BGP 4-Byte AS Number

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!