BGP loop prevention with AS_PATH

[Kevin Dorrell]

Just a curious thought about AS_PATH.  The way loop prevention works in BGP is that if I see my own AS in the AS_PATH attribute, then I discard the route.

My curiosity is this ... why did my neighbor send me the route in the first place?  He knows my AS number.  He could see the route had my AS in the AS_PATH attribute.  So why did he bother to send it to me, if he knew I would just discard it?

Kevin Dorrell

Luxembourg

[marikakis]

Hi Peter,

Thanks for commenting on some points that I didn't previously. I am aware that my use of the rating system was more loose than generally accepted, but I sometimes do this to boost morale even for members with thousands of points such as you, Jon and Giuseppe (when you get some 1's or 2's for example). I hope this is not a problem. Paul seems really interested in the details of BGP. BGP is one of my favorite protocols. So, we have BGP in common Paul and hope to see you again in the BGP threads!

Kind Regards,

Maria

[marikakis]

It seems Paul decided to come back a few minutes before I posted! Unfortunately, I am unable to follow this thread anymore. It needs a lot of thinking and I am running out of time with some things I have to do. I will hopefully be able to read the thread sometime in the near future.

[raj gupta]

i read that BGP uses a loop prevention method using ASN no in ASpath length attribute , what like if we had two routers separated physically in our enterprise like rtr1 and rtr2   and we want that routes be exchanged at both sites using BGP and we also have ISP which provides the connectivity but if a route is advertised as follows through rtr1-- ISP -- rtr2 then it will have the same AS , and rtr2 should discard the routes ideally as it will already have ASN (as it was originally advertised by rtr1 of same enterprise) can someone help please ?

[Harold Ritter]

Hi @raj gupta ,

You can definitely accommodate the scenario you are describing by using the allows-in feature.

https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/112236-allowas-in-bgp-config-example.html

Regards,

[raj gupta]

Thanks @Harold Ritter , hoping this solves query