Flow Masks; Layer 3-Switched Packet Rewrite - Cisco 6500 Series Software Configuration Manual

Chapter 20 Configuring IPX Unicast Layer 3 Switching on Supervisor Engine 1
The state and identity of the flow are maintained while packet traffic is active; when traffic for a flow
ceases, the entry ages out. You can configure the aging time for IPX MLS entries kept in the MLS cache.
If an entry is not used for the specified period of time, the entry ages out and statistics for that flow can
be exported to a flow collector application.
The maximum MLS cache size is 128K entries. However, an MLS cache larger than 32K entries
increases the probability that a flow will not be switched by the PFC and will get forwarded to the MSFC.

Flow Masks

The PFC uses flow mask modes to determine how IPX MLS entries are created. The flow mask mode is
based on the access lists configured on the IPX MLS router interfaces.
These sections describe how the flow mask modes work:
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Flow Mask Modes
The PFC supports only one flow mask (the most specific one). When the PFC flow mask changes, the
entire MLS cache is purged. When the PFC exports cached entries, flow records are created based on the
current flow mask mode. Depending on the current mode, some fields in the flow record might not have
values. Unsupported fields are filled with a dash (-).
The flow mask modes for IPX MLS are as follows:
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Flow Mask Mode and show mls entry Command Output
The flow mask mode impacts the screen output of the show mls ipx command. In destination mode, the
destination IPX address of the switched flows are displayed, along with the packet rewrite information:
rewritten destination MAC, rewritten VLAN, and egress interface.

Layer 3-Switched Packet Rewrite

When a packet is Layer 3 switched from a source host to a destination host, the PFC performs a packet
rewrite, based on information learned from the MSFC and stored in the MLS cache.
If Host A and Host B are on different VLANs and Host A sends a packet to the MSFC to be routed to
Host B, the PFC recognizes that the packet was sent to the MAC address of the MSFC. The PFC checks
the MLS cache and finds the entry matching the flow in question.
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Flow Mask Modes, page 20-3
Flow Mask Mode and show mls entry Command Output, page 20-3
destination mode—The least-specific flow mask 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. This mode is used if there are no access lists configured based on source IPX
addresses on any of the IPX MLS router interfaces.
destination-source mode—The PFC maintains one MLS entry for each destination (network and
node) and source (network only) IPX address pair. All flows between a given source and destination
use this MLS entry regardless of the IPX sockets. This mode is used if there is an access list on any
of the IPX MLS interfaces that filters on source network.
Catalyst 6500 Series Switch Cisco IOS Software Configuration Guide—Release 12.1 E
Understanding How IPX MLS Works
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