Optimum Switching

Optimum Switching
Optimum switching also relies on a caching mechanism, but there are important differences
from fast switching. The first difference is in the operation of the tree. Instead of a binary tree,
with each level being a single comparison (1 or 0 in the binary string), optimum switching
employs a 256-way multi-way tree (mtree). Each level allows selection of a single octet in the
destination address, resulting in a maximum of four lookup probes to find any target.
FIGURE 18.10 Fast switching tree
0 ROOT 1
0 0 1
0 00 1
0 000 1 0 001 1
0 01 1
0 1 1
0 11 1
0 110 1 0 111 1
0 10 1
580 Chapter 18  Multilayer Switching (MLS)
Optimum switching is very fast, but still suffers from the same problems of cache invalidation
and therefore needs to be aged out regularly, interrupting the optimum flow while caches
are rebuilt from requests to the route processor again.
The optimum switching tree is shown in Figure 18.11. Each octet in the 32-bit dotted-decimal
address is matched individually, resulting in a far faster lookup process.