Bgp Deterministic-Med - Allied Telesis SwitchBlade x908 Series Software Reference Manual

bgp deterministic-med

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 bgp deterministic-med 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 deterministic-med
no bgp deterministic-med
Disabled by default
Default
Router mode
Mode
When the bgp deterministic-med command is enabled, routes from the same AS are
Usage
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:
Group 2:
Note:
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.
All routers in an AS should have the same setting for BGP deterministic-MED. All routers in an
AS should have BGP deterministic-MED enabled with bgp deterministic-med, or all
routers in an AS should have BGP deterministic-MED disabled with no bgp-deterministic-
med.
C613-50007-01 REV B
1: ASPATH 234, MED 120, internal, IGP metric to NEXT_HOP 40
3: ASPATH 234, MED 245, external
2: ASPATH 389, MED 190, internal, IGP metric to NEXT_HOP 35
Routes from the same AS are grouped together and ordered by MED
Software Reference for SwitchBlade® x908, x900 and x600 Series Switches
TM
AlliedWare Plus Operating System - Software Version 5.3.1
BGP Commands
38.19