3Com Switch 4800G 24-Port Configuration Manual page 374

374 C 30: BGP C
HAPTER ONFIGURATION
IBGP and IGP
Synchronization
Figure 124 Network diagram for BGP load balancing
AS 100
Router A
Router E
AS 200
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
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route to its peers.
A BGP speaker advertises only routes used by itself.
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A BGP speaker advertises routes learned through EBGP to all BGP peers,
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including both EBGP and IBGP peers.
A BGP speaker does not advertise IBGP routes to IBGP peers.
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A BGP speaker advertises IBGP routes to EBGP peers. Note that if BGP and IGP
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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.
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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 B
Router C
Router D