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

Basic STP
Objective:
To learn how to construct and understand Spanning Tree Protocol (STP) connections, to
view and understand spanning tree states with a protocol inspector, and to construct and
configure redundant backbones between switches.
Tools and Materials:
Three (3) cross-over cables
Three CISCO switches (1900 series)
Two (2) straight-through cables
Two Windows PC workstations with Hyperterminal and Ethereal installed
Lab Diagram:
1 bx ax bx ax bx ax 1
st xo xo st
NIC
xo
workstation “A”
workstation “B”
Background:
The main function of the Spanning-Tree Protocol (STP) is to allow us to set up
redundant back up lines in case of emergency between switches. When a main line
between two of the switches becomes dysfunctional the switch, through its STP states
(Blocking, Listening, Learning, Forwarding, Disabled), implements the Spanning Tree
Algorithm (STA) when a “link down” is detected. By default the switch checks the
condition of its ports every 30 seconds. In other words, when a main line goes down, the
redundant backbone should come up within 30 seconds (although sometimes it takes up
to about 60 seconds with default settings). STP is implemented on switches, by default,
for VLANs 1-64. This means all you have to do is plug in your redundant backbone (a
cross over cable) into any available port between switches because all switches in their
default state have all ports assigned to VLAN 1.
The switch uses priorities to determine which lines are the main lines and which
are the redundant backbones. The values can be 0 through 255. The lower number has
the higher priority (the main lines). By default each 10BaseT port is assigned a priority
of 128 and each 100BaseT port is assigned a priority of 10. On our 1900 series switches
this means that the Ax and Bx ports will be selected as main backup lines before ones
using the numbered (1-12 or 1-24) ports. In practice, we use the Ax and Bx lines to set
our “Trunks” or backbone lines. Since the Ax and Bx lines are typically used for high
speed this works best. In the next lab you will be configuring the backbone lines by
changing the settings (cost, priority, etc) on each port to determine statically which will
be the main backbones and which will be the redundant backbones.
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