Information Technology Reference
In-Depth Information
Think of MLS as a much more advanced form of route caching, with the cache kept separate from the
router on a switch. Both the MLS-RP and the MLS-SE, along with respective hardware and software
minimums, are required for MLS.
The MLS-RP can be internal (installed in a switch chassis) or external (connected via a cable to a trunk
port on the switch). Examples of internal MLS-RPs are the Route-Switch Module (RSM) and the
Route-Switch Feature Card (RSFC), which are installed in a slot or supervisor of a Catalyst 5xxx family
member, respectively; the same applies to the Multilayer Switch Feature Card (MSFC) for the Catalyst
6xxx family. Examples of external MLS-RPs include any member of the Cisco 7500, 7200, 4700, 4500,
or 3600 series routers. In general, to support the MLS IP feature, all MLS-RPs require a minimum IOS
version in the 11.3WA or 12.0WA trains; consult release documentation for specifics. Also,
MLS must
be enabled
for a router to be an MLS-RP
.
The MLS-SE is a switch with special hardware. For a member of the Catalyst 5xxx family, MLS requires
that the supervisor have a NetFlow Feature Card (NFFC) installed; the Supervisor IIG and IIIG have one
by default. In addition, a bare minimum of Catalyst OS 4.1.1 software is required. Note that the 4.x train
has “gone General Deployment (GD)”—that is, passed rigorous end-user criteria and field-experience
targets for stability—so check Cisco's web site for the latest releases. IP MLS is supported and
automatically enabled for Catalyst 6xxx hardware and software with the MSFC/PFC (other routers have
MLS disabled by default). Note that IPX MLS and MLS for multicasting may have different hardware
and software (IOS and Catalyst OS) requirements. More Cisco platforms do/will support the MLS
feature. Also,
MLS must be enabled
for a switch to be an MLS-SE
.
The third major component of MLS is the Multilayer Switching Protocol (MLSP). Because
understanding the basics of MLSP gets at the heart of MLS and is essential to performing effective MLS
troubleshooting, we will describe MLSP here more in detail. MLSP is utilized by the MLS-RP and the
MLS-SE to communicate with one another—tasks include enabling MLS; installing, updating, or
deleting flows (cache information); and managing and exporting flow statistics (NetFlow Data Export is
covered in other documentation). MLSP also allows the MLS-SE to learn the Media Access Control
(MAC, Layer 2) addresses of the MLS-enabled router interfaces, check the flowmask of the MLS-RP
(explained later in this chapter), and confirm that the MLS-RP is operational. The MLS-RP sends out
multicast “hello” packets every 15 seconds using MLSP; if three of these intervals are missed, then the
MLS-SE recognizes that the MLS-RP has failed or that connectivity to it has been lost.
Figure 23-7 illustrates three essentials that must be completed (using MLSP) for a shortcut to be created:
the candidate, enabler, and caching steps. The MLS-SE checks for a cached MLS entry; if MLS cache
entry and packet information match (a hit), the packet's header is rewritten locally on the switch (a
shortcut, or bypassing of the router) instead of being sent on to the router, as would normally happen.
Packets that do not match and that are sent on to the MLS-RP are
candidate packets
—that is, there is a
possibility of switching them locally.
After passing the candidate packet through the MLS flowmask (explained later in Step 7) and rewriting
the information contained in the packet's header (the data portion is not touched), the router sends it
toward the next hop along the destination path. The packet is now called an
enabler packet
. If the packet
returns to the same MLS-SE from which it left, an MLS shortcut is created and placed into the MLS
cache. Rewriting for that packet and all similar packets that follow (called a flow) is now done locally
by switch hardware instead of by router software.
The same MLS-SE must see both the candidate and
the enabler packets for a particular flow for an MLS shortcut to be created.
(This is why network
topology is important to MLS.) Remember, the point of MLS is to allow the communication path
between two devices in different VLANs, connected off the same switch, to bypass the router and thus
enhance network performance.
