Which criterion is evaluated first when selecting a BGP route?
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A. B. C. D.C
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
When a Border Gateway Protocol (BGP) speaker receives multiple paths to the same destination, it must decide which one to select as the best path. The BGP best path selection algorithm takes into consideration several criteria, in a specific order, until it identifies the optimal route.
The BGP best path selection criteria, in order of preference, are as follows:
Highest weight: This is a Cisco proprietary attribute and is not recognized by other vendors. It is the first attribute checked, and the highest weight value indicates the preferred path.
Highest local preference: Local preference is an optional, non-transitive attribute that is used within an Autonomous System (AS) to indicate the preferred path for traffic exiting the AS. The path with the highest local preference value is preferred.
Shortest AS-path: The AS-path attribute lists the sequence of AS numbers that the route has traversed, and the shortest AS-path is preferred.
Lowest origin type: The origin attribute indicates the source of the BGP route, and it can be either IGP (Interior Gateway Protocol), EGP (Exterior Gateway Protocol), or incomplete. The route with the lowest origin type is preferred.
Lowest Multi-Exit Discriminator (MED) value: The MED attribute is used to influence the inbound traffic flow when there are multiple entry points into an AS. The path with the lowest MED value is preferred.
eBGP path over iBGP path: If two paths are learned through both eBGP and iBGP, the eBGP path is preferred.
Path with the lowest IGP metric to the BGP next hop: If the above criteria are not sufficient to determine the best path, the path with the lowest IGP metric to the BGP next hop is preferred.
Path received from the BGP router with the lowest router ID: If all the previous criteria are equal, the path received from the BGP router with the lowest router ID is preferred.
Therefore, when selecting a BGP route, the first criterion evaluated is the highest weight, but since it is a Cisco proprietary attribute, it is not an option in Juniper Networks. The second criterion, the highest local preference, is evaluated first among the standard attributes, followed by the shortest AS-path, the lowest origin type, and the lowest MED value. Therefore, the correct answer to the given question is C. local preference.