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CLI-based Configuration Guide - IP Unicast Routing

AR100, AR120, AR150, AR160, AR200, AR1200, AR2200, AR3200, and AR3600 V200R009

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Interaction Between BGP and an IGP

Interaction Between BGP and an IGP

BGP and IGPs use different routing tables. To enable different ASs to communicate, you need to configure interaction between BGP and IGPs so that BGP routes can be imported into IGP routing tables and IGP routes can also be imported to BGP routing tables.

Importing IGP Routes to BGP Routing Tables

BGP does not discover routes and so needs to import the routes discovered by IGPs to BGP routing tables so that different ASs can communicate. When an AS needs to advertise routes to another AS, an Autonomous System Boundary Router (ASBR) imports IGP routes to its BGP routing table. To better plan the network, you can use routing policies to filter routes and set route attributes when BGP imports IGP routes. Alternatively, you can set the multi-exit discriminator (MED) to help EBGP peers select the best path for traffic entering an AS.

BGP imports routes in either import or network mode:

  • In import mode, BGP imports IGP routes, including RIP, OSPF, and IS-IS routes, into BGP routing tables based on protocol type. To ensure the validity of imported IGP routes, BGP can also import static routes and direct routes in import mode.

  • In network mode, BGP imports the routes in the IP routing table one by one to BGP routing tables. The network mode is more accurate than the import mode.

Importing BGP Routes to IGP Routing Tables

When an AS needs to import routes from another AS, an ASBR imports BGP routes to its IGP routing table. To prevent a large number of BGP routes from affecting devices within the AS, IGPs can use routing policies to filter routes and set route attributes when importing BGP routes.

Updated: 2019-12-27

Document ID: EDOC1000174069

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