Users Guide

8. If you enable the bgp bestpath router-id ignore command and:
● If the Router-ID is the same for multiple paths because the routes were received from the same route—skip this step.
● 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.
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Layer 3