How to configure BGP to send the default route to a neighbor

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Core Issue

Default routes are used by routing devices to forward packets to destination networks not available in the routing table.

These are the different ways to configure a router running the Border Gateway Protocol (BGP) to send a default route to its BGP peers:

• Configure a static route to the 0.0.0.0/0 network pointing to the null 0 interface by issuing the ip route command.      

  Inject into BGP by issuing the network command. 0.0.0.0

         
• Configure a static route to the 0.0.0.0/0 network pointing to the null 0 interface by issuing the ip route command. The static route needs to be redistributed into BGP by issuing the redistribute command.      

  Inject into BGP by issuing the redistribute static command and configure the default-information originate command in router configuration mode.

     

Both these mechanisms propagate the default route to all neighbors by default. If the requirement is to advertise the default route only to a specific neighbor, additional route filters may need to be configured.

Another way of advertising a default route to a specific BGP neighbor is by issuing the neighbor default-originate command. This method does not require the presence of the 0.0.0.0/0 network in the routing table of the advertising router. Additionally, the default route can be conditionally advertised based on the existence of another route by issuing the route-map option of the command.

Resolution

To configure a router running BGP to advertise a default route to a specific neighbor, issue the neighbor {ip-address, peer-group-name} default-originate [route-map map-name] command in router configuration mode.

For example, if a router in Autonomous Systems (AS) 100 needs to advertise a default route to its neighbor 172.16.1.1 in AS 200, then the command issued would be as shown:

router bgp 100
neighbor 172.16.1.1 remote-as 200
neighbor 172.16.1.1 default-originate

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Check my article; it helps you understand with a LAB demo: (3 Ways) How To Advertise A (BGP Default Route) In Cisco Switches