Tag: Local Preference

BGP Path Attributes: The BGP Path Selection Process

BGP Path Attributes

BGP has many attributes in choosing the best path. It is like an ice cream. It has many flavors. I bought Gianduia flavor from Gelato Messina while I was preparing this topic. I think I need loads of sugar to feed my brain as this BGP topic is robust and every attribute can be well-explained if we are going to lab it.

BGP’s attributes are mainly for path manipulation and these can influence either outbound or inbound traffic. It has a systematic process that it uses to choose the best path in the network.

BGP Path Selection Algorithm

The first thing that BGP checks is whether the WEIGHT is configured or not. WEIGHT is Cisco Proprietary so it is obvious that it prioritizes Cisco devices which has BGP WEIGHT configured. In short, if you are using Cisco devices, WEIGHT is the first thing it checks before it goes on with the series of standard BGP attributes. Keep in mind that WEIGHT is local to the router and doesn’t pass to other routers. The higher the value is more preferred. (more…)

BGP Path Attributes Types

BGP has many attributes in choosing the best path. However, not all attributes need to be present in every BGP update and need to be recognized or pass to other peers. There are some that can be silently discarded and ignored.

BGP attributes are classified into 4 types:

  1. Well-known Mandatory
  2. Well-known Discretionary
  3. Optional Transitive
  4. Optional Non-Transitive

Well-known Mandatory attributes, as the name suggests, it is a must in every BGP update to have it included. It should exist in every BGP update and must be recognized by all BGP speakers. Well-known mandatory attributes are AS Path, Next Hop address, and Origin.