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Re: Scalability issues in the Internet routing system

  • From: Patrick W. Gilmore
  • Date: Tue Oct 18 12:06:38 2005

On Oct 18, 2005, at 11:30 AM, Andre Oppermann wrote:

1. The number of prefixes*paths in the routing table and interdomain
   routing system (BGP)

This problem scales with the number of prefixes and available paths
to a particlar router/network in addition to constant churn in the
reachablility state.  The required capacity for a routers control
plane is:

 capacity = prefix * path * churnfactor / second

I think it is safe, even with projected AS and IP uptake, to assume
Moore's law can cope with this.
Especially since this does not have to be done in real time. BGP updates can take many seconds to process without end users thinking anything is amiss.

2. The number of longest match prefixes in the forwarding table

This problem scales with the number of prefixes and the number of
packets per second the router has to process under full or expected
load.  The required capacity for a routers forwarding plane is:

 capacity = prefixes * packets / second

This one is much harder to cope with as the number of prefixes and
the link speeds are rising.  Thus the problem is multiplicative to

Here I think Moore's law doesn't cope with the increase in projected
growth in longest prefix match prefixes and link speed.  Doing longest
prefix matches in hardware is relatively complex.  Even more so for
the additional bits in IPv6.  Doing perfect matches in hardware is
much easier though...
You are mistaken in one of your assumptions. The FIB is generated asynchronously to packets being forwarded, and usually not even by the same processor (at least for routers "in the core"). Therefore things like pps / link speed are orthogonal to longest match. (Unless you are claiming the number of new prefixes is related to link speed. But I don't think anyone considers a link which has nothing but BGP updates on it a realistic or useful metric.