BGP Attributes Used in Route-Selection Process

ACD

When a Junos device receives two similar route advertisements from different BGP peers, it will select the route will the lowest MED value. BGP route selection proceeds in this manner:

-> Prefer the highest local-preference value

-> Prefer the shortest AS-path length

-> Prefer the lowest origin value

Prefer the lowest MED value -

-> Prefer routes learned from an EBGP peer over an IBGP peer

-> Prefer best exit from AS

-> For EBGP-receive routes, prefer the current active route:

-> otherwise, prefer routes from the peer with the lowest RID

-> Prefer paths with the shortest cluster length

-> Prefer routes from the peer with the lowest peer ID

In BGP, the attributes are used to determine the best path for a particular destination network. The BGP route-selection process considers a variety of attributes to determine the best path. Three common BGP attributes used in the route-selection process are:

1. Local Preference: The Local Preference (LP) attribute is used to indicate the preference for a particular path among multiple paths to the same destination network within a single Autonomous System (AS). When multiple paths exist, the path with the highest Local Preference value is preferred.

2. MED: The MED (Multi-Exit Discriminator) attribute is used to indicate the preference for a particular path among multiple paths to the same destination network that are advertised by different AS's. MED is typically used to influence the path selection for outbound traffic. A lower MED value is preferred.

3. Origin: The Origin attribute is used to indicate the origin of the BGP route. The possible values for this attribute are IGP (Interior Gateway Protocol), EGP (Exterior Gateway Protocol), or Incomplete. The IGP value indicates that the route is learned from an internal routing protocol (such as OSPF or IS-IS), EGP indicates that the route is learned from an external BGP peer, and Incomplete indicates that the origin is unknown or not specified.

Note that while communities and atomic aggregate are also BGP attributes, they are not typically used in the route-selection process. Communities are used for policy control and atomic aggregate is used to indicate that the aggregate route is not an exact match for the specific routes being advertised.