Cisco Systems RJ-45-to-AUX manual MLS Flows, 230

MLS Flows

When a flow process begins, the MLS−RP starts sending out multicast hello messages every 15 seconds to all switches in the network that accept MLS−RP messages. These messages inform each switch that the MLS−RP (router or internal route processor) is available to provide routing information to the MLS switches, allowing them to cache learned routes.

MLSP is the protocol used between the MLS−SE and the MLS−RP. It uses a Cisco Group Management Protocol (CGMP) multicast address, so each MLS−SE (switch) enabled for CGMP will hear the hello message. To distinguish between normal CGMP messages and the MLS messages, the MLS−RP uses a special protocol type in the hello message itself.

The MLSP hello message (which is also known as an MLS−RP advertisement) can contain the following information:

∙The Media Access Control (MAC) addresses of the router interfaces participating in MLS

∙The router’s known virtual LAN (VLAN) information

∙The MLS−RP’s access lists

∙Any known or updated routing information

A switch participating in MLS has an MLS−SE component. This component processes the hello message and records the MAC address of the MLS−RP interfaces into its Content Addressable Memory (CAM) table. If multiple MLS−RPs exist in the network, the MLS−SE assigns a unique 1−byte identifier called an XTAG, as shown in Figure 11.3. The XTAG is a number that distinguishes the network flows of each MLS−RP.

Figure 11.3: An individual XTAG number is assigned to each MLS router in the network.

When a host from one VLAN on the network begins a network flow that is destined for a host on another VLAN, the MLS switch that received the first packet in the flow extracts the Layer 3 information for the flow. This information includes the destination address, source address, and protocol port numbers. The MLS−SE then forwards the first packet to the MLS−RP for a routing resolution. MLSP is used to inform the MLS−SE of the path to the destination hosts communicating in the flow. Because this is the first packet, no cache entry exists; a partial MLS entry for this Layer 3 flow is created in the MLS cache.

When the MLS−RP receives the packet, it looks at its route table to determine the destination of the packet and applies any applicable policies, such as an inbound or outbound access list. The MLS−RP will then rewrite the MAC header, adding the MAC address of the destination host and using its own MAC address as the source address. The MLS−RP then sends the packet back to the MLS−SE.

At this point, an MLS router has resolved the first packet with either a VLAN or Layer 3 logical address to a Layer 2 MAC address. The MLS−SE can now use this address to make a forwarding decision and send the packet out the correct port connected to the destination node based on the entries the switch has in its CAM table. The MLS−SE also determines that the MAC address of the MLS router is the source address in the packet and that the packet’s flow information matches a candidate entry in its MLS cache.

Now that the entry for the flow has been added to the MLS cache, any further packets that are identified as belonging to the same flow are handled by MLS−SE and switched based on the cached information. The MLS−SE rewrites the headers, reconditions the checksums, and forwards the packets without their having to

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