H3C S7500E Series Ethernet Switches Operation Manual

Operation Manual – IPv4 Routing
H3C S7500E Series Ethernet Switches Chapter 5 BGP Configuration
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only once, with AS_PATH unchanged, NEXT_HOP changed to Router C’s address.
Other BGP transitive attributes apply according to route selection rules.
III. BGP route advertisement rules
BGP supports the following route advertisement rules:
z When multiple feasible routes exist, a BGP speaker advertises only the best route
to its peers.
z A BGP speaker advertises only routes used by itself.
z A BGP speaker advertises routes learned through EBGP to all BGP peers,
including both EBGP and IBGP peers.
z A BGP speaker does not advertise IBGP routes to IBGP peers.
z 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.
z A BGP speaker advertises all routes to a newly connected peer.
5.1.4 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 5-11, 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 route recursion.
Router C has no idea about the route 8.0.0.0/8, so it discards the packet.
Figure 5-11 IBGP and IGP synchronization
If synchronization is configured in this example, the IBGP router (Router D) checks the
learned IBGP route from its IGP routing table first. Only the route is available in the IGP