pvst/rapid pvst port question

[Daniel Wyman]

Today a coworker and I were working on showing a few new guys some spanning tree info and we disagreed on if pvst/rpvst treats access ports ad trunk ports differently.  I know if you add commands like portfast or uplink fast that affects it but without any extra commands on the end access and trunk ports will spanning tree treat them differently? and if it does or doesn't how/why?

[Masoud Pourshabanian]

Hello,

in PVST, switches maintain one instance of STP for each VLAN allowed on the trunks. It means 1 BPDU is sent every two seconds for each VLAN. Since access ports are assigned to one VLAN, only 1 bpdu is sent every two seconds.

Access port which is assigned to one VLAN is always either in forwarding state or blocking state, while a trunk port is in blocking state for some VLANs and in forwarding state for others.

Hope helps,

Masoud

[Jon Marshall]

It depends on what is on the end of the trunk and access ports.

Without any extra configuration a switch cannot know what is on the other end of the link so it has to treat all links equally in that sense.

Whether the port is a trunk port or an access port tells the switch nothing about what is connected to the other end eg. it could be an access port connected to another switch or an end client, or it could be a trunk port connecting to another switch or a server, router etc.

As Masoud points out, a BPDU is transmitted per vlan so you will get tagged BPDUs being sent on a trunk link.

The more important thing though is whether BPDUs are received on that port or not because this affects how STP behaves in terms of working out a loop free topology.

Jon

[ed_fair]

I too have noticed unusual and unexpected behavior between Cisco PVST and non-Cisco MST equipment when a port is changed from access to trunk.  I believe (!) the Cisco sends BPDU's to 01:80:c2:00:00:00 when in access mode, but 01:00:0c:cc:cc:cd in trunk mode; the latter are not consumed by the non-Cisco gear but are instead forwarded (flooded) out all ports.   I never confirmed this due to not having access to a lab with sufficient hardware in it, and no packet capture available between the two.

[paul driver]

Hello

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@Daniel Wyman wrote:

 but without any extra commands on the end access and trunk ports will spanning tree treat them differently?

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 Depending on what stp mode is applied (ie:RSTP natively applies uplink/portfast) and if the edge ports are dynamic ( not administratively set to access) then both ports will go through listening learning/forwarding states,as such when they transition into the stp forwarding states topology changes will be incurred.

[Flavio Miranda]

Spanning-tree or its flavors consider only three thing: Bridge, Ports and links between Bridges. And act according to port state. 

The link below is from IEEE:

https://www.ieee802.org/1/files/public/docs2009/aq-seaman-merged-spanning-tree-protocols-0509.pdf 

This document explain STP in 'almost' coding level and you not going to see Trunk or Access in there.