3Com Switch 8800 Advanced Software V5 Configuration Guide

428 CHAPTER 35: BGP CONFIGURATION
Figure 140 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 adds 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 rule
The current BGP implementation supports the following route advertisement rules:
■ When multiple available 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 from EBGPs to all BGP peers, including
both EBGP and IBGP peers.
■ A BGP speaker does not advertise routes learned from IBGPs to IBGP peers.
■ A BGP speaker advertises routes learned from IBGPs to EBGP peers. Note that if
information synchronization is disabled between BGP and IGP, IBGP routes are
advertised to EBGP peers. If enabled, only IGP advertises the IBGP routes can
these routes be advertised to EBGP peers.
■ A BGP speaker advertises all routes to a newly connected peer.
IBGP and IGP
Information
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 unreachable destination. As shown in
Figure 141, 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