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Re: [GROW] LISP at GROW

From: Dino Farinacci [mailto:dino at cisco.com]
Sent: Tuesday, August 05, 2008 1:27 AM
To: PAPADIMITRIOU Dimitri
Cc: grow at ietf.org
Subject: Re: [GROW] LISP at GROW


i) EID value space variation can result in triggered updates (e.g.
permanently remove a contiguous set [j,..,m] of intermediate EID
from an
aggregate [a,..,z] results into two aggregates [a,..,i] and

[n,..,z])

Well EID-prefix assignments will be relatively static. So I don't
think we'll nearly have the flapping as the underlying
network. And it
seems the underlying network holds together fairly well with
the issue you state above.


"relatively static" is certainly dependent on needs/usage and which
entity will manage the EID space and its associated policy. also, it
suffices that an application requires permanence of the EID across a
sequence of RLOCs to invalidate this assumption.
The site goes to get one more EID-prefix that is a PI prefix. So it will never renumber again. That means it won't ever change. It may get more EID-prefixes if the site needs more addresses but that would *add* a new prefix to the ALT. 


iii) otoh you have the reverse issue, true negative. the BGP ALT is
flapping preventing EID reachability while the RLOC are actually
reachable via the BGP. to solve this BGP ALT must be at least as
stable as the underlying BGP.


The ALT will not flap, I don't know why you think that. We have
logical tunnels between eBGP peers that are very resilient
since there is a robust network underneath it.


Even if (logical) links are failure-safe (at least at the extend they
can be recovered within a timeframe that does not impose any
notification/update or their recovery does not impose a topological
change) just think about failures of the leafs of the BGP-based ALT
distribution tree i.e. TR like indicated in response to Dave or root
failure since the ALT topology structure is a tree.
I am saying those GRE tunnels will be more robust than physical links. An xTR at a site that loses it's physical connection can still keep it's eBGP ALT peering up because the GRE encap'ed packets will follow the physical path through the other xTR. 

Dino


And since the EID-prefixes are added and deleted based on
subscription
based contracts, the flap rate is more like months and years, then
what you are thinking.


See above.

Thanks,
-d.


Hope this clarifies that issue,
-dimitri.


Well, not really.

Dino




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