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Multiple Sites Using Same BGP AS with Different Providers?

David Lagace
Level 1
Level 1

Hello,

Apologies in advance if this has been answered somewhere but I'm seeing an issue where I have multiple independant sites running BGP off my ASN with different advertised IP blocks and different ISPs.

Site 1

ASN XXXXX

Route Advertised: yyy.yyy.62.0/24

Site 2

ASN XXXXX

Route Advertised: yyy.yyy.63.0/24

Site 3

ASN XXXXX

Route Advertised: yyy.yyy.56.0/23

If I check the routing table from site 1 to see the routes from site 3, I don't see the route at all.  I'm guessing this is normal.  Is there a way to bypass this behaviour to get it to advertise inside the same AS?  If not, is there someway to get all 3 sites to see each other?

Thanks for your help

David

1 Accepted Solution

Accepted Solutions

Hello

By default to avoiding routing loops BGP will not allow prefixes that have its own ASN in them, however fro a CE perspective you can overide this by using the allow-as command

CE perspective

router bgp xx

neighbor x.x.x.x allowas-in

PE Perspective

neighbor x.x.x.x as-overide

http://ccieblog.co.uk/bgp/bgp-as-override-vs-allow-as-in

res

Paul

Please don't forget to rate any posts that have been helpful.

Thanks.


Please rate and mark as an accepted solution if you have found any of the information provided useful.
This then could assist others on these forums to find a valuable answer and broadens the community’s global network.

Kind Regards
Paul

View solution in original post

2 Replies 2

Hello

By default to avoiding routing loops BGP will not allow prefixes that have its own ASN in them, however fro a CE perspective you can overide this by using the allow-as command

CE perspective

router bgp xx

neighbor x.x.x.x allowas-in

PE Perspective

neighbor x.x.x.x as-overide

http://ccieblog.co.uk/bgp/bgp-as-override-vs-allow-as-in

res

Paul

Please don't forget to rate any posts that have been helpful.

Thanks.


Please rate and mark as an accepted solution if you have found any of the information provided useful.
This then could assist others on these forums to find a valuable answer and broadens the community’s global network.

Kind Regards
Paul

Good morning all,

Wanted to add onto the discussion over BGP not allowing same AS routes to be installed.

BGP default behavior is to verify the AS path in prefix/Route and discard if sae AS Number is seen. This is an essential part of BGP loop prevention mechanism and helps keep unwanted route looping at bay.

 

Such scenario happens generally when a customer has many sites across locations which are connected via MPLS Service provider.Since all the cusotmer sites use the same AS Number , the trouble arrives when one sites advertises its own routes via Service provider over BGP.The other sites drops the route considering default behavior of seeing its own AS in AS path of Received Route.

 

BGP default behaviour can be addressed by any of the 2 BGP features - "AS-Override" and "Allowas-in"

While "As-Override is used by PE to modify the AS Number in AS Path so that prefix is not dropped, "Allowas-in is implemented in CE device to introduce an exception in BGP AS path loop prevention mechanism.

 

Further differnce and functioning may be available in below links -

http://www.ipwithease.com/as-override-vs-allowas-in/

http://www.ipwithease.com/understanding-bgp-as-override-feature/

 

 

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