Settlements For Problems In Large Scale Bgp Networks - HP 6125G Configuration Manual

Figure 53 BGP and IGP synchronization in an AS
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.
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.
When a route flap occurs, the routing protocol sends an update to its neighbor, and then the neighbor
must recalculate routes and modify the routing table. Frequent route flaps consume large bandwidth and
CPU resources, which could affect network operation.
In most cases, BGP is used in complex networks, where route changes are more frequent. To solve the
problem caused by route flaps, BGP route dampening is used to suppress unstable routes.
BGP route dampening, as shown in
bigger the value, the less stable the route. Each time a route flap occurs, BGP adds a penalty value (1000,
which is a fixed number and cannot be changed) to the route. When the penalty value of the route
exceeds the suppress value, the route is suppressed from being added into the routing table or being
advertised to other BGP peers.
The penalty value of the suppressed route will decrease to half of the suppress value after a period of time.
This period is called "Half-life". When the value decreases to the reusable threshold value, the route is
added into the routing table and advertised to other BGP peers.
Figure 54, uses a penalty value to judge the stability of a route. The
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