Introducing MLS

MLS was first introduced for the Catalyst 5000 in version 4.1(1) of the switch operating system, and a NetFlow Feature Card (NFFC) was also required. The MLS-enabled switch keeps track of the initial packet destined to the router, and any subsequent packets from the same flow destined to the router are intercepted by the switch. The switch now does the forwarding for the router. The process greatly increases the number of packets switched between VLANs.

MLS offers low latency at Layer 3. It uses the existing network infrastructure in place, assuming the Catalyst 5000 product line. Aside from a daughter card and software upgrade, MLS does not have any other major requirements. MLS handles packet switching and rewrite function at the ASIC level, whereas the router does this at the software level. A Catalyst 6500 switch automatically is configured for MLS, and its architecture is vastly different from the Catalyst 5000 product line.

The following are the requirements for enabling MLS on the Catalyst 5000 product line:

  • Switching Engines (MLS-SE)

    - Cat5k?Supervisor Engine SW 4.1(1) or later

    - Supervisor IIG, or IIIG

    - Supervisor III w/NFFC I or II

    - CAt6k?Supervisor Engine SW 5.3(1)CSX or later

    - Supervisor IA w/MSFC and PFC

  • Routing Processors (MLS-RP)

    - RSM, Cisco 7500, 7200, 4700, 4500 series routers

    - SW 11.3(2)WA4(4)

    - SW 11.3(8)WA4(11) or later (ATM Media)

    - RSFC?SW 12.0(3c)W5(8a) or later

    - 3600?12.0(2) or later

    - MSFC?12.0(3)XE or later

Example 6-1 shows the output from a Catalyst 5500 device. As the output shows, the supervisor is a Supervisor III, WS-X5530, running 4.5(5) Catalyst Operation System (Catalyst OS) software, which means it can support MLS. Furthermore, it has an NFFC II, WS-F5531A, card installed.

Example 6-1. Supervisor Information

Switch3> (enable) show module 1

Mod Module-Name         Ports Module-Type           Model    Serial-Num Status

--- ------------------- ----- --------------------- --------- --------- -------

1                       0     Supervisor III        WS-X5530  030061500 faulty

Mod MAC-Address(es)                        Hw     Fw         Sw

--- -------------------------------------- ------ ---------- -----------------

1   00-90-86-66-50-00 to 00-90-86-66-53-ff 3.5    5.1(2)     4.5(5)

Mod Sub-Type Sub-Model Sub-Serial Sub-Hw

--- -------- --------- ---------- ------

1   NFFC II  WS-F5531A 0030060943 2.2

There are three components to MLS:

  • Multilayer Switching Route Processor (MLS-RP)? The router component responsible for the initial packet forwarding. It informs the Multilayer Switching Engine (MLS-SE) of what MAC address it used for rewrite, and any changes to the interface, such as routing, access list, and so on.

  • Multilayer Switching Engine (MLS-SE)? Understands Layer 3 flow, rewrites MAC, and switches packets.

  • Multilayer Switching Protocol (MLSP)? MLSP is the means by which MLS-RP and MLS-SE communicate. MLSP uses Cisco Group Management Protocol (CGMP) multicast MAC address 01-00-0c-dd-dd-dd. The MLSP packet contains VLAN Trunking Protocol (VTP) domain name, version number, router ID, and sequence number. It also sends information regarding access list and routing table changes/updates. The switch adds a 1-byte XTAG for the purpose of identifying all MAC addresses associated with a single router. Hellos are sent every 15 seconds, with dead timer set at 45 seconds.

MLS can be programmed on an internal or external router. Cisco introduced various routing capable modules for the Catalyst 5000 product line. These modules supported almost all features and functions of a typical router. These modules include Route Switch Module (RSM) and Route Switch Feature Card (RSFC), with RSM being the most popular. RSM could software switch packets at 114,000 pps. It is basically an RSP2 card in the Catalyst 5000/5500 chassis. The combination of fast internal router and the switching engine allowed for robust forwarding of packets with low latency. MLS can also be programmed on an external router such as 75xx, 72xx, and so on that is connected to a Catalyst switch. Many Catalyst 5500 MLS implementations used an RSM.