BGP Path Attributes – MED
MED (Multi Exit Discriminator) attribute is the BGP Path attribute which provides information to the external neighbours, about how to come their Autonomous System. This is opposite of Local Preference. Local Preference says to the internal neighbours, “How to Exit AS”. MED says to the external neighbours, “How to Enter AS”.
MED (Multi Exit Discriminator) attribute is applied to the outbound interface and shows the best inbound interface into its AS (Autonomous System).
If BGP Best Path is selected via MED (metric) attribute, the lowest MED (metric) value is better and it is selected as BGP path. The default MED (metric) value is 0.
Local preference was sending only to IGP neighbours. MED attribute can be sent only EGP neighbours. In other words, Local Preference can exchanged in the AS, MED attribute can exchange between ASs.
Here, one of the important point is this, MED attribute can be carried into an AS and used inside this AS. But it does not leave that AS. If leaved, it is set as metric 0.
MED (Multi Exit Discriminator) attribute is an Optional and Non-transitive BGP Path attribute. This means that it can be or can not be supported in any BGP implementation and in BGP update the unsupported attribute is discarded.
MED (Multi Exit Discriminator) attribute is indicated as metric in the BGP table. When you use “show ip bgp” command, you can see the metric value after the neighbour value.
As an important note, there are two commands used with MED attribute. These commands are:
“bgp deterministic-med” command provides to compare different MED values that advertised by neighbours in the same AS for routes selection.
“bgp always-compare med” command provides to compare different MED values that advertised by neighbours in different ASs for routes selection.
Let’s do an example configuration and understand MED (metric) attribute better.