This is the scenario in which two or more paths from EBGP neighbors have identical BGP attributes and BGP best-path selection is done based on the RID. The BGP best-path selection rule states that, in case all other attributes are identical, the path with the lowest RID should be selected as best. In this case, the path with the highest RID is selected as best.
In Cisco IOS Software, if BGP selects a best path based on the RID and a new path comes in with a lower RID, with all other attributes being equal, the previously selected best path will not be toggled and will remain unchanged. This is done intentionally in Cisco IOS Software to maintain stability in BGP paths because newly selected paths must be advertised to all BGP neighbors, and the previous one must be withdrawn. To avoid this churn, BGP in Cisco IOS Software does not select a new best path if the previous path selected was done based on RID.
debugs and verification:
show ip bgp a.b.c.d - resuls shows that the one with highest RID is the best route.
Solution:
bgp bestpath compare-routerid
this command compare the RIDs of all the paths and pick the lowest RID as the best in BGP best-path calculation. The effect of this configuration change takes place when the BGP scanner runs. (It runs every minute in Cisco IOS Software.)
this command compare the RIDs of all the paths and pick the lowest RID as the best in BGP best-path calculation. The effect of this configuration change takes place when the BGP scanner runs. (It runs every minute in Cisco IOS Software.)
2. Problem: Lowest MED Not Selected as Best Path
One BGP rule that must be kept in mind is the rule of MED comparison. By default, Cisco IOS Software will not compare the MEDs if two paths came from different autonomous systems.
One BGP rule that must be kept in mind is the rule of MED comparison. By default, Cisco IOS Software will not compare the MEDs if two paths came from different autonomous systems.
debugs and verification:
show ip bgp a.b.c.d
example scenario:
The output in 15-95 shows that R1 has three paths in this order:
Path 1: This path is from R5 (RID 5.5.5.5), with a MED of 30.
Path 2: This path is from R4 (RID 4.4.4.4), with a MED of 40.
Path 3: This path is from R3 (RID 3.3.3.3) with a MED of 50.
If the best-path selection algorithm described in Chapter 14 were run, the following would be the selection process:
- Path 1 is compared with Path 2. All BGP attributes are the same except for the MED. However, these two paths came from different autonomous systems—110 and 111, respectively—so the MED will not be the tiebreaker and will be ignored. The tiebreaker will be the RID. Based on the RID, path 2 has a lower RID (4.4.4.4) than path 1 (5.5.5.5). Therefore, path 2 is the winner.
- The winner of Step 1, path 2, is compared with path 3. Again, the MED will be ignored because of a different AS_PATH. The lower RID of path 3 (3.3.3.3) will win again path 2's RID (4.4.4.4).
- Path 3 is selected as best even though it has a higher MED than any of the paths (MED 50).
The output in 15-95 shows that R1 has three paths in this order:
Path 1: This path is from R5 (RID 5.5.5.5), with a MED of 30.
Path 2: This path is from R4 (RID 4.4.4.4), with a MED of 40.
Path 3: This path is from R3 (RID 3.3.3.3) with a MED of 50.
If the best-path selection algorithm described in Chapter 14 were run, the following would be the selection process:
- Path 1 is compared with Path 2. All BGP attributes are the same except for the MED. However, these two paths came from different autonomous systems—110 and 111, respectively—so the MED will not be the tiebreaker and will be ignored. The tiebreaker will be the RID. Based on the RID, path 2 has a lower RID (4.4.4.4) than path 1 (5.5.5.5). Therefore, path 2 is the winner.
- The winner of Step 1, path 2, is compared with path 3. Again, the MED will be ignored because of a different AS_PATH. The lower RID of path 3 (3.3.3.3) will win again path 2's RID (4.4.4.4).
- Path 3 is selected as best even though it has a higher MED than any of the paths (MED 50).
Solution:
bgp always-compare-med
The best path is the one that has the lowest MED. As stated earlier, choosing the path with the lowest MED could be crucial if links between autonomous systems are of different bandwidth and a path from a higher-bandwidth neighbor is sending a lower MED.
In addition, one important design recommendation is that the command bgp always-compare-med should be enabled on all the routes in an AS running BGP; otherwise, packet forwarding loops might occur. For example, Router A running this command might point its best path to Router B, whereas Router B without this command might point the best path back to Router A, resulting in a routing loop.
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