iBGP/eBGP Interaction?

[wpalumbo06]

Hello,

Quick question regarding BGP route advertisement behavior.  This should be a simple question but my brain is not cooperating with me today.  As shown in the diagram, R1 and R2 have eBGP peerings with the service provider and iBGP peerings with each other.  I am trying to remember why R1 and R2 are not re-advertising the iBGP learned routes back to the service provider.  This is not happening, which is good but I cannot remember the mechanism that prevents this from happening.  Can anyone shed some light on this?

Thanks,
 

[Jon Marshall]

If R1 and R2 are receiving the same routes from the SP then those routes will be EBGP with AD of 20 whereas routes advertised between the routers with IBGP will be AD 200 so the EBGP routes would be considered the best path within BGP. 

Of course if, for example,  R1 received some routes from the SP not received by R2 then R1 would advertise to R2 and R2 would advertise back to SP so you need to make sure you do not act as transit. 

Basically it is wise to use prefix lists etc. to only advertise your local subnets via EBGP to the SP. 

Jon

[wpalumbo06]

Thanks Jon but I am trying to understand what is preventing the routers from re-advertising the routes they learn via iBGP, to their eBGP peers.  I agree that outbound filtering is a good idea but that is not configured in this example.  This behavior seems to resemble split-horizon but I don't believe eBGP supports that.

[wpalumbo06]

Thanks Jon but I am trying to understand what is preventing the routers from re-advertising the routes they learn via iBGP, to their eBGP peers.  I agree that outbound filtering is a good idea but that is not configured in this example.  This behavior seems to resemble split-horizon but I don't believe eBGP supports that.

[Alex Pfeil]

You can check the advertised router with the show ip bgp neighbor "neighbor ip address" advertised-routes.

By default, BGP will not install routes in it's routing table when it receives those routes with its own AS number in the path.

Please mark helpful posts.

[Alex Pfeil]

Please review this document:

https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html

#4 is shortest AS_Path.  You have to remember that BGP uses the shortest AS_Path in order to makes its routing decsision unless the first three are manually configured. 

If BGP receives a route with its own AS_Path number, 65001 in your example, it will not install it in the routing table. Since you are dealing with the same provider and same AS number, you would not have to worry about routes being advertised out to the ISP unless you were modifying the AS_Path.

[Alex Pfeil]

Please mark helpful posts.

[Jon Marshall]

Sorry I misunderstood. 

This has been discussed a few times before, start here - 

https://community.cisco.com/t5/routing/bgp-why-does-cisco-router-sometimes-advertise-routes-to-the-peer/td-p/2590184

Jon