Dell PowerSwitch S4112F-ON OS10 Enterprise Edition User Guide Release 10.4.1.0 - Page 298

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. Origin The origin indicates how the prefix came into BGP. There are three origin codes-IGP, EGP, and INCOMPLETE. IGP Prefix originated from information learned through an IGP. EGP Prefix originated from information learned from an EGP, which Next Generation Protocol (NGP) replaced. 298 Layer 3