Specifications

Multilayer Switching Overview
Introduction to IPX MLS
12
IPX MLS Flows
Layer 3 protocols such as IP and IPX are connectionless—they deliver every packet independently of
every other packet. However, actual network traffic consists of many end-to-end conversations, or flows,
between users or applications.
A flow is a unidirectional packet sequence between a particular source and destination that share
identical protocol and network-layer information. Communication flows from a client to a server and
from the server to the client are distinct.
Flows are based only on Layer 3 addresses. If a destination IPX address identifies a flow, then IPX traffic
from multiple users or applications to a particular destination can be carried on a single flow.
Layer 3-switched flows appear in the MLS cache, a special Layer 3 switching table is maintained by the
NFFC II. The cache contains traffic statistics entries that are updated in tandem with packet switching.
After the MLS cache is created, packets identified as belonging to an existing flow can be Layer 3
switched. The MLS cache maintains flow information for all active flows.
MLS Cache
The MLS-SE maintains a cache for IPX MLS flows and maintains statistics for each flow. An IPX MLS
cache entry is created for the initial packet of each flow. Upon receipt of a packet that does not match
any flow in the MLS cache, a new IPX MLS entry is created.
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 128,000 entries. However, an MLS cache larger than 32,000 entries
increases the probability that a flow will not be switched by the MLS-SE and will get forwarded to the
router.
Note The number of active flows that can be switched using the MLS cache depends on the type of access lists
configured on MLS router interfaces (which determines the flow mask). See the “Flow Mask Modes”
section later in this document.
Flow Mask Modes
Two flow mask modes—destination mode and destination-source mode—determine how IPX MLS
entries are created for the MLS-SE.
You determine the mode when you configure IPX access lists on the MLS-RP router interfaces. Each
MLS-RP sends MLSP messages about its flow mask to the MLS-SE, which performs Layer 3 switching.
The MLS-SE supports only the most specific flow mask for its MLS-RPs. If it detects more than one
mask, it changes to the most specific mask and purges the entire MLS cache. When an MLS-SE exports
cached entries, it creates flow records from the most 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 zero (0).
The two modes are described, as follows: