H3C LS-3100-52P-OVS-H3 Operation Manual page 673

Figure 1-10 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 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.
If a non-BGP router works in an AS, it may discard a packet due to an unreachable destination. As
shown in Figure
sends a packet to 8.0.0.0/8 through Router D, which finds from its routing table that Router B is the next
hop (configured using the peer next-hop-local command). Because Router D has learned the route to
Router B via IGP, it forwards the packet to Router C through route recursion. Router C has no idea
about the route 8.0.0.0/8, so it discards the packet.
1-11, Router E has learned a route of 8.0.0.0/8 from Router D via BGP. Then Router E
1-10