North American Network Operators Group

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Re: multi-homing fixes

  • From: Alex Bligh
  • Date: Fri Aug 31 11:05:11 2001

The bitmaps are generated inside the source AS (presumably, iBGP will
still carry regular routes) and the bitmaps are transmitted from one
network to another, so there is no requirement for full interconnetion at
the routing level.
The trouble with using 1 bit to represent 1 prefix is that there is
a need to move more than 1 bit of information per route between
AS's (think AS paths for loop detection, communities etc.).

In iBGP the situation is worse as you have more information
you want to carry (next hop, localpref), but you seem to
envisage this only to replace eBGP. So all you are doing
is compressing the data stream (after making some simplifying
assumptions some of which I don't believe hold up). As you have to
translate your bitmap back to/from iBGP in order to propagate
announcements across the AS, you might as well consider the
simpler alternative of just compressing the eBGP. However,
you're increasing processor power here, rather than decreasing

It would probably be possible to compress information on
stub nodes, or nearly stub nodes much further (but you
can do that effectively with outbound route filters),
and, to a limitted extent reduce their visibility in
the middle of the network (think proxy-aggregation)
but we have existing tools to do this.

The 'real' solution is to hierachicalize (sp?) or indirect
the routing tree such that reachability information
for common multihomed configurations does not in
general reach the core's of most people's networks [*].
I suspect technologies similar to mobile IP may
have application here.

[*] or reduce the routes held in most of the routers
in most people's networks, which, without wishing
to start another flame war, is a claim occasionally
made for MPLS networks in that non-edge LSR routers
need not carry any BGP table, 'merely' an LIB. However,
you still need to carry the prefixes in edge LSRs
so, stepping neatly around the flame-fest, this
seems to me an incomplete solution.

Alex Bligh
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