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Ozelo
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BGP routing filter help

Mon Jul 19, 2010 9:32 pm

Ive got some questions while playing with filters, but couldn't figure the most important thing... anyone could help me please?

Consider the following scenario:

AS1<---BGP--->AS2<---BGP--->AS3

AS3 is actually a traffic exchange point. AS2 already receive routes from AS3 without its ASN in AS_PATH. How to advertise these routes to AS1 without AS2 number? Also, Ive put a "Route TAG" on routes from AS3, but noticed that AS1 peer just received two routes less than that peer inside AS2 to AS3. Guess its not like that the correct way to do this, right?

Please, thank you in advance.
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mrz
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Re: BGP routing filter help

Tue Jul 20, 2010 9:11 am

That is the whole point of BGP, it append AS to as_path to prevent loops and to calculate best path.
 
Ozelo
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Re: BGP routing filter help

Tue Jul 20, 2010 2:30 pm

Thank you for reply, mrz.

I just didn't know what is the setting required to achieve the right result. There is a router with two peers, one of them I receive a AS_PATH with the ASN of this peer(A) in front of all route's AS_PATH from this peer(A), but the other peer(B) (traffic exchange point) does not show the ASN of that peer(B) in front of the AS_PATH of routes coming from this peer(B).

So, I have two difficulties here: 1) I want a router to become a traffic exchange point, so, a peer with this router should show routes like peer(B), 2) In the other hand I need a router with a peer on this traffic exchange point to act as transit for a third router AS also. I hope have explained my situation.

That's why I asked on how to remove that ASN from AS_PATH, but seems from what you said that may be a way to just not append its router ASN on his advertise route's AS_PATH? Am I maybe trying to do something with MT that only route servers or looking glasses do?
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Ozelo
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Re: BGP routing filter help

Wed Jul 21, 2010 8:18 pm

I found the feature I've been trying a MT ROS 4.10 to do! Hope anybody knows if that is even possible to configure on ROS. Can I configure this on a Mikrotik router?

CISCO feature:
no bgp enforce-first-as
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mrz
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Re: BGP routing filter help

Thu Jul 22, 2010 9:05 am

Isn't this option quite opposite from what you want?
The bgp enforce-first-as command is used to deny incoming updates received from eBGP peers that do not list their AS number as the first segment in the AS_PATH attribute
As far as I understood you wan to advertise routes without adding your own AS.
 
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Re: BGP routing filter help

Thu Jul 22, 2010 1:59 pm

In short, YES, I want Mikrotik to do that routers (other AS) with eBGP peers connected to our router (our AS) do not list our AS number as the first segment in the AS_PATH attribute for them. "Disabling" that option on CISCO router seems to do the job, but I want to disable it on Mikrotik routers. You got it right.
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Ozelo
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Re: BGP routing filter help

Mon Jul 26, 2010 8:43 pm

"bump" Guess the worse, the answer if I can configure a Mikrotik to do that requirement above is a clear silence. Wonder if quagga also do it...
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Re: BGP routing filter help

Sat Aug 07, 2010 11:25 pm

It is possible by adding routing filters to "set-bgp-prepend=0".
If value is set to 0 then peer's own AS is removed from AS_PATH (Similar to CISCO feature "no bgp enforce-first-as")

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