H3C Technologies H3C S12500 Series Switches - Manual (Page 215)

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Settlements for problems in large-scale BGP networks
A large-scale BGP network has a great number of BGP peers and large routing table size. To facilitate
management and improve route distribution efficiency, you can use the following methods:
• Route summarization
Route summarization can reduce the BGP routing table size, 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 specific routes.
• 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 recalculates routes and modifies the routing table. Frequent route flaps consume large
bandwidth and CPU resources, which could affect network operations.
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 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 BGP 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 BGP routing table and advertised to other BGP peers.
Figure 78 BGP route dampening
• Peer group
You can organize BGP peers with the same attributes into a group to simplify their configurations.
Suppress
threshold
Reusable
threshold
Penalty
value
Time
Half-life
Suppress time