Router A Route Selection
Question
Router A receives two similar route advertisements from different BGP peers. What would cause a route to be selected in this scenario?
Answers
Explanations
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A. B. C. D.A
When a router receives multiple route advertisements for the same destination from different BGP peers, it needs to select one of these routes to be installed in the routing table and used for forwarding traffic. The selection process is based on the BGP route selection algorithm, also known as the path selection process.
The BGP route selection algorithm evaluates a number of attributes for each candidate route and assigns a preference value to each attribute. The route with the highest preference value is selected as the best path and installed in the routing table.
The attributes evaluated by the BGP route selection algorithm are:
Weight: A locally significant value assigned by the local router to the route. The highest weight value is preferred.
Local preference: An indication of the degree of preference given to a route by the local AS. The highest local preference value is preferred.
Locally originated routes: Routes originated by the local router are preferred over routes learned from other routers.
AS path length: Shorter AS paths are preferred over longer AS paths.
Origin type: IGP (Interior Gateway Protocol) routes are preferred over EGP (Exterior Gateway Protocol) routes, which are in turn preferred over incomplete routes.
MED (Multi-Exit Discriminator): A metric used to signal the preferred exit point for traffic leaving an AS. The lowest MED value is preferred.
External BGP (eBGP) over Internal BGP (iBGP): eBGP-learned routes are preferred over iBGP-learned routes.
Peer IP address: The BGP speaker with the lowest IP address is preferred.
Based on the above attributes and their preference values, the BGP route selection algorithm selects the best path for each destination. In the case of receiving two similar route advertisements from different BGP peers, the selection would be based on the above attributes, where the route with the best value for any attribute would be preferred.
Therefore, based on the provided answer options, the answer to the question would be:
A. having a lower MED value - The route with the lowest MED value is preferred, therefore, if one of the routes has a lower MED value than the other, it would be selected.
B. having a lower local preference - The route with the highest local preference value is preferred, therefore, if one of the routes has a higher local preference value than the other, it would be selected.
C. learning a route from a peer with a higher peer ID - Peer ID is not one of the attributes used by the BGP route selection algorithm.
D. learning a route from a peer with a higher IGP metric - IGP metric is not one of the attributes used by the BGP route selection algorithm.
