H3C Technologies H3C SR8800 - Manual (Page 219)

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• A BGP speaker advertises routes learned through IBGP to EBGP peers. 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 81 , Router E has learned a route of 8.0.0.0/8 from Router D via BGP. Router E then 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 is not aware of the route
8.0.0.0/8, so it discards the packet.
Figure 81 IBGP and IGP synchronization
For this example, if synchronization is enabled, and the route 8.0.0.0/24 received from Router B is
available in its IGP routing table, Router D adds the route into its BGP routing table and advertises the
route to the EBGP peer.
You can disable the synchronization feature in the following situations:
• The local AS is not a transitive AS (AS20 is a transitive AS in the above figure).
• Routers in the local AS are IBGP fully meshed.
Settlements for problems in large scale BGP networks
Route summarization
Route summarization can reduce the routing table size on a large network, and allow BGP routers to
advertise only summary routes rather than more specific routes.
The system supports both manual and automatic route summarization. Manual route summarization
allows you to determine the attribute of a summary route and whether to advertise the route.
Route dampening
BGP route dampening solves the issue of route instability such as route flaps—a route comes up and
disappears in the routing table frequently.