This topic will cover configuring, verifying and troubleshooting the rapid spanning tree protocol.
To enable rapid spanning tree protocol on our three switches:
Switch A:
configure terminal spanning-tree mode rapid-pvst
Switch B:
configure terminal spanning-tree mode rapid-pvst
Switch C:
configure terminal spanning-tree mode rapid-pvst
We will also enable portfast on our access ports (PCs connected to the switches) – This will enable the ports to go straight to a forwarding state meaning the ports will instantly come up. You do not want to enable this on trunk links, this may cause issues with switching loops.
Switch A:
configure terminal interface fa0/3 spanning-tree portfast
Switch B:
configure terminal interface fa0/3 spanning-tree portfast
You will get a warning message about enabling portfast on trunk links.
%Warning: portfast should only be enabled on ports connected to a single host. Connecting hubs, concentrators, switches, bridges, etc... to this interface when portfast is enabled, can cause temporary bridging loops. Use with CAUTION %Portfast has been configured on FastEthernet0/3 but will only have effect when the interface is in a non-trunking mode.
That’s all that’s needed to enabled rapid spanning-tree and portfast. Lets go one step further and lets force switch C become the root bridge for VLAN 1. Currently Switch B is the root.
We can do this two ways, one to set the priority of VLAN 1 to a much lower value for example 4096 or force the switch as the root.
Set the priority lower on Switch C:
configure terminal spanning-tree vlan 1 priority 4096
Force switch as root:
configure terminal spanning-tree vlan 1 root primary
I have gone with changing the priority to 4096, The above will only change the root bridge for VLAN 1.
Now if we do a show spanning-tree vlan 1 on Switch C:
The above shows a show spanning-tree output from Switch C. We can see the priority address has been changed to 4096+VLAN 1 (4097) and the bridge is the root for VLAN 1.
Verifying:
To verify RSTP has been configured correctly we can go through the configuration by using the show running-config command and verifying spanning-tree mode rapid-pvst has been entered on all three switches.
The next command we can verify RSTP with is show spanning-tree, this will show what type of spanning-tree is enabled and root bridge/interface statuses.
Show spanning-tree summary can be used to give a quick indication of what mode spanning tree is configured for and some other useful information such as what VLANs are taken part in spanning-tree.
Troubleshooting:
- Ensure all switches are configured for rapid-spanning tree – rapid-spanning tree is backwards compatible meaning it will match the normal spanning-tree timers and can cause slowness
- Check cables between switches are correct – Crossover cable
- Check you haven’t made a trunk link portfast by mistake
- Using the verify commands above can greatly help with understanding why RSTP isn’t work (root bridge incorrect, port blocked, rapid-spanning tree not configured)