Spanning tree

[harmanrms]

Hello Tutor,

Could you please help us out with these 2 queries.

1)As per Cisco documentation, we know that PVST and PVST+ are 2 separate spanning-tree modes.

While configuring the 4507 Switch;having IOS ver 12.1(20)EW1 we see that we can configure spanning-tree with 3 modes

a)MST b)PVST c) R-PVST .

Now does this PVST work similar to a PVST+ under this IOS ver and later, or does Cisco recommend a separate configuration for PVST+?

2)What is the default spanning-tree mode on a 4507 swith with IOS version 12.1 and later.

The reason for the above stated question is that for the same category of switches with IOS version of 12.2 and above, when we run the "sh run" command, we can see the type of spanning tree mode that is running.

But for the 4507 with an IOS version of 12.1(20)EW1, the "sh run" command does not show us the type of spanning-mode that is being run. We need to run the "sh spanning-tree summary" to see the mode.

The PVST mode is being run on the above stated switch.

Hoping to get a resolution for the same at the earliest.

Thank you,

Sonali

[globalnettech · Accepted solution]

Hello Sonali,

on a 4507 switch running IOS version 12.1(20)EW1 or later, when you enable 'spanning-tree mode pvst', you actually enable PVST+. The previous PVST mode is not available at all anymore, but is, as you correctly stated, fully compatible with PVST.

Keep in mind that PVST works only on ISL trunks, since an ISL trunk natively supports multiple spanning trees per vlan. PVST+ is a modification of PVST which allows per vlan spanning trees over standard 802.1q links.

Does that make sense ?

HTH,

GNT

[harmanrms]

yes ofcourse this had cleared my doubt..

But still one part of the question is remaining.

What is the default spanning-tree mode on a 4507 swith with IOS version 12.1 and later.

Is it PVST+ ?

The reason for the above stated question is that for the same category of switches with IOS version of 12.2 and above, when we run the "sh run" command, we can see the type of spanning tree mode that is running. ie it shows in the runn config that "spanning tree mode is pvst"

But for the 4507 with an IOS version of 12.1(20)EW1, the "sh run" command does not show us the type of spanning-mode that is being run. We need to run the "sh spanning-tree summary" to see the mode.

The PVST mode is being run on the above stated switch.

Hoping to get a resolution for the same at the earliest.

Thanks,

Sonali

[Roberto Salazar]

All Cisco Switches that supports 802.1Q runs PVST+ by default since Per VLAN Spanning Tree Plus (PVST+) provides the same functionality as PVST using 802.1Q trunking technology rather than ISL. PVST+ is an enhancement to the 802.1Q specification. PVST+ interoperates with 802.1Q mono Spanning Tree via the so-called Common Spanning Tree (CST) over an 802.1Q trunk. The CST is always on VLAN 1, so this VLAN needs to be enabled on the trunk to interoperate with other vendors. CST BPDUs are transmitted, always untagged, to the IEEE Standard Bridge-Group (MAC Address 01-80-c2-00-00-00, DSAP 42, SSAP 42). For completeness of description, a parallel set of BPDUs are also transmitted to the Cisco shared Spanning Tree MAC address for VLAN 1. Now, some older switch did nto support 802.1Q and only supported ISL naturally these switches will not supports PVST+ since there is no need. In newer switches there is no PVST anymore, so what you are seeing is PVST+, remember PVST+ is an extension of PVST to support 802.1Q, so if the switches you are seeing "PVST" supports 802.1Q it would not make sense that it's not PVST+. I could not find any documentation on this evolution from PVST to PVST+ and when Cisco decided to just simply put "PVST",In any case for 4507 or any switches that supports 802.1Q, PVST+ is the default mode which is compatible with PVST.

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