• Skip to content
  • Skip to footer

Packet6

San Francisco Bay Area Wi-Fi Professional Services

  • About Us
  • Blog
  • Services
  • Case Studies
  • Contact Us

Alternate Port Role Shows in STP

January 5, 2014 by Rowell Dionicio Leave a Comment

I have four physical switches I use for studying the CCNP SWITCH. While labbing spanning-tree protocol, I noticed an RSTP particular port role in IEEE spanning-tree mode. Note that this is not RSTP. My switches are running in 802.1D, STP.

DLS2#show spanning-tree

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    24577
             Address     000d.bdd3.4e80
             Cost        19
             Port        2 (FastEthernet0/2)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     0011.bba6.b000
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1               Altn BLK 19        128.1    P2p
Fa0/2               Root FWD 19        128.2    P2p
Fa0/47              Altn BLK 19        128.47   P2p
Fa0/48              Altn BLK 19        128.48   P2p

DLS2#sh run | i span
spanning-tree mode pvst
spanning-tree extend system-id

I have redundant links between my switches. I highlighted the text in bold. VLAN 1 is running IEEE STP and you can see F0/1, F0/47, and F0/48 in Altn port role. The switch command spanning-tree mode pvst is shown in the running-config.

The Alternate port role is actually found in RSTP, 802.1W, as you can see in this Cisco document.

I went out to the networking-forum to see if anyone else had this issue and was then lead to the Cisco Support forums.

A Cisco employee gave this response regarding this topic:

It is true that the port roles were introduced with RSTP. However, they definitely make sense for STP (for instance, the difference between an alternate and a backup port is useful to uplinkfast, and it’s also nice to see clearly what the root port is) so we decided to display the role in STP mode also (something we did not do for CatOS afaik).

Now, just for the sake of nitpicking, STP has been in fact deprecated when 802.1w amended 802.1D. An 802.1D switch is an RSTP switch! Running STP nowadays should be in fact running RSTP in STP compatibility mode. And in that case, the switch would definitely display the RSTP roles.

I think that PVST is a sufficient deviation from 802.1D that it makes irrelevant arguing about whether the display of the RSTP role impact the compliance to the standard, so I definitely don’t want to insist on that;-)

Regards,

Francois

So there you have it. TL;DR: IEEE STP on Cisco switches really runs RSTP in STP compatibility mode.

Related

Filed Under: Certification Tagged With: ccnp switch, port role, spanning-tree

About Rowell Dionicio

I am Rowell Dionicio, expert in helping companies improve Wi-Fi performance, reliability, and efficiency. Follow me on Twitter, LinkedIn, Facebook and YouTube.

Footer

LET’S TALK

Are you ready to improve your wireless network?

WE'RE LISTENING

© Copyright 2019 Packet6 · All Rights Reserved · Privacy Policy · Terms of Use