Elements in BGP Path Selection Algorithm

BD

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

The BGP (Border Gateway Protocol) path selection algorithm is used by routers to determine the best path to reach a given destination network. The algorithm takes into account various attributes of the BGP routes to select the most preferable path. Two of the attributes that are used in the path selection algorithm are:

1. Local preference value: The Local preference value is a non-transitive attribute used within a single Autonomous System (AS) to indicate the preferred path for outbound traffic. The higher the Local preference value, the more preferred the path is. The Local preference is typically set by the network administrator and is used to influence the traffic flow within the AS.

Therefore, the correct options for elements that are valid in the BGP path selection algorithm are: A. Lowest Local preference value D. Highest Local preference value

2. Multi-Exit Discriminator (MED): The Multi-Exit Discriminator (MED) attribute is an optional, non-transitive attribute that is used to convey to an external neighboring AS the preferred entry point into the local AS. The lower the MED value, the more preferred the path is. The MED is typically set by the receiving AS and is used to influence the inbound traffic flow.

Therefore, option B is correct: Lowest Multi-exit Discriminator. Option C (highest multi-exit discriminator) is incorrect because the lower MED value is more preferred.