Case Study: Using the MULTI_EXIT_DISC Attribute
What if, rather than influencing outgoing path selection, you want to influence incoming path selection? That is, when you advertise a prefix to multiple external peers in a neighboring AS, you want to “tell” the neighboring AS which route to prefer. One way of accomplishing this is to attach a MED attribute to the advertised prefixes.
The MULTI_EXIT_DISC attribute, or MED, is used to influence the routing decisions in neighboring autonomous systems. The MED is also known as the external metric and is labeled as “metric” in the BGP table. Like LOCAL_PREF, the MED is a 4-octet number and therefore can be any number from 0 to 4294967295.
The MED is a relatively weak attribute. In the BGP decision ...
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