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Figure 83
Network diagram for BGP load balancing
In Figure 83, Router D and Router E are iBGP peers of Router C. Router A and Router B both
advertise a route destined for the same destination to Router C. If load balancing is configured,
and the two routes have the same AS_PATH attribute, ORIGIN attribute, LOCAL_PREF, and MED,
Router C installs both the two routes to its route table for load balancing. Afterwards, Router C
forwards to Router D and Router E the route that has AS_PATH unchanged but has NEXT_HOP
changed to Router C; other BGP transitive attributes are those of the best route.
BGP route advertisement rules
The current BGP implementation supports the following route advertisement rules:
•
When multiple feasible routes to a destination exist, the BGP speaker advertises only the
best route to its peers.
•
A BGP speaker advertises only routes used by itself.
•
A BGP speaker advertises routes learned through eBGP to all BGP peers, including both
eBGP and iBGP peers.
•
A BGP speaker does not advertise routes from an iBGP peer to other iBGP peers.
•
A BGP speaker advertises routes learned through iBGP to eBGP peers. Note that if BGP and
IGP synchronization is disabled, those routes are advertised to eBGP peers directly. If the
feature is enabled, only after IGP advertises those routes can BGP advertise the routes to
eBGP peers.
•
A BGP speaker advertises all routes to a newly connected peer.
iBGP and IGP synchronization
Routing information synchronization between iBGP and IGP avoids giving wrong directions to
routers outside of the local AS.
