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BGP-Same AS

Joe Lee
Level 1
Level 1

We are using the private AS to run BGP in our network.

Router B, C and D will be connected to router A. Can Router C and Router D use the same AS number?

Router A -> AS100

Router B -> AS 200

Router C -> AS 300

Router D -> AS 300

Thank you,

7 Replies 7

cadet alain
VIP Alumni
VIP Alumni

Hi,

Yes they must be in the same AS as you're doing iBGP in your own network.

Regards.

Alain.

Don't forget to rate helpful posts.

Alain,

It is eBGP, not iBGP.

Thank you.

Hi,

if it's eBGP then the ASes must be different.Can you explain what you want to do exactly?

Regards.

Alain.

Don't forget to rate helpful posts.

Hi Alain,

Can you explain why they must be different?

TKS.

Hello Joe,

RC and RD would not accept routes with an AS path attribute containing their AS number  ( BGP loop avoidance)

from RC the AS path would be:

100 300 i

but 300 = local AS number so the route would be silently discarded

in eBGP session the AS path attribute is updated before sending to the eBGP neighbor so RA sees an AS path 300 i and before advertising to RC it puts its own AS path in leftmost position

Hope to help

Giuseppe

I guess what Joe means by C & D being in an eBGP session is that they are not at the same location. They can still carry the same AS but like Giuseppe pointed any BGP router that sees his own AS in the AS_PATH attribute will drop the route, You can however use the allowas-in feature to overide this:

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a0080b59d08.shtml

http://www.cisco.com/en/US/docs/ios/12_3t/mpls/command/reference/mp_n5gt.html#wp1007547

Hello,

If you need to use same AS for whatever the reason you can "hide" real as with BGP Local-AS Feature. This will still allow you to configure eBGP. Otherwise as mentioned before it will be iBGP.

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a00800949cd.shtml

NB: when you quote private AS (for internal use like RFC1918 IP range), these are on the 64512 to 65535 range. Public AS are assigned by RIR are on the 1 to 64511.

I hope it helps.

Regards

Gonçalo

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