Use of SD Access Affinity-ID

[bclounie]

In reviewing Border Node selection for SD Access Transit (LISP Pub/Sub), the Affinity-ID feature determines the BN selection using the lowest value of Affinity-ID.  Evaluating these values in the following manner determines the border node selected.
1. The BN with the lowest Prime value
2. If a tie with Prime, the BN with the lowest Decider value
3. If a time with both Prime, and Decider, then the BN with the lowest Priority value.

Would there be any issues with leaving the Prime and Decider values at 0 and just use the Priority value ?

[ChrisNewnham_]

Does changing the Border Priority alter the path within the local fabric, so the local fabric edge's will prefer the local border with a lower priority? All the examples given seem to be talking about SDA transit between sites, not intra-site.

I honestly can't work out why you would use Affinity ID over Borer Priority, aren't they both just arbitrary numbers used to determine a path preference?

I find Cisco's documentation really lacking in this area. A few examples with diagrams wouldn't hurt.

Thanks!

[andy!doesnt!like!uucp]

there are many contributors to your (& my frankly saying) poor experience on the topic. 1st of all: whatever LISP RFCs declares there r simply couple cases of LISP-in-action in the iIPworld, most of them r niche ones like noticed in Should We Use LISP? « ipSpace.net blog & only one market-achievable  is represented by SDA. i'm asking myself would i use it beyond the niche use-cases until solution gets maturity...
2nd: why Cisco cant produce good documentation on technology? there was site lisp.cisco.com Cisco forgotten of like poor mom does to its orphaned child. Lack of resources? not sure. rather poor managing it.

etc

[jedolphi]

Hi Chris, you are correct, if a Fabric Site has the same external reachabilitiy via an IP Transit on two site-local Border Nodes then the BN with the lower priority value will be preferred over the BN with the higher priority value. In a single site use case LISP priority is the right tool to use.

Affinity-ID has a more advanced use case that is applicable only to SDA Transit. It guides traffic to the closest non-fabric-site-local default route over SDA Transit, Affinity-ID does not steer traffic within a Fabric Site site. E.g. you have a global SDA Transit, and on that Transit you have 100s of Fabric Sites. A default route is injected into the SDA Transit from a hub site Europe, a hub site in Asia and a hub site in the Americas. Affinity-ID allows the Fabric Sites without a local default route to find the "closest" default route over the SDA Transit. This means Asian Fabric Sites use Asian default route, European Fabric Sites use European default route, etc.

Priority is more blunt, lowest priority always wins. In above example if the Asian hub site on SDA Transit had lowest priority then it would attract internet traffic from Europe, Asia the Americas, and 99% of the time this is not a desirable outcome.

I'm putting together an SDA Transit design presentation for Cisco Live in February 2024. Thanks for the discussion and questions here, I'll be sure to unpack this thread in Feb/24. In the mean time please ask more questions here. Cheers, Jerome