BGP automatic route aggregation

BGP Is a classless routing protocol , Support CIDR、VLSM（ Classless inter domain routing , Variable length subnet mask ） And route aggregation . The use of route aggregation technology , The number of routing entries can be reduced to a certain extent , At the same time, it can also reduce the problem of network instability caused by routing shock .BGP There are two ways of route aggregation , One is automatic route aggregation , One is manual route aggregation . Automatic route aggregation is performed automatically on natural network boundary routers . By default ,BGP The automatic route aggregation function of is turned off , also BGP Will not automatically aggregate BGP Routes sent by neighbors and their use network Route of command advertisement
Use BGP Automatic route aggregation , A rigorous IP Address planning . In a network with disorderly address planning , Automatic route aggregation can cause many unexpected problems . for example , In networks with discontinuous subnet planning , Automatic route aggregation may lead to routing problems in packet forwarding , Or generate a routing loop .

Three routers use directly connected physical interfaces IP Address to establish EBGP Neighborhood ,R2 and R3 Of Loopback 1 The interface is used to simulate each other ISP A network segment in .R1、R2、R3 Enable the automatic route aggregation function on ,R3 Of Loopback 1 The network segment where the interface is located will use network Order notification to BGP process ,R2 Of Loopback 1 The network segment where the interface is located will be introduced into BGP In progress , Final realization R2 Of Loopback 1 And R3 Of Loopback 1 Can communicate with each other .

Interface IP The configuration will not be repeated .
To configure BGP agreement , Each router uses Loopback 0 Interface IP Address as your own Router-ID.

see R1 Of BGP Neighborhood

R1 And R2、R3 The neighbor status between is Established, The neighbor relationship has been established normally .

Turn on BGP Automatic route aggregation



When BGP When automatic route aggregation is on , The system will give the following prompt ：Info: Automatic summarization is valid only for the routes imported through the import-route command. （ Automatic rollup is only available for import-route Command route is introduced in the way of route introduction ）

Notification routing into BGP in , Use network command , take R3 Of Loopback 1 The network segment where the interface is located announces the entry of BGP process .

stay R1、R2、R3 Upper use display bgp routing-table Command view BGP Routing table .



You can see , stay R1、R2、R3 Of BGP In the routing table ,33.33.33.0/24 Not aggregated .R3 take 33.33.33.0/24 Inform to R1,R1 Then inform R2, The route was not aggregated in the whole process . This means that , Use network The order is announced to BGP The routing , At the boundaries of natural networks ,BGP No automatic aggregation .

Bring in external routing to BGP Agreement . stay R2 Upper use import-route Command to introduce a direct route

see R1、R2、R3 Of BGP Routing table



You can see , stay R1 and R3 Of BGP In the routing table , There were 22.0.0.0, And no mask information , This is the one after aggregation A Like natural networks .
Automatic routing aggregation pair IP The requirements of address planning are relatively harsh , And in the BGP In the network environment ,IP The planning of the address is difficult to be orderly , So in the actual project , Rarely enabled BGP Automatic route aggregation function . Just need to know ,BGP After the automatic route aggregation function of is turned on , Who will be interested in BGP Route can be automatically aggregated