 as their IGP.  Now imagine that R3's link to R0 was severed, but R3 was sending packets to  It has to go through R7 to reach now, but R7 does not have that in its routing table.  It drops the packet, creating the black hole that synchronization works to prevent.  This is how you tell if a route is not synchronized:

R3#show ip route
% Network not in table
R3#show ip bgp
BGP routing table entry for, version 19
Paths: (1 available, no best path)
Flag: 0x820
Not advertised to any peer
1200 (metric 30720) from (
Origin IGP, metric 0, localpref 100, valid, internal, not synchronized

Notice the not synchronized at the bottom of the show ip bgp output. This shows that synchronization has not been accomplished with bgp and the running igp (eigrp). This is true in this case. But say R7 has a default route out the two border routers...it would then be safe to turn off synchronization via the no synch command. Another instance where it would be safe to turn it off is if all routers in the local as were running IBGP.

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