BGP Route Selection Criteria

D

In this scenario, we have received the same BGP route from two different peers, both learned using IBGP and Local-Preference. MED, Origin, and AS-path are equal.

To determine which route should be installed in the routing table, BGP uses a set of tie-breaking rules known as the BGP best path selection algorithm. The algorithm considers multiple attributes in a predefined order to select the best path among multiple paths to the same destination.

The criteria used by BGP in the best path selection algorithm are as follows:

1. Prefer the path with the highest Weight attribute.
2. If the Weight attribute is the same for multiple paths, prefer the path with the highest Local-Preference attribute.
3. If the Local-Preference attribute is the same for multiple paths, prefer the path with the shortest AS-Path attribute.
4. If the AS-Path attribute is the same for multiple paths, prefer the path with the lowest Origin attribute.
5. If the Origin attribute is the same for multiple paths, prefer the path with the lowest MED attribute.
6. If the MED attribute is the same for multiple paths, prefer the path learned from an EBGP neighbor over a path learned from an IBGP neighbor.
7. If all attributes are the same, prefer the path with the lowest IGP cost to the next hop.
8. If the IGP cost is the same for multiple paths, prefer the path with the lowest BGP router ID.
9. If the BGP router ID is the same for multiple paths, prefer the path with the lowest cluster list length.
10. If the cluster list length is the same for multiple paths, prefer the path received from the neighbor with the lowest peer address.

In this scenario, since the MED, Origin, and AS-path attributes are equal, the next attribute considered by the algorithm would be the EBGP/IBGP attribute. However, both peers have been learned using IBGP, so this criterion does not apply. The next attribute is IGP cost, but this is not likely to be relevant in this scenario since the routes have been received via IBGP, and IGP cost is typically only used when selecting between routes learned via different IGP protocols.

Therefore, in this scenario, the next attribute used by the BGP best path selection algorithm would be the BGP router ID (RID). Each BGP router has a unique RID assigned to it, and the router with the lowest RID is preferred. Therefore, the answer to this question is B. router ID (RID). The router with the lowest RID would be selected to install the BGP route in the routing table.