Learning by Doing: CISCO Certified Network ... - SCN Research

8. Plug one end of a crossover cable into port “Ax” on SW-D and the other end into port
“Bx” on SW-A. You have now created a loop in your switches.
9. Turn on the power. After the switches cycle through their start-up procedures one by
one the lights over the Ax and Bx ports should change from amber-colored (Problem
or not functioning) to green-colored (OK-operational). One of the lights should
change back to amber. Were you right? Remember different groups on different
groups of switches will have different answers…it all depends upon the MAC
addresses.
Manual selection of main and redundant backbones by changing port costs and priorities
1. Disconnect the backbone cable that is not connected to the root bridge and is not
selected as the redundant backbone.
SW-A Ax Bx Root bridge (lowest MAC address)
SW-B Ax Bx unplug this one
SW-C Ax Bx
SW-D Ax Bx (redundant backbone darkened)
If you lab setting appears like the above drawing, then select the line between SW-B
(Ax) and SW-C (Bx) to be disconnected. All remaining lights should be green.
2. Switch the crossover cable which you just disconnected to any two ports on SW-C
and SW-D (let’s just use port #7 on each). Note: this will vary dependent upon which
one is the root bridge. This line should become a redundant backup, mostly because
of the lower priority for the slower speed (10BaseT instead of 100BaseT). This line
will now become the redundant backbone. We just forced it to be by using our
knowledge of default port priority settings. (Just like we did in the last lab).
3. Reconnect that cable back into the Ax and Bx ports.
4. Remove one of the main crossover cables that is attached to the root bridge (like the
one between SW-A (Ax) and SW-B (Bx) above).
5. Give it about 60 seconds for the STP to switch the redundant backbone to a main
backbone.
6. Connect that crossover cable to ports #7 on SW-A and SW-B. This should
reconfigure as the new redundant backbone because of the lower port priority of
10BaseT connections. It should change back almost immediately.
7. Now let’s go in and change the port costs for these ports. Put the console cable into
the switch with the amber light of the redundant backbone line. Use [M] menus, [P]
port configuration, [select port number 7], and then [C] cost. Change this value to 1.
When you hit enter you should almost immediately see the line change from amber to
green (from backup to main). The line with the next lowest priority will become the
redundant backup line. If you change the end of the line at the port where you
changed the priority (for example from port 7 to port 5) the line will become a
redundant backbone again.
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