Operation Manual – IPv4 Routing
H3C S3610&S5510 Series Ethernet Switches
Chapter 5 BGP Configuration
5-13
You can disable the synchronization feature in the following cases:
z
The local AS is not a transitive AS (AS20 is a transitive AS in the above figure).
z
IBGP routers in the local AS are fully meshed.
5.1.5 Settlements for Problems Caused by Large Scale BGP Networks
I. 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 any more specific routes.
Route summarization can be manual or automatic. The latter provides for controlling
the attribute of a summary route and deciding whether to advertise the route.
II. 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.
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 (the state change of a
route from active to inactive is a route flap), 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. That is, it is neither added into
the routing table, nor advertised to other BGP peers.
The penalty value of the suppressed route will reduce 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 in update packets.
