The value of tBgpInstanceAlwaysCompareMED specifies if the
Multi-Exit Discriminator (MED) path attribute is used in the BGP route
selection process.
When the value of tBgpInstanceAlwaysCompareMED is 'zero' or 'infinity',
the MED attribute is always used in the route selection process
regardless of the peer AS that advertised the route.
A value of 'off' specifies that only the MEDs of routes that have the
same peer ASs are compared.
A value of 'zero' specifies that a value of zero is used in the MED
comparison, this in effect makes these routes the most desirable.
A value of 'infinity' specifies that a value of infinity (2^32-1) is
used in the MED comparison, this in effect makes these routes the least
desirable.
The value of 'on' specifies that the Multi-Exit Discriminator (MED)
path attributes of the two routes are compared if both the routes have
MEDs whether or not they are from the same neighbor-AS.
The original BGP RFC was unclear what to do if a route had no MED
attribute. This was clarified in the Internet-Draft, sec 9.1.2.1
(Breaking Ties) specifies that if a route has no MED metric it is
assigned the highest possible value (2^32-1 since it is a 32-bit number)
which is the least preferable.