Users Guide

• If the Router-ID is not the same for multiple paths, prefer the path that was first received as the Best Path. The path selection
algorithm returns without performing any of the checks detailed.
9. Prefer the external path originated from the BGP router with the lowest router ID. If both paths are external, prefer the oldest path—
first received path. For paths containing an RR attribute, the originator ID is substituted for the router ID. If two paths have the same
router ID, prefer the path with the lowest cluster ID length. Paths without a cluster ID length are set to a 0 cluster ID length.
10. Prefer the path originated from the neighbor with the lowest address. The neighbor address is used in the BGP neighbor configuration
and corresponds to the remote peer used in the TCP connection with the local router.
In Non-Deterministic mode, the bgp non-deterministic-med command applies. Paths compare in the order they arrive. This
method leads to system selection of different best paths from a set of paths. Depending on the order they were received from the
neighbors, MED may or may not get compared between the adjacent paths. In Deterministic mode, the system compares MED. MED is
compared between the adjacent paths within an AS group because all paths in the AS group are from the same AS.
Weight and local preference
The weight attribute is local to the router and does not advertise to neighboring routers. If the router learns about more than one route to
the same destination, the route with the highest weight is preferred. The route with the highest weight is installed in the IP routing table.
The local preference — LOCAL_PREF represents the degree of preference within the entire AS. The higher the number, the greater the
preference for the route.
LOCAL_PREF is one of the criteria that determines the best path — other criteria may impact selection, see Best path selection. Assume
that LOCAL_PREF is the only attribute applied and AS 100 has two possible paths to AS 200. Although the path through Router A is
shorter, the LOCAL_PREF settings have the preferred path going through Router B and AS 300. This advertises to all routers within AS
100, causing all BGP speakers to prefer the path through Router B.
Multiexit discriminators
If two autonomous systems connect in more than one place, use a multiexit discriminator (MED) to assign a preference to a preferred
path. MED is one of the criteria used to determine best path—other criteria may also impact selection.
One AS assigns the MED a value. Other AS uses that value to decide the preferred path. Assume that the MED is the only attribute
applied and there are two connections between AS 100 and AS 200. Each connection is a BGP session. AS 200 sets the MED for its Link 1
exit point to 100 and the MED for its Link 2 exit point to 50. This sets up a path preference through Link 2. The MEDs advertise to AS 100
routers so they know which is the preferred path.
Layer 3
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