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Can all customer side MPLS Sites have the same AS number

Hi,

We currently have a global MPLS network that is provided by Verizon, with a backup MPLS network provided by AT&T. At each of our sites, we utilize a different AS#, and Verizon provides us with the site of origin to route all internet traffic to our major DC's (we have two per region), the AT&T setup is the same.

My question is, would it be possible to have all of global sites on each MPLS network use one AS# (that would be a total of two AS#'s, one for each provider), and then from our main DC sites we would tag the default route to make it look more preferrable to the locations in that region?

I mentioned this, and was met by a lot of push back stating that we could not have multiple sites within the same MPLS network and provider on the same AS# unless we ran "routing over routing" aka enabled our routers to be VRF aware routers.

My suggestion was to have all the sites use one AS# per provider, then use some BGP attributes to tag traffic advertised out of the DC to make it's default route more preferable than the other DC, but at the same time have BGP maintan a list of all the options in the order we want (for failover). My simple suggestion was to use Local Preference at the sites, since they would all be on the same AS it would iBGP and the attribute would be allowed to pass.

I have been out of the routing world for several years (i'm on the security remote access side), so I'm asking if anyone out here can provide some insight on this and any links to documentation regarding this as I would like to have an real answer to my conterparts since I was met by very strong objection with no basis.

Thank you,

Sami

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Re: Can all customer side MPLS Sites have the same AS number

Hello Sami,

yes it is possible if the AS number is private (64512 - 6500 range for classical 16 bit AS number) with the MPLS SP using features like as override that allows for changing the private AS on another appearance of SP AS number

so for example AS path 1234 65000 i is seen on another MPLS site as 1234 1234 i and this allows for the related advertisement to be accepted on the other sites.

However, you have two MPLS SP that could expect two different private AS numbers on customer side.

This adds complexity on the picture. If the routers that connect to the two MPLS SPs are different (that is you have node redundancy) you can manage by using different AS number and you would end up with two iBGP meshes one in AS1 connected to SP1 and one in AS2 connected to SP2.

The problem may arise if there is a need of routing exchange between the two iBGP meshes.

There is also  a chance to use the local-as feature on your routers to pretend toward SP2 to be using AS2 even if the locally defined AS number is AS1 this can be done per eBGP session basis as the local-as feaure is an option of neighbor command (under router  bgp AS1 process)

Generally speaking it should be feasible combining as-override on SP side (quite typical not a special case I would say) and eventually other tricks on customer side if needed

in an eBGP scenario (two meshes) you should be able to use AS path prepending to build less preferable routes from DR site. each private AS number instance should be changed by SP router in SP's AS number with as-override (to be checked, but it should be so)

Edit:

another possible option is the use of BGP communities to tag routes so that different remote sites can make different choices if desired/needed

Hope to help

Giuseppe

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