Settlements For Problems In Large Scale Bgp Networks - HP 4800G Series Configuration Manual

Figure 1-11 iBGP and IGP synchronization
If synchronization is enabled in this example, only when the route 8.0.0.0/24 received from Router B is
available in its IGP routing table, can Router D 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).
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.
Currently, 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 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 and even affect network normal operation.
In most cases, BGP is used in complex networks, where route changes are very frequent. To solve the
problem caused by route flaps, BGP route dampening is used to suppress unstable routes.
BGP route dampening uses a penalty value to judge the stability of a route. The 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 a 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.
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