[40387] in North American Network Operators' Group
Re: MPLS VPNs or not?
daemon@ATHENA.MIT.EDU (Craig Partridge)
Wed Aug 8 08:35:06 2001
Message-Id: <200108081234.f78CYZ306075@aland.bbn.com>
To: Scott Brim <sbrim@cisco.com>
Cc: nanog@merit.edu
In-Reply-To: Your message of "Wed, 08 Aug 2001 10:38:21 BST."
<20010808103821.H1592@SBRIM-W2K>
Date: Wed, 08 Aug 2001 08:34:35 -0400
From: Craig Partridge <craig@aland.bbn.com>
Errors-To: owner-nanog-outgoing@merit.edu
In message <20010808103821.H1592@SBRIM-W2K>, Scott Brim writes:
>> * Per hop policy decisions can be made more effectively in MPLS than
>> in IP. Not true in theory unless you want to look very deep in
>> the packet to identify the policy association, though it may be
>> true in practice on certain current systems.
>
>MPLS doesn't require per-hop policy decisions. Policy decisions only
>need to be made at the edge, re FEC inclusion. Intelligence at the edge
>etc. Parallels with the diffserv model of classifying & marking packets
>at the edge so you only need to look at PHBs in the middle.
Hi Scott:
Sorry I was too cryptic here -- sure MPLS makes a policy decision -- it
decides how to forwarding based on the tag (e.g. the policy is embedded in
the tag). My point is that you could just as easily associate the forwarding
rule with a key, made up, say from source and destination address (which in
some route lookup schemes requires only one more memory access than looking
up purely on destination).
>> * Instantiation of per-hop policy information via MPLS is more scalable
>> than it would be in IP (not quite said above but an implied issue).
>> Almost certainly not true (see above about general policy being hard
>> being why IP doesn't do it).
>
>Instantiation of per-hop policy in MPLS consists of forwarding by LSP,
>except at the edge router.
Except that something has to decide where the the path goes (and thus,
has to execute the policy at something close to a network wide level in
terms of analyzing the network and instantiating the path). If you're
suggesting we can do policy purely at the edges, then presumably a
routing protocol could equally well force its policy information to only
be computed at the edges. Yes? Or am I missing something?
Craig
