3Com 4800G Series, Configuration Manual

Page: 375 / 1246

BGP Overview 375
route recursion. Router C has no idea about the route 8.0.0.0/8, so it discards the
packet.
Figure 125 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 routing table can the IBGP router add the route into its BGP routing table
and advertise the route to the EBGP peer.
You can disable the synchronization feature in the following cases:
■
The local AS is not a transitive AS (AS20 is a transitive AS in the above figure).
■
IBGP routers in the local AS are fully meshed.
Settlements for
Problems Caused by
Large Scale BGP
Networks
Route summarization
The size of BGP routing tables on a large network is very large. Using route
summarization can reduce the routing table size.
By summarizing multiple routes with one route, a BGP router advertises only the
summary route rather than all routes.
Currently, the system supports both manual and automatic summarization. The
latter provides for controlling the attribute of a summary route and deciding
whether to advertise the route.
Route dampening
BGP route dampening is used to solve the issue of route instability such as route
flaps, that is, a route comes up and disappears in the routing table frequently.
When a route flap occurs, the routing protocol sends an update to its neighbor,
and then the neighbor needs to recalculate routes and modify the routing table.
Therefore, frequent route flaps consume large bandwidth and CPU resources even
affect normal operation of the network.
In most cases, BGP is used in complex networks, where route changes are very
frequent. To solve the problem caused by route flaps, BGP uses route dampening
to suppress unstable routes.
Router A
Router B
Router C
Router D
Router E
EBGP IGP IGP
EBGP
AS 20
IBGP
AS 10 AS 30