BGP Essentials: Configuring Internal BGP Sessions
Internal BGP (IBGP) sessions (BGP sessions within your autonomous system) are identified by the neighbor’s AS number being identical to your AS number. While the external BGP (EBGP) sessions are usually established between directly-connected routers, IBGP sessions are expected to be configured across the network.
The current best practice is to configure IBGP sessions between the loopback interfaces of the BGP neighbors, ensuring that the TCP session between them (and the BGP adjacency using the TCP session) will not be disrupted after a physical link failure as long as there is an alternate path toward the adjacent router.
To configure IBGP session on a Cisco router, specify the neighbor’s loopback address in all neighbor commands and use the neighbor update-source command to specify the source IP address of the TCP session. Without the neighbor update-source configuration command, the TCP session will use the IP address of the outgoing physical interface and the neighbor will reject the incoming TCP SYN packet as it’s not coming from a recognized BGP neighbor.
The following table shows the configuration commands necessary to configure an IBGP session between loopback interfaces of two routers:
| AS 11 | AS 12 |
|---|---|
interface Loopback 0 | interface Loopback 0 |
If you're looking for more in-depth BGP knowledge, try our Configuring BGP on Cisco Routers e-learning solution. If you just need to enhance your hands-on skill, the BGP Remote Lab Bundle is the perfect choice.
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2 comments:
Hi, thank you for the article. Could you go over some reasons why you would require IBGP in general as well as in a non-internet or non-mpls environment. For eg in typical extranet type enterprise environment often IBGP is used between redundant edge routers and redundant inside routers and EBGP between the edge and inside through a firewall. Is this a recommended design and if so what is the real advantage of running IBGP between the inside routers. Thx
The nice thing about running BGP across firewall is that it's just another TCP session from the firewall's perspective, so you don't need any extra security rules. When I get a few hours of spare time, I'll write a post about typical design scenario.
See also this thread in NIL forums for other reasons to use BGP in enterprise network.
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