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Bronze

BGP behaviour "remove-private-as"

Scenario :

R1--IBGP--R2--EBGP--R3

R3 with private AS

If we use this command "remove-private-as" at R2 to remove private AS while it advertise R3 prefixes to its neighbors.

I think if there is IBGP between R2 and R1 then R2 wonnt strip the private AS from R3 prefixs.

Its only when R1 and R2 have an EBGP session then R2 will strip off the private AS from R3 prefixs.

I read the cisco doc saying that this command is only applicable to the EBGP neighbors, that OK but it doesnt discuss this kinda scenario ..

expecting a discussion frm experts or anybody who has already tested this :)

1 ACCEPTED SOLUTION

Accepted Solutions

Re: BGP behaviour "remove-private-as"

Hello,

What did you mean when you said: "If we use this command "remove-private-as" at R2 to remove private AS while it advertise R3 prefixes to its neighbors." ? If you use the command at R2 for the R3 neighbor, its ok (eBGP session), but it will remove the private AS numbers for routes that are advertised towards R3 (not from routes received from R3). You cannot configure the command in the session between R2 and R1, since this is iBGP. You should have removed the private ASs in R3, before they reach R2. This is more of an outbound command (for updates sent to the neighbor). In the documentation this is refered to as "removal of private AS numbers from outgoing eBGP updates". Note the word "outgoing".

Kind Regards,

M.

4 REPLIES

Re: BGP behaviour "remove-private-as"

Hello,

What did you mean when you said: "If we use this command "remove-private-as" at R2 to remove private AS while it advertise R3 prefixes to its neighbors." ? If you use the command at R2 for the R3 neighbor, its ok (eBGP session), but it will remove the private AS numbers for routes that are advertised towards R3 (not from routes received from R3). You cannot configure the command in the session between R2 and R1, since this is iBGP. You should have removed the private ASs in R3, before they reach R2. This is more of an outbound command (for updates sent to the neighbor). In the documentation this is refered to as "removal of private AS numbers from outgoing eBGP updates". Note the word "outgoing".

Kind Regards,

M.

Bronze

Re: BGP behaviour "remove-private-as"

Hi Maria,

You have actually nailed the ambiguity in my problem statement that made me read the doc again :)

Yeah, I got it now ....thx for your help

rgds

Cisco Employee

Re: BGP behaviour "remove-private-as"

Rajat,

This is correct. R2 will not be able to strip the private AS if it is part of that same private AS.

If the private AS was received via eBGP from R1, then R2 would be able to remove it from the AS-PATH as it sends it to R3.

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
Bronze

Re: BGP behaviour "remove-private-as"

Hi Harold,

You have as always hit the bulls eye :)

yes, my understanding was correct but application of command was wrong, I was applying that command for filtering the incoming private AS rather than for outgoing advertisements towards eBGP neighbors..

Thanks for your reply..

rgds

you are correct if i am egting

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