Specifications

Multilayer Switching Overview
MLS Implementation
5
The MLS-RP routes the packet to Host B. When the packet appears back on the Catalyst 5000 series
switch backplane, the MLS-SE recognizes the source MAC address as that of the MLS-RP, and that the
packet’s flow information matches the flow for which it set up a candidate entry. The MLS-SE considers
this packet an enabler packet and completes the MLS entry (established by the candidate packet) in the
MLS cache (see Figure 65).
Figure 65 MLS Implementation
After the MLS entry has been completed, all Layer 3 packets with the same flow from Host A to Host B
are Layer 3 switched directly inside the switch from Host A to Host B, bypassing the router
(see Figure 66). After the Layer 3-switched path is established, the packet from Host A is rewritten by
the MLS-SE before it is forwarded to Host B. The rewritten information includes the MAC addresses,
encapsulations (when applicable), and some Layer 3 information.
The resultant packet format and protocol behavior is identical to that of a packet that is routed by the
RSM or external Cisco router.
Note MLS is unidirectional. For Host B to communicate with Host A, another Layer 3-switched path needs to
be created from Host B to Host A.
Figure 66 MLS Implementation
See the Catalyst 5000 Series Multilayer Switching User Guide for additional network implementation
examples that include network topologies that do not support MLS.
MLS-RP
(MLS-SE)
12002 Host A Host B
The MLS-RP routes this packet to Host B. Because the
MLS-SE has learned both this MLS-RP and the Layer 3
flow in this packet, it completes the MLS entry in the
MLS cache. The first routed packet is called the
enabler packet
Enabler packet
MLS-RP
(MLS-SE)
12003
Host A
Host B
Layer 3-switched packets
With the MLS entry from Host A to B established, the
Layer 3 traffic for this flow is switched directly inside
the Catalyst switch without going to the router