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

For each port, there are five parameters that may be changed for each port. Each of these
affects which port connections are utilized as the main backbones and which are the
redundant backbones:
1. State—Blocking, Listening, Learning, Forwarding, Disabled
2. Forward Transitions—number of times STP changing forwarding states. This
number increases when STP detects network loops
3. Path Cost—inversely proportional to LAN speed; path costs range from 1 to
65,535—lower number means higher speed connection; default is 100.
4. Priority—ranges from 0 to 255 (used in basic lab); 10BaseT priority is 128;
100Bast T priority is 10
5. Port Fast Mode—using this will accelerate the time it takes to bring a port into the
forwarding state from blocking; Use Port Fast-Mode enabling on ports only for
end station attachments; default for 10BaseT is enabled; default for 100BaseT is
disabled; by default STP discovery is 30 seconds (don’t confuse this with BPDU’s
every 2 seconds)
With all switches reset to their factory defaults how do you think one backbone takes
priority over the others if we use all 100BaseT connections? If all costs are equal, then
the switch uses the MAC addresses to determine which ones will be the main and which
ones will be the backup (redundant) lines.
There are three steps involved in the Spanning Tree process: (1) Electing a root
bridge, (2) electing root ports, and (3) electing designated ports.
The root bridge is the bridge from which all other paths are decided. Only one
switch can be the root bridge. The selection process uses the lowest bridge priority
number first and then uses the lowest bridge ID number (the MAC address). The
switches use the BPDU’s to elect a root bridge. When a switch first powers up, it will
assume the role of root bridge until it is told otherwise. The default setting for CISCO
1900 series switches is 32768.
Next the switches will search for any redundant paths or loops using BPDU’s. An
election of main and backup paths is made using costs. By default, port cost is usually
based upon bandwidth (as we saw in the basic lab). The port with the lowest root path
cost will be elected as the root port/path. Any time a switch has a direct connection to the
root switch it will serve as the root port, regardless of path cost.
The designated port is the port that is advertising the lowest costs to the root
bridge. When all three steps are complete the Spanning Tree is finished being set up.
For this lab we will use private IP addressing with one subnet. You can use
mixed subnet addresses but only by activating more complicated settings on the switches
and/or using routers. Using different subnets will not allow you to ping with this
topology.
Step-By-Step Instructions:
You should set each switch back to its factory default settings. The power should be
turned off when you are finished re-setting.
Calculate and identify root bridge and main and redundant backbones:
1. Now then…this is a bit different than our three-switch configuration in the last lab.
In that lab no matter which line was disconnected, each line still had a direct
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