Multilayer switching, as its name implies, allows switching to take place at different protocol layers. Switching can be performed only on Layers 2 and 3, or it can also include Layer 4. MLS is based on network flows. Key Point
A network flow is a unidirectional sequence of packets between a source and a destination. Flows can be very specific. For example, a network flow can be identified by source and destination IP addresses, protocol numbers, and port numbers as well as the interface on which the packet enters the switch.
The three major components of MLS are as follows131:
• MLS Route Processor (MLS-RP) The MLS-enabled router that performs the traditional function of routing between subnets
• MLS Switching Engine (MLS-SE) The MLS-enabled switch that can offload some of the packet-switching functionality from the MLS-RP
• Multilayer Switching Protocol (MLSP) Used by the MLS-RP and the MLS-SE to communicate with each other
MLS can be implemented in the following two ways:
• Within a Catalyst switch Here both the MLS-RP and the MLS-SE are resident in the same chassis. An example of an internal MLS-RP is a Route Switch Module (RSM) installed in a slot of a Catalyst 5500 Series switch.
• Using a combination of a Catalyst switch and an external router An example of a router that can be an external MLS-RP router is a Cisco 3600 Series router with the appropriate IOS software release and with MLS enabled.
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