Re: [RRG] Long term clean-slate only for the RRG?

["William Herrin" <bill@herrin.us>]

On Thu, Jul 10, 2008 at 1:59 PM, Tony Li <tony.li@tony.li> wrote:
> MPLS requires that you install state _per tunnel_ on intermediate
> forwarders.  In other packet based tunnelling protocols, this is not
> necessary.
>
> Both MPLS and normal L3 protocols require routing state on intermediate
> forwarders.


Hi Tony,

I'm still not following the distinction.


I thought the way MPLS worked was along these lines:

Router A announces, "I know what to do with label 555." This label is
distributed to the rest of the systems speaking MPLS. Any packets they
see where the top label is 555, they forward them in the direction
they consider optimal to reach router A.

Router A then speaks a routing protocol (e.g. BGP) to router B. There
are multiple MPLS routers in between the two but routers A and B are
speaking BGP directly to each other. Router A tells router B: "Any
IPv4 packets you want to send to me, label them 555 and move them
through MPLS."

As a result, router B can move packets to router A without the
intervening routers needing the full BGP table. They only need to know
label 555.


But that's map-encap in a nutshell. You map multiple discontiguous
address spans into a single label that represents those spans' shared
exit point. You then route through the intermediate (core backbone)
nodes on the (presumably) much smaller number of labels. Whether that
label is part of a completely different protocol (MPLS) or just a new
address in the same protocol, the map-destination-to-a-label,
encapsulate and route-on-the-label process is semantically the same.

Have I grossly misunderstood how MPLS works? What's the story?

-Bill


-- 
William D. Herrin ................ herrin@dirtside.com bill@herrin.us
3005 Crane Dr. ...................... Web: <http://bill.herrin.us/>
Falls Church, VA 22042-3004

--
to unsubscribe send a message to rrg-request@psg.com with the
word 'unsubscribe' in a single line as the message text body.
archive: <http://psg.com/lists/rrg/> & ftp://psg.com/pub/lists/rrg