BGP : bgp deterministic-med and bgp always-compare-med
Today I am going to discuss about some of the BGP commands, starting from one of the common parameters used by service providers to route the traffic over the specific path
The MED is one of the parameters that is considered when
selecting the best path among many alternative paths. The path with a lower MED
is preferred over a path with a higher MED.
By default, during the best-path selection process, MED
comparison is done only among paths from the same autonomous system. This command changes the default behavior by
allowing comparison of MEDs among paths regardless of the autonomous system from which the paths
are received.
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| Fig 1.1- BGP MED Parameter |
“bgp always-compare-med”
You heard about this command but some of you know about the functionality of
this command. Let’s talk about “bgp always-compare-med”
When you want to allow the comparison of the Multi Exit
Discriminator (MED) for paths from neighbors in different autonomous systems,
use the bgp always-compare-med router configuration command.
The important thing to note here is “different autonomous
systems”
Configuring “bgp always-compare-med”
R1(config)#router bgp 500
R1(config-router)#bgp always-compare-med
“bgp deterministic-med”
The other command you heard about is “bgp deterministic-med” , When using
the “bgp deterministic-med” comparison
is made across all routes received from the same autonomous system.
The important thing to note here is “same autonomous
system”
Configuring “bgp deterministic-med”
R1(config)#router bgp 600
R1(config-router)# bgp deterministic-med
So, let’s understand if “bgp deterministic-med” Enabled
Group1Entry1: ASPATH 1, MED 500, internal, IGP metric to
NEXT_HOP 10 >>>> AS500
Group1Entry 2: ASPATH 2, MED 550, internal, IGP metric to NEXT_HOP 5>>>>AS500
Group2Entry3: ASPATH 1, MED 600, external >>>AS600
So, in group 1, there are two entries and the best path is
entry 1, Why ? because of lower MED value.
In group 2, we have only one entry, so it is best path in another group
So now the question, are these two entries which is best
from each group compared ? The answer is no because “bgp deterministic-med” only
compare with in same AS number. So how can these be compared then ? As we
discuss above to compare with two different AS you need “bgp
always-compare-med”
Ok, let’s understand one more thing, if we disable bgp
deterministic-med, the order in which routes are received may impact MED-based
best path decisions. This can occur when the same route is received from
multiple ASs or confederation sub-ASs, with the same path length, but different
MEDs.
“bgp bestpath med missing-med-worst”
If the MED is not attached to a BGP route, it is interpreted as value 0, and
thus as the best metric. With this command, a missing MED is interpreted
as infinity (worst).
R1(config-router)# bgp bestpath med missing-med-worst
“bgp bestpath med-confed”
To enable MED comparison among paths learned from confederation peers, use
the bgp bestpath med-confed router configuration command. To prevent the software from
considering the MED attribute in comparing paths, use the no form of this command
R1(config-router)# bgp bestpath med-confed
Recommendation
Well, if you say you have more than 3 entries and having two or three different
AS numbers, we would recommend to enable both “bgp deterministic-med” and
“bgp always-compare-med”
