Issues with RSTP and Rapid PVST (CISCO and DELL hardware)

[Zeeshan Siddiqui]

Can you please help with the following

we are adding few Powerconnect 8024F Dell switch on our Network.

The problem we have is we are running RSTP  on Dell switches and are using RSTP PVST on cisco 3750 stacks that directly connects to the del switches when we connect the two using trunk ports ideally it should recognise the cisco switch as the root as it has lower spanning tree priority.

What we see on the DEL switch is

Regional Root Path Cost:  0

ROOT ID

              Priority        32768

              Address         5C26.0AD3.8D06

              This Switch is the Root.

              Hello Time 2 Sec Max Age 20 sec Forward Delay 15 sec TxHoldCount 6

What we see on the cisco switch is

KB-SR1-6509-1#sh spanning-tree vlan 185

VLAN0185

  Spanning tree enabled protocol rstp

  Root ID    Priority    8192

             Address     0011.bc7c.08b9

             Cost        3

             Port        1669 (Port-channel10)

             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    16384

             Address     0011.bc6b.d0b9

             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

             Aging Time 300

Interface        Role Sts Cost      Prio.Nbr Type

---------------- ---- --- --------- -------- --------------------------------

Po8              Desg FWD 3         128.1667 P2p

Po10             Root FWD 3         128.1669 P2p

Po12             Desg FWD 3         128.1671 P2p

Vlan 185 exist both on the Del switch and cisco switch and is allowed over the trunk ports adn as you can see we have two Root bridge switches fro the same vlan (we only have a single link connected to the DELL switch)

Is there a risk with the above setup

Kind Regards,

Zeeshan Siddiqui

[krahmani323]

Hello Zeeshan,

Intersting conversation...Regarding your initial question, the reason why we saw the DELL switch to consider itself as ROOT RSTP is due to the differents format of the BPDUs sent by the Cisco switches (and also depending on the switchport configuration) and their interpretation …..

In fact in the (Rapid)-STP ([R]PVST+) Cisco switch sends two different "types" of BPDUs.

In your scenario =>

1/  Access VLAN ( or Native VLAN in case of a trunk) STP BPDUs are sent untagged to the IEEE STP MAC address (0180.c200.0000) -.

==> Standardized BPDU undestood and interpreted by the DELL switch (as the standard format).

2/ Tagged VLAN (vlan 185 ...) STP BPDUs are sent to the [R]PVST+ MAC address (also called the Shared Spanning Tree Protocol [SSTP] MAC address, 0100.0ccc.cccd), tagged with a corresponding IEEE 802.1Q VLAN tag.

==> Those BPDU are not understood by DELL as Spanning-tree BPDUs and as a result the DELL (non-Cisco switches in general) do not treat and interpret them (these BDPU are forwarded as mutlicast).

This is explaining why when you were in trunk mode with only vlan 185 tagged, the BPDUs were not understood by the DELL thus considering itself as ROOT [BPDUs sent in point 2].

I think also the "either tagged or untagged" port configuration mode in the DELL has a role to play in this context..

When in access mode the Cisco was sending the BPDU [described in point 1] the DELL undestood correctly the BPDU IEEE BPDU of the [native vlan] trunk going from the 6509 and updating its root bridge information .

There is a good link here explaining the different type of BPDU in regards to the different Cisco interface configurations (mac are the same between PVST/RPSTP) => http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00801d11a0.shtml#topic1

================

Anyway and as discussed througout the previous post => In a multiple-vendor environment the use of MSTP for interoperability is a good choice (note the pro&cons in your particular situation and test like you did has to be performed before deciding)...

Kind regards.

Karim

[alfredobosca]

Hello,

I have find out that interoperability between Cisco - Dell is possible configuring PVT on Dell side and rapid PVST on Cisco devices.

Best regards,

[Farhan Javed]

Well 2 answers:

1. You need to allow Vlan 1 on the trunk link to ensure RSTP works in both switches and whoever switch has the lowest priority on the vlan 1 will become the root-bridge and your problem will go away to have 2 root bridges.

2. even if you dont allow the vlan 1 in the trunk links, it wont impact anything since they will remain root-bridges in their respective vlan 1 and unless you are using any IP/L3 network on Vlan 1, there wont be any issue.

Hope that will clarify a little more.