Cisco Support Community
Community Member

2 connections to same ISP. Can I stop re-advertising their blocks to them?

I have two connections to the same ISP. I am running eBGP sessions to them.

Why is it the when I do "show ip bgp neigh x.x.x.x advertised-routes" I am re-advertising learned routes back to them on both links?

Can I stop this from showing up in that command?

I do know that the ISPs router is filtering out any learned route that has its ASN.

But this is making it very difficult to see if new routes I add to my router are being advertized.


Everyone's tags (6)
Hall of Fame Super Gold

2 connections to same ISP. Can I stop re-advertising their block

The way to keep that output from showing up in the command is to stop those advertisements. (and those advertisements are an indication that you are acting like a transit network that would forward traffic through your network for them) The most common way to prevent these advertisements is to configure an AS filter and to only advertise prefixes that originate in your own network.



Hall of Fame Super Blue

Re: 2 connections to same ISP. Can I stop re-advertising their b


An example of what Rick was referring to -

router bgp

neighbor x.x.x.x  filter-list 1 out <-- x.x.x.x is the IP address of the ISP peer router

ip as-path access-list 1 permit ^$ 

The regex ^$ matches AS PATHs with no AS numbers in it which is what you want because for your internal networks in the BGP table there is no AS in the AS_PATH attribute. Any routes received from your ISP will have at least one AS in the path and so will not be advertised back out.

Edit - just to be on the safe side it's always worth checking your BGP table before doing this. When you do a "sh ip bgp" all your networks should have no AS numbers in the path and all the ISP learned routes will have.


CreatePlease to create content