Hi Tony

See section 3.3.1 of RFC 8402 SR Architecture


It has a nice graphical explanation of SR-MPLS Anycast usage.

The concept of Anycast SID is similar to Multicast group GDA conceptually
and similar to IGP routing anycast construct of proximity routing but more
like the former GDA based routing.

Basically you can have a group of nodes in the SR domain that can be
assigned an Anycast SID A and another group an Anycast SID B for the nodes
and a common group outer label Anycast SID is assigned to forward to the
Group of nodes represented by the Anycast SID.  The inner label is
basically a common label to forward to the group of nodes.

You can think of Anycast as similar to SR-TR prefix sid steering ECMP
pathing where all paths are active to get to the unicast destination.

In this case its all paths active but now to a pseudo multicast like
“anycast sid” group destination.

The concept of Anycast SID exists with SRv6 as well.

I think to your point I am not sure why it matters and can the Area prefix
be either unicast or Anycast.  As far at the area leader if that is a
single node then the node sid could be utilized and if the area leader is a
group or more then one node then a Anycast sid could be utilized.  I think
flexibility would be best so it can be either as you pointed out.

Kind Regards


On Thu, Aug 6, 2020 at 12:32 PM Tony Li <tony1ath...@gmail.com> wrote:

> Les,
> * There then remains the question as to whether the “Area Prefix” is
> anycast or unicast i.e., is it common to all IERs or is it unique to
> whomever gets elected Area Leader?*
> Does it matter? We have no clear semantics for this prefix. A difference
> that makes no difference is no difference.
> *[Les:] This question needs to be directed at those who prefer the Area
> Prefix approach. It matters as it impacts configuration and advertisement
> semantics. An anycast prefix is NOT a Node Prefix.*
> *And it impacts how traffic is forwarded into the area.*
> How so?  Traffic will be directed to the SID value (modulo PHP).
> *[Les3:] If the prefix is private to a single router then traffic has to
> pass through that router. If it is anycast the traffic could arrive at any
> one of the routers supporting the anycast address.*
> I must be missing something. My understanding of SR is that you forward
> based on the label.  Please educate me.
> Tony
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*Gyan Mishra*

*Network Solutions A**rchitect *

*M 301 502-134713101 Columbia Pike *Silver Spring, MD
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