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BGP AS_PATH Attribute

Hello.

What is the maximum number of AS #s that can be included in the AS_PATH Attribute of a particular route ?

In the RFC it states:

"Total Path Attribute Length: This 2-octet unsigned integer indicates the total length of the Path Attributes field in octets."

So it's 512 ?

Thanks.

-Rob

4 REPLIES
Cisco Employee

Re: BGP AS_PATH Attribute

The "Total Path Attribute Length" you are referring to is the the length of the sum of all the path attributes for a specific NLRI, not only the AS PATH length.

RFC1771 makes the following statement though:

+++++++++++++++++++++++++++++++++++++++++++++++++

The path segment length is a 1-octet long field containing the number of ASs in the path segment value field.

+++++++++++++++++++++++++++++++++++++++++++++++++

This would limit the number of AS in the ASPATH to 255.

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
Purple

Re: BGP AS_PATH Attribute

That's not quite true. It is possible to have more than one AS path segment triple in a single UPDATE message. You can have a number of AS_SEQUENCEs or AS_SETs in a single UPDATE message. However, the maximum number of ASs that you can fit into one message depends on how many optional attributes you have and the maximum BGP message length of 4096 octets.

Cheers,

Paresh.

Cisco Employee

Re: BGP AS_PATH Attribute

Paresh,

I should have said the limit is 255 ASs given that you don't have an AS_SET. I don't believe the IOS implementation will create an additional AS_SEQUENCE triple to allow more than 255 ASs.

That being said, this is a theorical discussion since the maximum ASPATH length seen on the Internet is more in the order of 15 to 20.

Thanks for keeping me honest.

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
Purple

Re: BGP AS_PATH Attribute

Yeah, I can't imagine that I would want a route with an AS_PATH length of more than 255 in my routing table !!

Paresh.

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