Reference Guide

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.
MEDs are nontransitive attributes. If AS 100 sends the MED to AS 200, AS 200 does not pass it on to AS 300 or AS 400. The
MED is a locally relevant attribute to the two participating AS — AS 100 and AS 200. The MEDs advertise across both links—if
a link goes down, AS 100 has connectivity to AS 300 and AS 400.
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