Scalability issues in the Internet routing system

Tue Oct 18 16:46:40 UTC 2005

Patrick W. Gilmore wrote:
> On Oct 18, 2005, at 11:30 AM, Andre Oppermann wrote:
>> 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
>> quadratic.
>>
>> 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.

I'm not talking about BGP here but the actual forwarding on the line
card or wherever it happens for any particular architecture.  The ASIC
thingie.  It has to do longest-match lookups for every packet that comes
in to figure out the egres interface.

-- 
Andre