7.1 MLS Processes
7.1.5
Step 2: The MLS-SE identifies candidate packets
After Step 1 has allowed the MLS-SE to acquire the addresses of the MLS-capable routers, the MLS-SE starts using its pattern-matching capabilities to look for packets that are destined to these addresses. If a packet is headed to the router and does not have an existing shortcut entry, it is classified as a candidate packet (if it did have a shortcut entry, it would skip this step and be MLS Switched). The packet uses the normal Catalyst Layer 2 Forwarding (L2F) process and gets forwarded out the port connected to the router.

Candidate packets must meet the following criteria:

  • They have a destination address equal to one of the router's MAC addresses learned via MLSP or the HSRP (Hot Standby Router Protocol) Virtual IP address learned via MLSP.
  • They do not have an existing shortcut entry.

Note: HSRP provides network redundancy for IP networks, ensuring that user traffic immediately and transparently recovers from first hop failures in network edge devices or access circuits.

For example, refer to Figure and assume that Host-A initiates a Telnet to Host-B. Recognizing that Host-B is in a different subnet, Host-A sends the packets to its default gateway, subinterface 1/0.1 on the router. Figure illustrates the relevant fields in this packet as it traverses the ISL link to the router.

The ISL header contains a VLAN ID of 1. The Ethernet header contains a source MAC address equal to Host-A and a destination MAC address equal to 00-00-0C-11-11-11, the MAC address of subinterface 1/0.1 on the router. The source and destination IP addresses belong to Host-A and Host-B, respectively. The switch uses the destination MAC address to perform two actions:

It forwards the packet out Port 1/1 toward the router using Layer 2 switching.  It recognizes the MAC address destination address as one of the addresses of the router learned in Step 1. This triggers a lookup for an existing Layer 3 shortcut entry based on the destination IP address (other options are available, but these are discussed later). Assuming that a shortcut does not exist (it is a new flow), the packet is flagged as a candidate packet and a partial shortcut entry is created.