Information Technology Reference
In-Depth Information
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Always preferring the path with the lowest MED might not be optimal, because this
does not take into account the intra-AS topology. For example, R4 needs to forward
traffic destined for 172.16.0.0/16 via R1 to R3 instead of directly to R6.
Enabling a
deterministic-MED
Comparison
When
deterministic-MED
is enabled,
the order in which the paths are received is unimportant. (For more information, refer to
Chapter 2.) Path selection is affected in Steps 2 and 3, as described in the section “Problem
Description: Incomparable Inter-AS Metrics.”
Table 7-8 shows the new BGP RIB on R1. Path 1 becomes the best path because it has a
lower RID than Path 2. Now the path via R2 is sent to R4.
Table 7-8
Initial Paths on R1
Path
BGP Next Hop
AS_PATH
MED
RID
1
*
R2
200 400
10
192.168.45.4
2
R3
300 400
5
192.168.45.5
3
R4
300 400
6
172.16.67.6
Table 7-9 shows the new BGP RIB on R4. Path 2 is the best path because
it is external.
Step 7
Table 7-9
New Path Information on R4
Path
BGP Next Hop
AS_PATH
MED
1
R2
200 400
10
2
*
R6
300 400
6
Notice that the best paths on R1 and R4 are not affected by new updates. So there is no best
path oscillation. Because there is no negative impact, it is almost always a good practice to
enable
deterministic-MED
in a network.
Resetting MEDs to 0s
One common solution for removing the impact of MED alto-
gether on path selection is to reset incoming MEDs to 0s. Configurations are typically made
on the edge of the network as updates are received from other autonomous systems.
Example 7-4 shows a configuration sample for R4 (only relevant commands are shown).
An inbound route map named Med-reset is configured on R4's session with R6 (IP address
192.168.46.6). The route map resets MED to 0. Because similar configurations are made
on R2 and R3, R1 has a consistent metric (0) in path selection.
