Re: [Lsr] I-D Action: draft-ietf-lsr-isis-area-proxy-03.txt

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Hi Les,

> [Les:] Any one of the IERs can be elected Area Leader, therefore all of them have to be configured with the Area Prefix and associated SID.


The Area Leader may not be an IER.  In fact, in an important use case for us, the area is a leaf-spine topology.  The Area Leader is one of the spines.  The leaves are the edge routers.  For resource reasons, we do NOT want the Area Leader to be a leaf.


We do NOT require that the Area Leader candidates have identical configurations.  In fact, if there is a configuration change, it may be beneficial to configure one candidate differently and then raise its priority.  It’s a simple way of effecting an area-wide configuration change.  

> Perhaps you are allowing that each IER could choose a different Area Prefix/SID. Not sure why you would want to do that – but even if you did, the behavior of the winning prefix/SID is analogous to an anycast address.
> The difference here is that the advertisement of the Prefix Reachability associated with the area prefix is within the Proxy LSP – which appears to OERs as if it was originated by all of the IERs i.e., the set of IERs appears as a single node to the OERs. Still, all IERs are aware of the winning prefix reachability advertisement and will do what is required in forwarding based on that content.


They will not be aware of it unless we tell them via the Area Proxy TLV.  For obvious reasons, the Inside Nodes do NOT do anything with the Proxy LSP other than flood it.


> Which is why we’re using the Prefix SID.
>  
> [Les:] You are using the prefix-SID, but the advertisement is not associated with a prefix reachability  advertisement, yet you want nodes to install forwarding entries based on this advertisement. This is what seems inappropriate.


We want outside nodes to install forwarding entries on the Prefix SID.  This is entirely backward compatible.  How is that inappropriate?


> The only current case where a prefix-SID is advertised and is NOT associated with prefix reachability is in the Binding TLVs. This has two use cases:
> Advertising SIDs for prefixes associated with nodes which are NOT SR capable
> As an alternative to per prefix advertisement if the operator prefers to use a centralized SID assignment service
>  
> In both of these cases if a SID were to be advertised in prefix reachability TLV for the same prefix the SID in the prefix reachability advertisement would be preferred.
> You don’t discuss this at all in the draft i.e., what happens if the SID in the prefix reachability advertisement for the Area Prefix differs from that advertised in the Area Proxy TLV. What I am pushing for is eliminating the need to do so by relying on the existing prefix SID advertisements and not introducing a new one in the Area Proxy TLV.


The existing ones do not have the required semantics.

> [Les:] The semantics you require are functionally equivalent to anycast behavior – which is supported already.


Please point me to anycast semantics that will ONLY be selected by IERs. 

Tony