Partial vs Full tables

William Herrin bill at
Mon Jun 8 18:44:27 UTC 2020

On Mon, Jun 8, 2020 at 10:52 AM <ljwobker at> wrote:
> Every "fast" FIB implementation I'm aware of takes a set of prefixes, stores them in some sort of data structure, which can perform a longest-prefix lookup on the destination address and eventually get to an actual physical interface for forwarding that packet.  Exactly how those prefixes are stored and exactly how load-balancing is performed is *very* platform specific, and has tons of variability.  I've worked on at least a dozen different hardware based forwarding planes, and not a single pair of them used the same set of data structures and design tradeoffs.


AFAIK, there are two basic approaches: TCAM and Trie.  You can get off
in to the weeds fast dealing with how you manage that TCAM or Trie and
the Trie-based implementations have all manner of caching strategies
to speed them up but the basics go back to TCAM and Trie.

TCAM (ternary content addressable memory) is a sort of tri-state SRAM
with a special read function. It's organized in rows and each bit in a
row is set to 0, 1 or Don't-Care. You organize the routes in that
memory in order from most to least specific with the netmask expressed
as don't-care bits. You feed the address you want to match in to the
TCAM. It's evaluated against every row in parallel during that clock
cycle. The TCAM spits out the first matching row.

A Trie is a tree data structure organized by bits in the address.
Ordinary memory and CPU. Log-nish traversal down to the most specific
route. What you expect from a tree.

Or have I missed one?

Bill Herrin

William Herrin
bill at

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