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Re: MPLS VPNs or not?
In message <[email protected]>, 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