Which role does the router ID (RID) have in the BGP route selection process?
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A. B. C. D.C
http://www.juniper.net/techpubs/en_US/junos10.0/information-products/topic-collections/nog-baseline/bgp-routes-and-selection-introduction.htmlIn the Border Gateway Protocol (BGP), the Router ID (RID) is a unique identifier that is used to differentiate between BGP-speaking routers in a network. The RID is determined by the highest IP address on the router, typically the loopback interface.
Regarding the BGP route selection process, the RID plays a significant role in the tie-breaking process if multiple paths to the same destination have the same attributes, including the same AS_PATH length, origin code, and MED value. In this scenario, BGP will compare the RID of the neighboring routers to determine which path to select.
The correct answer to the question is B. The router ID is evaluated last in the route selection process. BGP uses a well-defined algorithm to select the best path among multiple paths to the same destination. This algorithm is commonly known as the BGP route selection process, or the BGP decision process.
The BGP route selection process evaluates BGP attributes in a specific order to select the best path. The first attribute that BGP evaluates is the WEIGHT attribute, which is a Cisco proprietary attribute used to influence the routing decision. If multiple paths have the same weight, BGP will then evaluate the LOCAL_PREF attribute, which is used to influence the routing decision within a single AS.
If multiple paths have the same LOCAL_PREF, BGP will evaluate the AS_PATH attribute, which is used to prevent routing loops and to provide information about the path of a BGP route. BGP prefers paths with shorter AS_PATH lengths.
If multiple paths have the same AS_PATH length, BGP will evaluate the ORIGIN attribute, which is used to indicate how BGP learned about a route. BGP prefers routes with a lower origin code, where IGP < EGP < INCOMPLETE.
If multiple paths have the same ORIGIN code, BGP will evaluate the MULTI_EXIT_DISC (MED) attribute, which is used to influence the routing decision when multiple paths to the same destination exist through the same neighboring AS. BGP prefers paths with lower MED values.
If multiple paths still have the same attributes, including the same MED value, BGP will then compare the RID of the neighboring routers to determine which path to select. BGP prefers paths through the neighboring router with the lower RID.
Finally, if all of the above attributes are the same, BGP will select the path with the lowest IGP metric cost to the neighboring router that advertised the path.
To summarize, the RID plays a crucial role in the BGP route selection process as a tie-breaker if multiple paths to the same destination have the same attributes. However, it is evaluated only after all other attributes have been compared, and it is evaluated last in the BGP decision process.