| MLS cache is an area in memory where
the MLS entries are stored. To prevent the MLS cache from
overflowing, an aging process must be run. This is a
software-controlled operation that runs in the background. Although
the architecture of the current MLS-SE can theoretically hold
128,000 entries, it is recommended to keep the total number of
entries below 32,000 on current versions of the card. MLS supports
three separate aging times:
Quick aging is utilized to age out
partial shortcut entries that never get completed by an enable
packet. The aging period for these entries is fixed at five seconds.
Normal aging is used for the typical
sort of data transfer flow. This is a user-configurable interval
that can range from 64 to 1920 seconds with the set
mls agingtime [agingtime] command.
The default is 256 seconds. When changing the default value, it is
rounded to the nearest multiple of 64 seconds.
Fast aging is used to age short-term
data flows such as Domain Name System (DNS), ping, and Trivial File
Transfer Protocol (TFTP). The fast aging time can be adjusted with
the set mls agingtime fast
[fastagingtime] [pkt_threshold]
command. If the entry does not have more than pkt_threshold
packets within fastagingtime
seconds, the entry is removed. By default, fast aging is not enabled
because the fastagingtimeme
parameter is set to 0. The possible fastagingtime
values are 0, 32, 64, 96, and 128 seconds (it uses the nearest value
if a different value is entered). The pkt_threshold
parameter can be set to 0, 1, 3, 7, 15, 31, or 63 (again, other values
can be entered and it uses the closest possible value).
Other events, such as applying access
lists, routing changes, or reconvergence, normal cam aging, or disabling MLS on the switch, can cause
MLS entries to be purged.
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