01-25-2011 07:48 AM - edited 03-04-2019 11:12 AM
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,
01-25-2011 07:52 AM
Hi,
Yes they must be in the same AS as you're doing iBGP in your own network.
Regards.
Alain.
01-25-2011 08:52 AM
Alain,
It is eBGP, not iBGP.
Thank you.
01-25-2011 10:59 AM
Hi,
if it's eBGP then the ASes must be different.Can you explain what you want to do exactly?
Regards.
Alain.
01-25-2011 12:33 PM
Hi Alain,
Can you explain why they must be different?
TKS.
01-25-2011 01:10 PM
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
01-25-2011 07:41 PM
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
01-26-2011 12:50 AM
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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