Modifying Fast Aging Time

Modifying Fast Aging Time
When the layer 3 cache grows greater than 32KB in size, the possibility increases that the PFC
or NFFC will not be able to perform all layer 3 switching, causing some packets to be forwarded
to the router. To aid in maintaining a layer 3 cache smaller than 32KB, you can enable and
adjust fast aging times.
Because some flows can be very short—a DNS query, for example—you can enable packet
thresholds that can be used in correlation with the fast aging time to quickly age out these
entries. Both of these attributes are thresholds. When you set the fast aging time, you specify the
amount of time for which n number of packets (defined by the packet threshold) must have used
the cache entry.
When a flow is initialized, the switch must see a number of packets equal to or greater than
the packet threshold set within the time specified by the fast aging time. If this criterion isn’t
met, the cache entry is aged out immediately.
Valid values for the fast aging time are 32, 64, 96, and 128. Valid values for the packet
threshold are 0, 1, 3, 7, 15, 31, and 63. Let’s try an example to illustrate how this works.
Suppose you configured a fast aging time of 64 seconds and set the packet threshold to
31 packets by using the set mls agingtime fast 64 31 command on the switch. This
is telling the MLS-SE that a layer 3 cache entry has 64 seconds in which 31 packets or more
must utilize the entry. If this doesn’t happen, the cache entry is removed.
The actual syntax for the command is set mls agingtime fast fastagingtime pkt_
threshold. An example configuration follows:
Terry_6506> (enable) set mls agingtime fast 64 31
Multilayer switching fast aging time set to 64 seconds for
entries with no more than 31 packets switched.
Terry_6506> (enable)