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Understanding How Layer 3 Switching Works
MLS Cache Aging
The state and identity of flows are maintained while the packet traffic is active; when the traffic for a
flow ceases, the entry ages out. You can configure the aging time for the MLS entries that are kept in the
MLS cache. If an entry is not used for the specified period of time, the entry ages out and the statistics
for that flow can be exported to a flow collector application.
MLS Cache Size
The maximum MLS cache size is 128,000 entries. The MLS cache is shared by all MLS processes on
the switch (IP MLS, IP MMLS, and IPX MLS). An MLS cache that is larger than 32,000 entries
increases the probability that a flow will not be Layer 3 switched but will be forwarded to the MSFC.
Understanding Flow Masks
The PFC uses flow masks to determine how the MLS entries are created.
These sections describe the flow mask modes:
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Flow Mask Modes—Prior to Software Release 8.5(1)
The PFC supports only one flow mask (the most specific one) for all MSFCs that are Layer 3 switched
by that PFC. If the PFC detects different flow masks from the different MSFCs for which it is performing
Layer 3 switching, it changes its flow mask to the most specific flow mask detected.
When the PFC flow mask changes, the entire MLS cache is purged. When the PFC exports the cached
entries, the flow records are created based on the current flow mask. Depending on the current flow
mask, some fields in the flow record might not have values. The unsupported fields are filled with a
zero (0).
The MLS flow masks are as follows:
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Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
14-6
Flow Mask Modes—Prior to Software Release 8.5(1), page 14-6
Flow Mask Modes—Software Release 8.5(1) and Later Releases, page 14-7
Flow Mask Modes and show mls entry Command Outputs, page 14-9
destination-ip—The least-specific flow mask. The PFC maintains one MLS entry for each Layer 3
destination address. All flows to a given Layer 3 destination address use this MLS entry.
destination-ipx—The only flow mask mode for IPX MLS is destination mode. The PFC maintains
one IPX MLS entry for each destination IPX address (network and node). All flows to a given
destination IPX address use this IPX MLS entry.
source-destination-ip—The PFC maintains one MLS entry for each source and destination IP
address pair. All flows between a given source and destination use this MLS entry regardless of the
IP protocol ports.
source-destination-vlan—For IP MMLS. The PFC maintains one MMLS cache entry for each
{source IP, destination group IP, source VLAN}. The multicast source-destination-vlan flow mask
differs from the IP unicast MLS source-destination-ip flow mask in that, for IP MMLS, the source
VLAN is included as part of the entry. The source VLAN is the multicast reverse path forwarding
(RPF) interface for the multicast flow.
full flow—The most-specific flow mask. The PFC creates and maintains a separate MLS cache entry
for each IP flow. A full flow entry includes the source IP address, destination IP address, protocol,
and protocol ports.
Chapter 14
Configuring MLS
OL-8978-04
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