The router receives and routes the
packet as normal. Recognizing the destination address as being
directly connected on subinterface Fast Ethernet1/0.2, the router
sends the packet back across the ISL link encapsulated as a VLAN 2
frame, as illustrated in Figure .
Figure
shows the relevant fields contained in the packet as it crosses the
ISL link between the router and the switch.
The router has rewritten the Layer 2
header. Not only has it changed the VLAN number in the ISL header,
it has also modified both MAC addresses. The source MAC address is
now equal to 00-00-0C-22-22-22, the MAC address used on the Fast
Ethernet1/ 0.2 subinterface of the router, and the destination
address is set to Host-B. Although the IP addresses have not been
changed, the router must modify the IP header by decrementing the
Time-To-Live (TTL) field and update the IP checksum.
As the packet traverses the Catalyst
Switch on its way from the router to Host-B, five functions are
performed:
- The destination MAC address is
used to Layer 2 switch the packet out Port 3/1.
- The MLS-SE recognizes the source
MAC address as one of the entries created in Step 1 via the
hello process.
- The MLS-SE uses the destination IP
address to look up the existing partial shortcut entry created
in Step 2.
- The MLS-SE compares the XTAG
values associated with the source MAC address of this packet and
the partial shortcut entry. Because they match, the MLS-SE knows
that this is the enable packet coming from the same router
targeted by the candidate packet.
- The MLS-SE completes the shortcut
entry. This entry will contain all the information necessary to
rewrite the header of future packets (in other words, the fields
shown in Figure
).
|