| 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.
|