remove private-as from private-as neighboure

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Jul 17th, 2008
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how one can remove the private-as # not to adviste to the other neighbour in same private -as. for example i have as 20 and its contect to router X in private-as 64444 and its ibgp neighboure is router Y in private-as 64444. in router Y bgp table the route learn from 20.only show as 20 not 64444 with it. but it should be appear in private-as 64445. how it possible. when i try to remove the private-as command i have a messeage that private-as can't b remove with in private-as.

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lee.reade Thu, 07/17/2008 - 02:16
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Hi,


Im not sure if I understand what you are trying to do here.


You have


AS20---AS644444

(X--iBGP---Y)


You want Y to see the routes from AS20 with an as path of 64445 20 ?


If so then im not sure why you are asking to remove private as??? Y will see the routes from AS20 with as path of 20?


If you want the path on Y to be 64445 20 then you can configure bgp confedaration on X and Y, with bgp as 64445 on X and 64444 ON y, this will give you as path 64445 20.


HTH


LR

Kevin Dorrell Thu, 07/17/2008 - 02:24
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If I understand your requirements correctly, I think you need to set up a confederation.


So, router X should be in AS 64445, and router Y in AS 64444, and they should peer with each other using those AS numbers. Then, in router X, you add:

</p><p>router bgp 64445</p><p>  bgp confederation identifier 20</p><p>  bgp confederation peers 64444</p><p>

In router Y:

</p><p>router bgp 64444</p><p>  bgp confederation identifier 20</p><p>  bgp confederation peers 64445</p><p>

In this way, X and Y will see the private AS numbers of each other, but to the ouside world they will both look like just AS 20, with the private AS numbers stripped.


Did I understand your requirements correctly?


Kevin Dorrell

Luxembourg

wasif.khan Thu, 07/17/2008 - 03:24
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Dear Sir,

thanks for ur response. please check the picture. i do't wante private-as information in router C bgp table only As 100 should be with the route learn from 100. but private-as and as 100 both should in router D bgp table???



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lee.reade Thu, 07/17/2008 - 03:34
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Hi,


Sorry its really not clear what you are trying to do.


Just to point out however, the private-as range for BGP is 64512 - 65535, so you are not using an AS within this range.


Can you advise what you are trying to achieve with this?


Here is a link for info on remove private-as feature.


http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080093f27.shtml


HTH


LR

Kevin Dorrell Thu, 07/17/2008 - 04:37
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Good point on he 64512-65535. Sorry, I missed that!


Kevin Dorrell

Luxembourg


milan.kulik Thu, 07/17/2008 - 07:40
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Hi,


it's really not clear what required here.

But as the link you provided says:


"If the AS_PATH includes both private and public AS numbers, BGP doesn't remove the private AS numbers."


So no way to remove private AS numbers only from the as-path is some public ones present there.


BR,

Milan



Giuseppe Larosa Fri, 07/18/2008 - 06:08
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Hello Wasif,

I understand your concerns but this how BGP confederations work.


If you put a router E in AS 300 outside confederation of AS 200 you will see an AS path : 100 200 from RE (an eBGP session RD-RE is supposed here)


When using BGP confederations an additional AS path attribute is added to the advertisement. In this additional AS path are stored the private AS numbers members of the BGP confederation.

This is needed to provide a loop-detection mechanism inside the BGP confederation.

Being a separate attribute is easy for the routers to strip it from updates sent to real eBGP neighbors that are not part of the BGP confederations.

For this reason you need to declare what AS numbers are members of the BGP confed.


So if you have a Router E in AS300 with a eBGP session with RD or RC automatically its AS path will be 100 200 only and you don't need to configure any removal of private AS numbers it is automatically done.


Removing of private AS numbers inside the BGP confederations like in RD would be wrong and dangerous and I hope it isn't possible.


Hope to help

Giuseppe

wasif.khan Fri, 07/18/2008 - 06:54
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thanks alot sir, for your response. i know what your going to explain. what i m ask is basically my question in ie exam which i was unable to solve that y i put this question here. thanks

Giuseppe Larosa Fri, 07/18/2008 - 09:19
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Hello Wasif,

it was a task in the CCIE lab I suppose.

sometimes you can be asked to do something that routing protocols do already automatically and it is why it is important to study theory very well to avoid traps.


however, I made a mistake AS path from RE's point of view would be 200 100 not the opposite.


For example you can remove private AS info from AS path when sending updates to an EBGP neighbor with a public AS number


See

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a0080093f29.shtml


If it was a question in CCIE written you can guess what was the right answer


Hope to help

Giuseppe


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