Explore The BGP Path Selection Attributes [Explained with Labs]

BGP Attributes Categories

There are four categories of BGP attributes (CiscoPress):

• Well-known mandatory: Must be recognized by all BGP routers, present in all BGP updates, and passed on to other BGP routers. For example, AS path, origin, and next hop.
  
• Well-known discretionary: Must be recognized by all BGP routers and passed on to other BGP routers but need not be present in an update, for example, local preference.
  
• Optional transitive: Might or might not be recognized by a BGP router but is passed on to other BGP routers. If not recognized, it is marked as partial, for example, aggregator, or community.
  
• Optional nontransitive: Might or might not be recognized by a BGP router and is not passed on to other routers, for example, Multi-Exit Discriminator (MED), originator ID.
  

BGP Attributes Explained with configuration examples [Labs]

If A received BGP route/prefix doesn’t include AS loop, next hop is reachable and Synchronized is TRUE, we will go the following Attribute to select the Best path for this prefix:

0- Next Hop

1- WEIGHT

2- LOACL PREF

3- Locally ORIGINATED

4 – AS PATH

5- ORIGIN

6 – MED

7- EBPG over IBGP

8- IGP Metric to the neighbor

(If No tie breaker at this point, multipathing can be considered).

Multipath consiedered

9- Oldest EBGP Peering AGE

10- Lowest Router ID

Order of BGP Path Attributes

Manipulation of BGP attributes: