parameter to the maximum of 40 seconds. A physical
port on a router or switch may be part of more than one
spanning tree if it is a trunk. Note: VTP runs on
Catalyst switches not routers. It is advisable to configure a
Catalyst switch neighboring a router to operate in VTP
transparent mode until Cisco supports VTP on its routers. The
Spanning-Tree Protocol (STP) is considered one of the most
important Layer 2 protocols on the Catalyst switches. By
preventing logical loops in a bridged network, STP allows Layer
2 redundancy without generating broadcast storms. Minimize
spanning-tree problems by actively developing a baseline study
of the network. Web Links VLAN Tech Notes
http://www.cisco.com/en/US/tech/
tk389/tk689/tech_tech_notes_list.html
Content 8.3
Troubleshooting VLANs 8.3.4 Troubleshooting
VLANs The show and debug commands can be
extremely useful when troubleshooting VLANs. Figure illustrates
the most common problems found when troubleshooting VLANs. To
troubleshoot the operation of Fast Ethernet router connections
to switches, it is necessary to make sure that the router
interface configuration is complete and correct. Verify that an
IP address is not configured on the Fast Ethernet interface. IP
addresses are configured on each subinterface of a VLAN
connection. Verify that the duplex configuration on the router
matches that on the appropriate port/interface on the switch.
The show vlan command displays the VLAN information on
the switch. Figure , displays the output from the show
vlan command. The display shows the VLAN ID, name, status,
and assigned ports. The CatOS show vlan keyword options
and keyword syntax descriptions of each field are also shown.
The show vlan displays information about that VLAN on
the router. The show vlan command followed by the VLAN
number displays specific information about that VLAN on the
router. Output from the command includes the VLAN ID, router
subinterface, and protocol information. The show
spanning-tree command displays the spanning-tree topology
known to the router. This command will show the STP settings
used by the router for a spanning-tree bridge in the router and
switch network. The first part of the show
spanning-tree output lists global spanning tree
configuration parameters, followed by those that are specific
to given interfaces. Bridge Group 1 is executing the IEEE
compatible Spanning-Tree Protocol. The following lines of
output show the current operating parameters of the spanning
tree: Bridge Identifier has priority 32768, address
0008.e32e.e600
Configured hello time 2, Max age 20,
forward delay 15 The following line of output shows that the
router is the root of the spanning tree: We are the root of the
spanning tree. Key information from the show
spanning-tree command creates a map of the STP network. The
debug sw-vlan packets command displays general
information about VLAN packets received but not configured to
support the router. VLAN packets that the router is configured
to route or switch are counted and indicated when using the
show sw-vlan command.
Content 8.3
Troubleshooting VLANs 8.3.5 VLAN troubleshooting
scenarios Proficiency at troubleshooting switched networks will
be achieved after the techniques are learned and are adapted to
the company needs. Experience is the best way of improving
troubleshooting skills.Three practical VLAN troubleshooting
scenarios referring to the most common problems will be
described. Each of these scenarios contains an analysis of the
problem to then solving the problem. Using appropriate specific
commands and gathering meaningful information from the outputs,
the progression of the troubleshooting process can be
completed. Scenario 1: A trunk link cannot be
established between a switch and a router. When having
difficulty with a trunk connection between a switch and a
router, be sure to consider the following possible causes:
- Make sure that the port is connected and not receiving
any physical-layer, alignment or frame-check-sequence (FCS)
errors. This can be done with the show interface
command on the switch.
- Verify that the duplex and
speed are set properly between the switch and the router. This
can be done with the show int status command on the
switch or the show interface command on the router.
- Configure the physical router interface with one
subinterface for each VLAN that will route traffic. Verify this
with the show interface IOS command. Also, make sure
that each subinterface on the router has the proper
encapsulation type, VLAN number, IP address, and subnet mask
configured. This can be done with the show interface or
show running-config IOS commands.
- Confirm that
the router is running an IOS release that supports trunking.
This can be verified with the show version command.
Scenario 2: VTP is not correctly propagating
VLAN configuration changes. When VTP is not correctly affecting
configuration updates on other switches in the VTP domain,
check the following possible causes: - Make sure the
switches are connected through trunk links. VTP updates are
exchanged only over trunk links. This can be verified with the
show int status command.
- Make sure the VTP
domain name is the same on all switches that need to
communicate with each other. VTP updates are exchanged only
between switches in the same VTP domain. This scenario is one
of the most common VTP problems. It can be verified with the
show vtp status command on the participating switches.
- Check the VTP mode of the switch. If the switch is in
VTP transparent mode, it will not update its VLAN configuration
dynamically. Only switches in VTP server or VTP client mode
update their VLAN configuration based on VTP updates from other
switches. Again, use the show vtp status command to
verify this.
- If using VTP passwords, the same
password must be configured on all switches in the VTP domain.
To clear an existing VTP password, use the no vtp password
password command on the VLAN mode.
Scenario 3: Dropped packets and loops. Spanning-tree
bridges use topology change notification Bridge Protocol Data
Unit packets (BPDUs) to notify other bridges of a change in the
spanning-tree topology of the network. The bridge with the
lowest identifier in the network becomes the root. Bridges send
these BPDUs any time a port makes a transition to or from a
forwarding state, as long as there are other ports in the same
bridge group. These BPDUs migrate toward the root bridge. There
can be only one root bridge per bridged network. An election
process determines the root bridge. The root determines values
for configuration messages, in the BPDUs, and then sets the
timers for the other bridges. Other designated bridges
determine the shortest path to the root bridge and are
responsible for advertising BPDUs to other bridges through
designated ports. A bridge should have ports in the blocking
state if there is a physical loop. Problems can arise for
internetworks in which both IEEE and DEC spanning-tree
algorithms are used by bridging nodes. These problems are
caused by differences in the way the bridging nodes handle
spanning tree BPDU packets, or hello packets, and in the way
they handle data. In this scenario, Switch A, Switch B, and
Switch C are running the IEEE spanning-tree algorithm. Switch D
is inadvertently configured to use the DEC spanning-tree
algorithm. Switch A claims to be the IEEE root and Switch D
claims to be the DEC root. Switch B and Switch C propagate root
information on all interfaces for IEEE spanning tree. However,
Switch D drops IEEE spanning-tree information. Similarly, the
other routers ignore Router D's claim to be root. The result is
that in none of the bridges believing there is a loop and when
a broadcast packet is sent on the network, a broadcast storm
results over the entire internetwork. This broadcast storm will