could anyone expain BGP synchronization?

aueddonline

I'm thinking it's a synchronization between an IGP and BGP of the routers that the IGP knows about right?

dtlokee

It is intended to prevent a BGP speaker from advertising BGP routes if the BGP speaker does not have an IGP path to the destination route. The goal was to prevent black hole routing inside an AS where the BGP speaking routers know about a destination because it is being advertised into BGP, but the non BGP speaking routers in the AS do not. In this case the BGP speaking routers can forward the packets, but then the non BGP speaking router can not so it will drop the packet or default route the packet. This usually only becomes a problem when the AS is providing transit services between other autonomous systems becuase the IGP is not used to exchange routes with the neighboring autonomous systems and therefore has no knowlege of the neighbors.

so say you had a network like this:

AS100---- R1 ---- R2 ----- R3 ----- R4 ------ R5 ----- AS 300
                         (AS200)

R1 and R2 are eBGP neighbors between AS100 and AS200. R3 and R5 are iBGP peers in AS200, and R4 and R5 are eBGP neighbors between AS200 and AS300. The problem comes in where R3 is not running BGP so it doesn't know about any prefixes from AS100 or AS200.

BGP synchronization is disabled by default in the current IOS versions and can be disabled without problems if your AS does not provide transit to other autonomous systems, or all of the routers in the transit path run BGP (this typicall means all the routers in the AS but could just be the core of the network and treat other pieces as stub and default route them)