Problem: Path with Lowest RID Is Not Chosen as Best

Problem: Path with Lowest RID Is Not Chosen as Best

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in BGP paths because newly selected paths must be advertised to all BGP neighbors, and the previous one must be withdrawn.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.

Figure 15-49 shows the flowchart to follow to resolve this problem.

Figure 15-49. Problem-Resolution Flowchart

Debugs and Verification

Figure 15-50 shows a network composed of R1 in AS 109, and R3 and R5 in AS 110. Both R3 and R5 are advertising 100.100.100.0/24. The RIDs of R3 and R5 are 3.3.3.3 and 5.5.5.5, respectively.

Figure 15-50. Network in Which Path with Lowest RID Is Not Chosen as Best

Example 15-91 shows the necessary configuration in R1, R3, and R5 to form a BGP neighbor relationship, and for R3 and R5 to advertise 100.100.100.0/24.

Example 15-91 Configuring R3 and R5 to Advertise 100.100.100.0/24 and to Form an EBGP Neighbor Relationship with R1

 R1#  router bgp 109    bgp router-id 1.1.1.1    neighbor 1.1.2.3 remote-as 110   neighbor 1.1.7.5 remote-as 110  _____________________________________________________________________________________ R3#  router bgp 110    bgp router-id 3.3.3.3    network 100.100.100.0 mask 255.255.255.0   neighbor 1.1.2.1 remote-as 109   neighbor 1.1.8.5 remote-as 110  _____________________________________________________________________________________ R5#  router bgp 110    bgp router-id 5.5.5.5    network 100.100.100.0 mask 255.255.255.0   neighbor 1.1.7.1 remote-as 109  neighbor 1.1.8.3 remote-as 110 

The highlighted commands show how RID can be configured manually in BGP. Each router is configured with recognizable RIDs. This is done to ease in understanding BGP outputs later in this section.

Now, R1 will receive 100.100.100.0/24 from R3 and R5. The output in Example 15-92 shows that R1 is receiving both the updates and that it prefers the update from R5.

All BGP attributes (LOCAL_PREF, MED, ORIGIN CODE, and EXTERNAL versus INTERNAL) are identical. According to BGP best-path calculation rules, as described in

Example 15-92 BGP Output in R1 to Show the Best Path for 100.100.100.0/24

 R1#  show ip bgp 100.100.100.0  BGP routing table entry for 100.100.100.0/24, version 2 Paths: (2 available, best #2, table Default-IP-Routing-Table)   Advertised to non peer-group peers:   1.1.2.3   110     1.1.2.3 from 1.1.2.3 (  3.3.3.3  )       Origin IGP, metric 0, localpref 100, valid, external   110     1.1.7.5 from 1.1.7.5 (  5.5.5.5  )       Origin IGP, metric 0, localpref 100, valid, external,  best  

Chapter 14, the best path should be the one with the lowest RID if all other attributes are identical. In this output, the path from RID 3.3.3.3 (R3) should have been the best, but the output in Example 15-92 shows that the best path is from 5.5.5.5 (R5).

To explain this problem, you must understand the sequence of events in R1. In R1, the path from R5 must have been received before the path from R3. If R1 has selected the path from R5 as best, when the path from R3 comes and the deciding factor for the best-path calculation is RID, R1 will keep R5 as the best even though R3 offered a lower RID.

Solution

If R1 wants the proper RID to be the deciding factor in best-path calculation, it must add a BGP knob in Cisco IOS Software, as shown in Example 15-93.

Example 15-93 BGP Knob to Compare RID in Best-Path Selection

 R1#  router bgp 109   bgp router-id 1.1.1.1    bgp bestpath compare-routerid    neighbor 1.1.2.3 remote-as 110  neighbor 1.1.7.5 remote-as 110 

The highlighted command enables R1 to 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.)

The output in Example 15-94 shows that R1 has selected the R3 path as best because the R3 path has a lower RID (3.3.3.3) than the R5 path (5.5.5.5).

Example 15-94 BGP Best-Path Selection Using RID

 R1#  show ip bgp 100.100.100.0  BGP routing table entry for 100.100.100.0/24, version 3 Paths: (2 available, best #1, table Default-IP-Routing-Table)   Advertised to non peer-group peers:   1.1.7.5   110     1.1.2.3 from 1.1.2.3 (  3.3.3.3  )       Origin IGP, metric 0, localpref 100, valid, external,  best  110     1.1.7.5 from 1.1.7.5 (5.5.5.5)       Origin IGP, metric 0, localpref 100, valid, external