Subject: Re: [lisp] ETR connectivity to site - question on EID reachability (section 6.4)

From: [email-masked] (Noel Chiappa)
Date: Tue, 24 May 2011 13:52:15 -0400 (EDT)

> From: Yakov Rekhter <[email-masked]>

    >> 6.4.  EID Reachability within a LISP Site
    >>
    >> A site may be multihomed using two or more ETRs.  The hosts and
    >> infrastructure within a site will be addressed using one or more EID
    >> prefixes that are mapped to the RLOCs of the relevant ETRs in the
    >> mapping system.  One possible failure mode is for an ETR to lose
    >> reachability to one or more of the EID prefixes within its own site.
    >> When this occurs when the ETR sends Map-Replies, it can clear the
    >> R-bit associated with its own locator.  And when the ETR is also an
    >> ITR, it can clear its locator-status-bit in the encapsulation data
    >> header.

    > the 1st paragraph in 6.4 applies only when xTR is a CPE router, or
    > also when xTR is a PE router ?

Whether a LISP box is a PE router or a CPE router should not matter when
it comes to how it responds to a particular situation, should it? Or am I
confused about that?

If you want to discuss how _any_ ETR should respond when it loses contact with
a group of EIDs it is currently advertising, that's different.

And we could also usefully discuss how an ETR is expected to detect that event
- or is that outside the scope of LISP (since it likely involves interaction
with IGPs, etc, etc - I assume we're ruling out having the ETR ping each
address it is advertising directly)?

Finally, to anticipate a likely response, how do current routers respond when
a group of hosts they are 'advertising' goes offline? Do they reliably detect
all such circumstances (e.g. a bridge failure), and update their routing
advertisements accordingly? Is a failure to do so regarded as a fatal flaw in
the packet switching paradigm - or just a low-probability event that
experience has shown to be not worth adding a lot of complication to handle?

	Noel

---

From: Darrel Lewis <[email-masked]>
Date: Tue, 24 May 2011 20:22:29 -0700

Noel,

> it comes to how it responds to a particular situation, should it? Or am I

Margaret and I did a presentation discussing the trade-offs between CPE and PE xTRs in Hiroshima.  One of the obvious differences is the fact that the xTRs are not under the administrative control of the site when the xTR is a PE.  This was the genesis of the 'deployment' draft that has tried to capture this (and other issues) about the trade-offs of implementing LISP network elements in different topologies.

Attached is the presentation, I think you can find the recording online as well.

-Darrel