BGP Attributes and Path Selection

This topic is to discuss the following lesson:

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

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

thanks very much Laz

1 Like

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

Hi Rene
I configure IBGP and EBGP to practice the path selection, I got a problem


Here in network to 8.8.8.8 the best path is through 192.168.111.3 (EBGP neighboor) even I changed Weight to 400 in IBGP neighboor (192.168.12.2) the path still is over EBGP. I wanted to influence that best path to 8.8.8.8 was 192.168.12.2 and not 192.168.111.3, I don´t know whatI am doing wrong. I am using GNS3 and GNS3 VM

Hello Carlo

When configuring attributes such as weight (or any attributes in BGP), you must keep in mind that they affect the choice of route based on the same routing protocol. A router will consider iBGP and eBGP as two separate routing protocols. For example, if you have two routes to 8.8.8.8 via iBGP, then the weight can be used to force the router to route via one path and not the other. If you have one route via iBGP and the other via eBGP, the weight will not affect the choice. It’s like trying to make the router choose a path via iBGP and not via EIGRP by using the weight attribute. It won’t work.

What the router does use, before any of these attributes, is the Administrative Distance. Since it views eBGP and iBGP as two different routing protocols, the router will always choose to use eBGP over iBGP because it’s AD is 20 as opposed to 200.

I hope this has been helpful!

Laz

I just finished reading articles on : AS Path, MED, Weight, Local preference, Origin. I found there big room for clarification.

  1. It would be helpful to explain how attributes can be used in typical BGP architectures : https://networklessons.com/bgp/singledual-homed-and-multi-homed-designs. Then ; routers should be named ISP1, ISP2, Enterprise, PE, CE, etc.

  2. Most of the articles start with a schema explaining the attribute. But ; configuration is based on a second schema, which is completely different. It’s confusing !