374 CHAPTER 30: BGP CONFIGURATION
Figure 124 Network diagram for BGP load balancing
In the above figure, 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. After that, Router C forwards routes to
Router D and Router E only once, with AS_PATH unchanged, NEXT_HOP changed
to Router C’s address. Other BGP transitive attributes apply according to route
selection rules.
BGP route advertisement rules
BGP supports the following route advertisement rules:
■ When multiple feasible routes exist, a 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 IBGP routes to IBGP peers.
■ A BGP speaker advertises IBGP routes to EBGP peers. Note that if BGP and IGP
synchronization is disabled, IBGP routes are advertised to EBGP peers directly. If
the feature is enabled, only IGP advertises the IBGP routes can BGP advertise
these routes to EBGP peers.
■ A BGP speaker advertises all routes to a newly connected peer.
IBGP and IGP
Synchronization
The routing information synchronization between IBGP and IGP is for avoidance of
giving wrong directions to routers outside of the local AS.
If a non-BGP router works in an AS, a packet forwarded via the router may be
discarded due to an unreachable destination. As shown in Figure 125, Router E
learned a route of 8.0.0.0/8 from Router D via BGP. Then Router E sends a packet
to Router A through Router D, which finds from its routing table that Router B is
the next hop (configured using the peer next-hop-local command). Since Router
D learned the route to Router B via IGP, it forwards the packet to Router C using
Router C
Router E
Router D
Router A
Router B
AS 100
AS 200
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