BGP Attributes and Path Selection


(Rene Molenaar) #1

This topic is to discuss the following lesson:


(Dominique R) #2

Hi René,
good lesson i read in detail
I am not a BGP expert as i am studying now (only) for CCNP
I read other sources for description of attributes (i dont know if i may give this source name)
This (well known) source give 2 more attributes

  • the first one between 8 (Shortest IGP path to BGP next hop) and 9 (Oldest Path) he call “Multipath”: determines wether or not multiple paths need to be injected into the routing table, and continues if the best path is not yet selected.
  • the second between 10 (router-id) and 11(neighbor IP address) he call “Minimum Cluster List Length”: a tie breaker used in a route reflector BGP configuration that prefers the route with the minimum cluster list length
    Do you think you should add these attributes in your list ?
    Thank you
    Best regards

(Lazaros Agapides) #3

Hello Dominique

Looking at Cisco’s official BGP documentation whose link is below, it does indeed include these two attributes that you mention. I’ll let Rene take a look and see if those should be added as well.

Thanks for pointing that out!

Laz


(Dominique R) #4

thanks very much Laz


(Rene Molenaar) #5

Hi Dominique,

The link @lagapides added is the most complete list of BGP attributes and the path selection:

I didn’t include multipath because by default, it’s not enabled in BGP. Internal and external BGP only install a single best path (unless you enable multipath). If you do use multipath, it’s good to know how BGP deals with it.

The minimum cluster list length is a similar example. It only applies when you use BGP route reflectors. There are also some exceptions when you use BGP confederations.

I’ll add a note in the lesson that explains these possible extra steps. I don’t want to confuse people new to BGP too much with the terminology that is a bit more advanced level :grin:

Rene