BCMSN Notes — STP States
I’ve decided to take on the CCNP certification, so I’m going to wind up with a few posts will be more my own notes than anything. 
A switch port on a 2960 comes up with a default configuration on VLAN 1. What happens from the perspective of spanning-tree?
- First, the port comes up on blocking mode. This is to make sure that loops aren’t created without first listening to the network to see what’s going on.
- Next, if the port may be a root or designated port, the port is moved to the listening state. In this state, the port can send and receives BPDUs only. It can’t send traffic, but it can discover the other switches participating in STP.
- After the forwarding delay, the port goes into the learning state. In this state, the port can send and receive BPDUs as in listening, but it can now receive traffic. It can’t yet send any.
- After the forwarding delay again, the port goes into the forwarding state. The port can now send and receive data.
If the port is configured with spanning-tree portfast, the mode goes from blocking directly to forwarding without going through these steps. Obviously you don’t want a switch plugged into a port configured for portfast since you may wind up with a loop.
Here’s the debug spanning-tree events output from one of my labs. F0/3 is configured for portfast. I shut/no shut it to see what happens.
*Mar 8 18:09:51.163: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/3, changed state to down sw01# *Mar 8 18:09:51.747: set portid: VLAN0007 Fa0/3: new port id 8003 *Mar 8 18:09:51.747: STP: VLAN0007 Fa0/3 ->jump to forwarding from blocking sw01# *Mar 8 18:09:53.739: %LINK-3-UPDOWN: Interface FastEthernet0/3, changed state to up *Mar 8 18:09:54.739: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/3, changed state to up
Notice the “jump to forwarding from blocking”.
Here’s the same output when the port is not in portfast mode. Notice the timestamps. It takes about 30 seconds (2 x default foward delay) to go from blocking to listening to learning to forwarding.
*Mar 8 18:13:05.313: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/3, changed state to down sw01# *Mar 8 18:13:06.013: set portid: VLAN0007 Fa0/3: new port id 8003 *Mar 8 18:13:06.013: STP: VLAN0007 Fa0/3 -> listening sw01# *Mar 8 18:13:06.381: %LINK-3-UPDOWN: Interface FastEthernet0/3, changed state to up *Mar 8 18:13:07.381: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/3, changed state to up sw01# *Mar 8 18:13:21.013: STP: VLAN0007 Fa0/3 -> learning sw01# *Mar 8 18:13:36.013: STP: VLAN0007 Fa0/3 -> forwarding
Send any obvious corrections and questions my way.
about 9 months ago
Please allow a fellow CCNP (BCMSN) seeker to vent about STP.
STP needs to die. 6500 Multi-chassis Etherchannel (VSS) and Cross-stack etherchannel on the 3750s will probably replace STP-based designed completely. This being said, I’m sure they’ll keep STP in the cert tracks for years and years because the consultants will need it when they work on out-dated networks. Why would anyone want to use an over-complicated technology that actually blocks one of the uplinks?! Etherchannel or L3 at the access layer are technologies that actually allow the use of *both* uplinks.
PS What out for weird wireless questions on the BCMSN test. I got like four or five questions regarding the LEDs and GUI of the relatively obscure Cisco PCMCIA wireless card (doesn’t everyone use integrated wireless now?!). The thing that really burned me was that the Official Exam Certification Guide didn’t cover this product.
about 9 months ago
I agree with you, Iain. STP is an old design of an old technology that needs to go. Like you said, the VSS or StackWise technologies are vastly superior in that they don’t require STP to run among the cluster of switches. Is it not possible to run the same stacking technology on an Ethernet port (within reason)?
Since these are pretty new, though, you’re right again to say that STP will be on the test for years and years.
Thanks for posting and keep contributing!
about 9 months ago
nice explanation.
regards
shivlu jain
about 3 months ago
Nice post. To add to it, it might be helpful to insert the timing events as I’ve seen that on tests before – Blocking (20s), Listening (15s), Learning (15s) and BPDUs every 2s.