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Setting Global BGP Parameters Configuring BGP
page 4-24 OmniSwitch 6800/6850/9000 Advanced Routing Configuration Guide December 2007
Synchronizing BGP and IGP Routes
The default behavior of BGP requires that it must be synchronized with the IGP before BGP may adver-
tise transit routes to external ASs. It is important that your network is consistent about the routes it adver-
tises, otherwise traffic can be lost.
The BGP rule is that a BGP router should not advertise to external neighbors destinations learned from
IBGP neighbors unless those destinations are also known via an IGP. This is known as synchronization. If
a router knows about a destination via an IGP, it is assumed that the route has already been propagated
inside the AS and internal reachability is ensured.
The consequence of injecting BGP routes inside an IGP is costly. Redistributing routes from BGP into the
IGP results in major overhead on the internal routers, and IGPs are really not designed to handle that many
routes.
The ip bgp synchronization command enables or disables BGP internal synchronization. Enabling this
command will force all routers (BGP and non-BGP) in an AS to learn all routes learned over external
BGP. Learning the external routes forces the routing tables for all routers in an AS to be synchronized and
ensure that all routes advertised within an AS are known to all routers (BGP and non-BGP). However,
since routes learned over external BGP can be numerous, enabling synchronization can place an extra
burden on non-BGP routers.
To enable synchronization, enter the following command:
-> ip bgp synchronization
The BGP speaker will now synchronize with the IGP. The default for synchronization is disabled.
To deactivate synchronization, enter the same command with the no keyword, as shown:
-> no ip bgp synchronization