Explanation/Reference:
Section: Addressing and Routing Protocols in an Existing Network Explanation Explanation:
The multi-exit discriminator (MED) is the Border Gateway Protocol (BGP) attribute that represents the external metric of a route. A MED value is basically the external metric of a route. BGP uses a complex method of selecting the best path to the destination. The following list displays the criteria used by BGP for path selection:
1. Highest weight
2. Highest local preference
3. Locally originated paths over externally originated paths
4. Shortest autonomous system (AS) path
5. Lowest origin type
6. Lowest MED
7. External BGP (eBGP) paths over internal BGP (iBGP) paths
8. Lowest Interior Gateway Protocol (IGP) cost
9. Oldest eBGP path
10.Lowest BGP router ID (RID)
When determining the best path, a BGP router first chooses the route with the highest weight. The weight value is significant only to the local router? it is not advertised to neighbor routers.
When weight values are equal, a BGP router chooses the route with the highest local preference. The local preference value is advertised to iBGP neighbor routers to influence routing decisions made by those routers.
When local preferences are equal, a BGP router chooses locally originated paths over externally originated paths. Locally originated paths that have been created by issuing the network command or redistribute command are preferred over locally originated paths that have been created by issuing the aggregate address command.
If multiple paths to a destination still exist, a BGP router chooses the route with the shortest AS path attribute. The AS path attribute contains a list of the AS numbers (ASNs) that a route passes through.
If multiple paths have the same AS path length, a BGP router chooses the lowest origin type. An origin type of i, which is used for IGPs, is preferred over an origin type of e, which is used for Exterior Gateway Protocols (EGPs). These origin types are preferred over an origin type of i, which is used for incomplete routes where the origin is unknown or the route was redistributed into BGP.
If origin types are equal, a BGP router chooses the route with the lowest MED. If MED values are equal, a BGP router chooses eBGP routes over iBGP routes. If there are multiple eBGP paths, or multiple iBGP paths if no eBGP paths are available, a BGP router chooses the route with the lowest IGP metric to the next hop router. If IGP metrics are equal, a BGP router chooses the oldest eBGP path, which is typically the most stable path.
Finally, if route ages are equal, a BGP router chooses the path that comes from the router with the lowest RID. The RID can be manually configured by issuing the bgp router-id command. If the RID is not manually configured, the RID is the highest loopback IP address on the router. If no loopback address is configured, the RID is the highest IP address from among a router's available interfaces.
Neither a confederation nor a route reflector are BGP attributes. Confederations and route reflectors are both a means of mitigating performance issues that arise from large, full-mesh iBGP configurations. A full- mesh configuration enables each router to learn each iBGP route independently without passing through a neighbor. However, a full-mesh configuration requires the most administrative effort to configure. A confederation enables an AS to be divided into discrete units, each of which acts like a separate AS.
Within each confederation, the routers must be fully meshed unless a route reflector is established. A route reflector can be used to pass iBGP routes between iBGP routers, eliminating the need for a full-mesh configuration. However, it is important to note that route reflectors advertise best paths only to route reflector clients. In addition, if multiple paths exist, a route reflector will always advertise the exit point that is closest to the route reflector.
Reference:
CCDA 200-310 Official Cert Guide, Chapter 11, BGP Attributes, Weight, and the BGP Decision Process, pp. 449-455
CCDA 200-310 Official Cert Guide, Chapter 11, Route Reflectors, pp. 446-448 CCDA 200-310 Official Cert Guide, Chapter 11, Confederations, pp. 448-449 Cisco: BGP Best Path Selection Algorithm
Cisco: Integrity Checks: IBGP Neighbors Not Fully Meshed