Operation Manual – IPv4 RoutingH3C S7500E Series Ethernet Switches Chapter 5 BGP Configuration5-13routing table can the IBGP router add the route into its BGP routing table and advertisethe route to the EBGP peer.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 NetworksI. Route summarizationThe size of BGP routing tables on a large network is very large. Using routesummarization can reduce the routing table size.By summarizing multiple routes with one route, a BGP router advertises only thesummary route rather than all routes.Currently, the system supports both manual and automatic summarization. The latterprovides for controlling the attribute of a summary route and deciding whether toadvertise the route.II. Route dampeningBGP 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 thenthe neighbor needs to recalculate routes and modify the routing table. Therefore,frequent route flaps consume large bandwidth and CPU resources even affect normaloperation of the network.In most cases, BGP is used in complex networks, where route changes are veryfrequent. To solve the problem caused by route flaps, BGP uses route dampening tosuppress unstable routes.BGP route dampening uses a penalty value to judge the stability of a route. The biggerthe value, the less stable the route. Each time a route flap occurs (the state change of aroute from active to inactive is a route flap), BGP adds a penalty value (1000, which is afixed number and cannot be changed) to the route. When the penalty value of the routeexceeds the suppress value, the route is suppressed. That is, it is neither added intothe routing table, nor advertised to other BGP peers.The penalty value of the suppressed route will reduce to half of the suppress value aftera period of time. This period is called Half-life. When the value decreases to thereusable threshold value, the route is added into the routing table and advertised toother BGP peers in update packets.
