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Using the same AS in BGP

Question: Is there any way possible to use the same AS in two different parts of the country if the customers need to communicate with one another? For example:

In NY we have AS100 in use. Now, in Boston we would like to roll out BGP and plan to use AS100.

I know that BGP should drop any traffic going from our NY customers to Boston, but AT&T is stating many of their customers do this and I'm wondering if there is something within BGP that we can do (AS masking/stripping) that would allow us to use a BGP AS in two different parts of the country.

1 ACCEPTED SOLUTION

Accepted Solutions
Cisco Employee

Re: Using the same AS in BGP

Rick,

As long as the prefixes advertised by each one of the two sites are different, one could use allowas-in to allow prefixes from the other site in.

I don't necesseraly see a need to run iBGP between the two sites provided they use different sets of addresses.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
9 REPLIES
Hall of Fame Super Gold

Re: Using the same AS in BGP

David

I would be very interested to know what ATT is talking about. If you are talking about EBGP relationships, one of the fundamental principles of BGP is that it rejects any routing update in which its autonomous system number is already present. So on the face of it I do not see how this could work.

But thinking a little deeper I can come up with a couple of theories where you might get this to work.

First theory: configure a GRE tunnel through the Internet between the two border routers and run IBGP through the tunnel. You would then present a basically discontiguous AS, but if the prefixes advertised by NY are unique (different from the prefixes advertised by Boston) then ATT could probably get the right next hop information and route correctly.

Second theory: instead of using public AS numbers perhaps ATT has in mind having you use unregistered AS numbers and they might use the strip private AS so that the prefixes advertised by NY and the prefixes advertised by Boston do not have your AS number as they are advertised to other peers.

Ask ATT for more details of how they make this work and let us know.

HTH

Rick

Cisco Employee

Re: Using the same AS in BGP

Rick,

As long as the prefixes advertised by each one of the two sites are different, one could use allowas-in to allow prefixes from the other site in.

I don't necesseraly see a need to run iBGP between the two sites provided they use different sets of addresses.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
Hall of Fame Super Gold

Re: Using the same AS in BGP

Harold

Thanks for the alternative. That had not occured to me.

It seemed to me that, while not necessary, that running a tunnel between the sites with IBGP would be a way to work around the discontinuity.

HTH

Rick

Cisco Employee

Re: Using the same AS in BGP

Rick,

I'm not sure I understand why you would want to run iBGP between the two sites since the only path between them is the ISP network anyway.

In my view, the iBGP session would only add complexity for no appearant benifits. AS discontinuity is not necesseraly bad assuming the the separate parts of the AS advertise separate prefixes.

My 2 cents,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
Silver

Re: Using the same AS in BGP

Just to carry this forward,can i use as-override in this case ? If iam right, i think it will overwrite the current As with provider's As. Pls correct me if iam wrong

Cisco Employee

Re: Using the same AS in BGP

The as-override command is only available in vrf mode (ie: under address-family ipv4 vrf ).

So unless the connection is to an MPLS VPN network, this command is not usable.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
New Member

Re: Using the same AS in BGP

Harold,

We are in a fairly similar situation also, iBGP only adding to complexity of configurations with possibility of management-only interPoP links being swamped!

Consider if two of three PoP's were connected with unreliable link, each advertising out their own space.

Would we rely on differint announcement, allowas-in to statically route between PoP's.

Any thoughts?

Thanks.

Cisco Employee

Re: Using the same AS in BGP

I don't see any point in using iBGP sessions between these POPs if they are only connected via the SP network.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
New Member

Re: Using the same AS in BGP

Thanks. I think this is the answer I'm looking for.

I appreciate all the input.

We are not advertising the same prefixes so this will work.

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