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Re: Scalability issues in the Internet routing system
Patrick W. Gilmore wrote:
On Oct 18, 2005, at 11:30 AM, Andre Oppermann wrote: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.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...
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