BGP Path Selection - Oldest Path Criterion

[Road2CCIE]

Hi,

I am learning about BGP Path Selection process and precisely about "Oldest Path" criterion. I am using Perplexity to do that and on one question I asked it about this, the AI replied with this sentence:

"To illustrate how the Oldest Path criterion actually works, consider a scenario where a router has three paths to the same destination with identical values for all other criteria. If these paths were learned in the order of Path A, then Path B, then Path C, the router would select Path A as the best path due to it being the oldest.

If Path A is lost, the router runs through the path selection process again with the remaining paths. At this point, both Path B and Path C already exist in the BGP table when the selection process occurs. Since they both already exist when the "Oldest Path" criterion is evaluated, they are considered to have the same "age" from the perspective of this selection step. The tie then continues to the next criterion, typically the Router ID.

This behavior explains why, in some cases, when the current best path is lost, the router does not select the next oldest path based on absolute age, but rather proceeds to evaluate other criteria like Router ID or neighbor IP address"

How true is that? I mean, it is possible that having 3 routes with timings 01:00, 01:05 and 01:10, after losing the one with 01:00, the router to just consider the other two, which are inherently different, the same? (Considering the previous criterions equal))

[David Ruess · Accepted solution]

Hello,

Im not sure if the time is based on the route being in the BGP table or routing table and if it ignores it but according to the Cisco docs it states:

STEP 10

When both paths are external, prefer the path that was received first (the oldest one).

This step minimizes route-flap because a newer path does not displace an older one, even if the newer path would be the preferred route based on the next decision criteria (Steps 11, 12, and 13).

Skip this step if any of these items is true:

• You have enabled the bgp best path compare-routerid command.

To me this says a couple of things:

1. It needs to be an eBGP learned route and this criterion is not for iBGP routes.

2. If you have the command bgp best path compare-routerid turned on then this step is skipped and the RID is compared.

Select BGP Best Path Algorithm - Cisco

BGP has may ways to determine a path and the algorithm can tie in many places. At some point you have to have a tie breaker that would be astronomical for them to be the same. You can even have the 2 routes from the same RID through multiple paths. Even that's not definite.

Hope this helps

-David

[Joseph W. Doherty]

Also, as @David Ruess 's reference describes, multi path is an interesting variant.

[Road2CCIE]

It really helps me, thank you David.

But still it is a bit unclear for me from Cisco's perspective.

Thank you again!

[David Ruess]

Yes, it snot quite clear on the circumstances. The best way I have found is to lab this in CML or other environment you have to test the results. You can be sure to get a clearer picture with configs and debugs helping you out, especially in your studies.

-David