MSTP vs RPVST

[harisapkota123]

Hi,

I have a network as shown below. Where, Sw1 is running R-PVST and other switches Sw2,Sw3&Sw4 are aunning MSTP. Sw2 is the IST and CIST root.

The issue is, whenever I try to ping from Sw1 to the switches running MSTP, I can't ping without allowing VLAN 1 on Sw1's trunk port. Am I missing something here? Or, i'm lacking concept behind compatibility of RSTP & MSTP?

Additional info: NATIVE VLAN IS 1

                         MANAGEMENT VLAN IS 300

Here, Management VLAN is allowed on the trunks but not VLAN 1.

Sw1----Sw2-----Sw3

               -          -

                    .    .

                     Sw4

Thanks!

[Kevin Dorrell]

What model are the switches? There are some switches that misbehave if you trim the native VLAN and/or VLAN 1 from the trunk.

Kevin Dorrell

Luxembourg

[harisapkota123]

The switches I have are Cisco2950 & Cisco2970.

IOS Version is :

c2950-i6k2l2q4-mz.121-22.EA12

c2970-lanbasek9-mz.122-44.SE6

Thanks!

[JohnTylerPearce]

MSTP uses VLAN1(Native VLAN) to communicate outside of it's region. Since, you have VLAN1 off of that trunk, this is going to cause some problems. I would make sure vlan1 is active on all your trunks.

[harisapkota123]

Hi,

Thanks for the reply!

Actually, I have tested it again and found an intersting thing.

I had read somewhere that if any MST region receives superior BPDU from the PVST, the port goes into Root Inconsistent mode. But, the intersting part is, if I allow VLAN 1 on the trunk, No root inconsistent is seen and it just pings other end too.

Struggling a lot to understand relation of MST with VLAN 1.

Any thoughts?

[JohnTylerPearce]

With MST communicates with other regions(other than its own locally configured region), PVST+ regionns, or RSTP region, on VLAN1 using Native VLAN1 (Common Spanning Tree). This is because on the BPDU sent from MST, it includes a hash of the vlan-to-instance mapping table for each instance configured, whihc is included in its own M-record on this BPDU using VLAN 1.

I'm not sure about a Root Inconsistent mode though.

Now, Cisco will send out "two" BPDUs on native VLAN1. One on(0180.c200.0000 IEEE Standard BPDU), and one on (0100.0ccc.cccd Cisco Proprietary PVST+ BPD). But this only happens on trunk ports (802.1q). If this is an access port it will always use IEEE standard BPDU (0180.c200.0000).

So if you have multiple vlans, for instance, vlan 10, vlan 20, the MST switch will not undestand these BPDUs, since by Cisco switches ALWAYS send out BPDUs for all vlans other than vlan1 (as 0100.0ccc.cccd Cisco PVST+ BPDU).

Now, there is backward compatibility between these two.

I know that if a MST switch is connected to PVST+, the MST port that is connected to the PVST+ switch will detect this, and after a few seconds around 4 I think ( could be wrong ) If it detects a PVST+ BPDU it will turn the port from MST to PVST+ mode.