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BGP and BGP4+ Commands
Software Reference Supplement for SwitchBlade® x8112, x908, x900 and x610 Series Switches
C613-50032-01 REV D AlliedWare Plus
TM
Operating System - Software Version 5.4.3-2.6 3.29
bgp deterministic-med (BGP and BGP4+)
Use this command to allow or disallow the switch to compare the Multi Exit Discriminator
(MED) variable when choosing among routes advertised by different peers in the same
autonomous system (AS).
Use the bgp deterministic-med command to enable this feature to allow the comparison
of MED variables when choosing among routes advertised by different peers in the same
AS.
Use the no variant of this command to disable this feature to disallow the comparison of
the MED variable when choosing among routes advertised by different peers in the same
AS.
Syntax [BGP/BGP4+] bgp deterministic-med
no bgp deterministic-med
Default [BGP/BGP4+]
Disabled
Mode [BGP/BGP4+] Router Configuration
Usage [BGP/BGP4+] When the bgp deterministic-med command is enabled, routes from the same AS are
grouped together and ordered according to their MED values, and the best routes of each
group are compared.
The main benefit of this is that the choice of best route then does not depend on the order
in which the routes happened to be received, which is rather random and arbitrary.
To see how this works, consider the following set of bgp table entries, all for the same
route:
1: ASPATH 234, MED 120, internal, IGP metric to NEXT_HOP 40
2: ASPATH 389, MED 190, internal, IGP metric to NEXT_HOP 35
3: ASPATH 234, MED 245, external
If bgp deterministic-med is not enabled, then entry 3: will be chosen, because it is an
external route.
But if BGP deterministic-MED is enabled, the entries will be grouped as follows:
Group 1: 1: ASPATH 234, MED 120, internal, IGP metric to NEXT_HOP 40
3: ASPATH 234, MED 245, external
Group 2: 2: ASPATH 389, MED 190, internal, IGP metric to NEXT_HOP 35
Entry 1: is chosen as the best route from Group 1, since this route has the lowest MED
value. Entry 2 has to be the best route in Group 2, since this is the only route in that
group. These two group winners are compared against each other, and Entry 2: is chosen
as the best route because Entry 2: has the lower metric to next-hop.
Note
Routes from the same AS are grouped together and ordered by MED.