BGP 4-Byte AS Number

(Rene Molenaar) #1

This topic is to discuss the following lesson:

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(Mohammad Hasanuz Zaman) #2

Hi Rene,
Very Good Stuff.A quick questions for you …
How R2 know R1 only 2 byte supported before sending any open message ??

br//zaman

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(Rene Molenaar) #3

Hi Zaman,

It doesn’t. When R2 receives a reply from R1 without the “support for 4-octet AS number capability” in its OPEN message, it knows that R1 doesn’t support it.

You can see it in the wireshark capture:

https://www.cloudshark.org/captures/d8e5e9240959

Take a look at the 1st packet from R2 and the 2nd packet from R1. R1 is missing the capability.

Hope this helps!

Rene

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(Minh D) #4

Hi Rene,
I have question in 2-Byte & 4-Byte AS compatibility situation.

In case of we adding R3 (AS3) and connect it to R2. R1 advertises one prefix (for example: 1.1.1.1/32) to R2 and R2 will forward to R3.
What will be in the AS path on R3? (22222222 1 or 23456 1)?

Thanks,
Minh

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(Lazaros Agapides) #5

Hello Minh

This is an excellent question. Essentially what you are asking (allow me to put it more generally) is how is the AS path displayed when there is a 2-byte AS compatible router in the mix?

Well, if you display the AS path on the 2-byte AS compatible router, then you will see the 23456 AS in place of the incompatible 4-byte AS number. So essentially the AS_TRANS attribute replaces the 4-byte AS number. However, the AS4 PATH number is an attribute that is received by the 2-byte AS compatible device, and is transitive. It may not understand it, but it will send it down the line. So if you have yet another router connected to your R3 that is 4-Byte AS compatible, it will receive the AS4 path attribute correctly and actually use the correct 4-Byte AS’s for the full path.

I hope this has been helpful!

Laz

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(Minh D) #6

Thanks Laz for the explanation.

Enjoy your weekend,
Minh

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(Staut S) #7

If we had another router R3 (4 byte support) connected to R1 (2 byte support), then on R1 we would configure R3 neighbor using the same 23456 AS (neighbor 192.168.13.3 remote-as 23456)? R1 would have believed that it is dual homed connected to the same AS?

Thanks,
Stefanita

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(Lazaros Agapides) #8

Hello Staut

Yes, that is correct. This is a limitation of the feature and thus must be used carefully,. This could result in routers using the 4-byte AS not being able to reconstruct the entire AS path information correctly.

I hope this has been helpful!

Laz

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