The first part of troubleshooting Spanning Tree Protocol (STP) is to gather additional information about the running protocol. Our good buddy show will get you started. If you have read other chapters on protocols, you will know that the show command has a great deal of information to display, and STP is no different.
Here is a list of the show options available to STP on Cisco switches:
Switch2> enable
Switch2#show spanning-tree ?
active Report on active interfaces only
backbonefast Show spanning tree backbonefast status
blockedports Show blocked ports
bridge Status and configuration of this bridge
detail Detailed information
inconsistentports Show inconsistent ports
interface Spanning Tree interface status and configuration
mst Multiple spanning trees
pathcost Show Spanning pathcost options
root Status and configuration of the root bridge
summary Summary of port states
uplinkfast Show spanning tree uplinkfast status
vlan VLAN Switch Spanning Trees
| Output modifiers
<cr>
Because PVST is the default STP version, it also includes statistical information about the VLANs for which STP is running.
Switch2> enable
Switch2#show spanning-tree summary
Switch is in pvst mode
Root bridge for: none
EtherChannel misconfig guard is enabled
Extended system ID is enabled
Portfast Default is disabled
PortFast BPDU Guard Default is disabled
Portfast BPDU Filter Default is disabled
Loopguard Default is disabled
UplinkFast is disabled
BackboneFast is disabled
Pathcost method used is short
Name Blocking Listening Learning Forwarding STP Active
---------------------- -------- --------- -------- ---------- ----------
VLAN0001 1 0 0 2 3
VLAN0002 1 0 0 1 2
VLAN0005 1 0 0 1 2
VLAN0010 1 0 0 1 2
VLAN0015 1 0 0 1 2
VLAN0020 1 0 0 1 2
---------------------- -------- --------- -------- ---------- ----------
6 vlans 6 0 0 7 13
Switch2> enable
Switch2#show spanning-tree root
Root Hello Max Fwd
Vlan Root ID Cost Time Age Dly Root Port
---------------- -------------------- ------ ----- --- --- ----------------
VLAN0001 32769 0006.d6ab.a040 19 2 20 15 Fa0/2
VLAN0002 32770 0006.d6ab.a040 19 2 20 15 Fa0/2
VLAN0005 32773 0006.d6ab.a040 19 2 20 15 Fa0/2
VLAN0010 32778 0006.d6ab.a040 19 2 20 15 Fa0/2
VLAN0015 32783 0006.d6ab.a040 19 2 20 15 Fa0/2
VLAN0020 32788 0006.d6ab.a040 19 2 20 15 Fa0/2
This output shows much more information. To abridge the output, I show only the STP information for VLAN1. Here are some of the major pieces of information in this output:
Bridge address: The MAC address of the current switch
Address of the Root Bridge: The MAC address of the current Root Bridge
Delays and forwarding times: The current configured values of the forwarding and delay timers
Port status for that VLAN: Status of the ports configured in the displayed VLAN
In the following output of show spanning-tree from Switch1, note that all listed ports are in a Forwarding (FWD) state. Currently, port 11 and port 12 are connected to Switch2, creating a loop.
Switch1>enable
Switch1#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0006.d6ab.a040
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0006.d6ab.a040
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1 Desg FWD 19 128.1 P2p
Fa0/2 Desg FWD 100 128.2 Shr
Fa0/3 Desg FWD 19 128.3 P2p
Fa0/11 Desg FWD 19 128.11 P2p
Fa0/12 Desg FWD 19 128.12 P2p
In the following output, you see what the ports look like on Switch2.
Switch2>enable
Switch2#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0006.d6ab.a040
Cost 19
Port 11 (FastEthernet0/11)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0006.d6ac.46c0
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 15
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/2 Desg FWD 19 128.2 P2p
Fa0/11 Root FWD 19 128.11 P2p
Fa0/12 Altn BLK 19 128.12 P2p
If you examine the state of the ports, you see almost the same output as Switch1, with a few exceptions. Notice that Switch2 knows that it is not the Root Bridge because the Root ID and Bridge ID on this switch do not match. Port Fa0/12 is in a Blocking (BLK) state, and a Priority value is defaulting to 32769.
If you want to force a switch to be the Root Bridge or not be the Root Bridge, you can change this value. A higher value will guarantee that you are not the Root Bridge, while a lower value will ensure that you are the Root Bridge.
The lowest value achieved by adding the priority MAC address will get to the Root Bridge. Therefore, if you change this value, you influence the Root Bridge assignment.
If you have a few core switches that never have a problem with rebooting, adjust the priority so that one of these core switches is your Root Bridge.
Avoid having an edge switch on the far side of a LAN extension as the Root Bridge. An edge switch regularly loses its connection or gets rebooted. When these occur, the entire topology is rebuilt.
dummies
Source:http://www.dummies.com/how-to/content/troubleshooting-spanning-trunk-protocol-stp.html
No comments:
Post a Comment