Cisco – LACP from Cisco 6509 to Force10 S48-10 – in default VLAN1

ciscoforce10ieee-802.1axspanning treevlan

A client has the following configuration:

The entire network is on VLAN1, with the subnet 172.16.0.0/16 – performing a redesign of their network is not an option at this point, as they're a 24/7 facility that cannot currently afford downtime (yeah, I know).

The client wishes to trunk two switches together:

1: Cisco 6509 running IOS 12.3

2: Dell F10 S48-10

The Cisco 6509 module 8 is equipped with 10gbps links – the client wants to use four of those links to trunk to the Force10. The problem that I have run into is that the Force10 does not allow tagged traffic on VLAN1, and spanning-tree pvst on the S48-10 rejects the untagged BPDU on VLAN1 from the 6509.

I've tried this configuration with the following settings – the LACP link works, but the Force10 rejects the BPDU from VLAN1 of the Cisco switch saying "Rejected untagged BPDU on VLAN1", and subsequently shuts the port down. Attempting to tag VLAN1 fails on the Force10 – that functionality is not allowed.

Force 10 Configuration

protocol spanning-tree pvst
no disable
!
!
interface range Te0/40 - 43
 description Port Channel 2 Member
 no ip address
!  
 port-channel-protocol LACP 
  port-channel 2 mode active 
 no shutdown
!

interface Port-channel 2
 description TenGig Uplink to Cisco 6509
 no ip address
 switchport
 no shutdown

Cisco 6509 configuration

!
spanning-tree mode pvst
!
interface range TenGigabitEthernet8/1 - 4
 switchport
 switchport mode trunk
 channel-protocol lacp
 channel-group 2 mode active
!
interface port-channel2
 no ip address
 switchport
 switchport trunk encapsulation dot1q
 switchport mode trunk 
 no shutdown
!

I feel that either I'm missing something very simple, or this may need to be fully converted off VLAN1 before the configuration will function. I was able to try every variant of this configuration that I could, except for forcing "trunk native vlan 1" on the Cisco side. I have not been able to obtain equipment to sandbox this configuration yet. 🙁

Thoughts?

Best Answer

Configure your Force10 with portmode hybrid and no spanning-tree pvst err-disable cause invalid-pvst-bpdu on the interface which faces the Cisco 6500... Reference: Force10 - Cisco STP Interoperability paper.

This is the complete configuration for your Port-channel (however, see the warning below)

interface Port-channel 2
 portmode hybrid
 switchport
 no spanning-tree pvst err-disable cause invalid-pvst-bpdu
!

As described in the aforementioned white paper, the Force10 "Hybrid Mode" is required for an untagged Vlan1 on the trunk port. Note that hybrid mode accepts both tagged and untagged frames; however, it will err-disable when it sees the PVST+ BPDU from the Catalyst 6500. Thus, I recommended that you also use no spanning-tree pvst err-disable cause invalid-pvst-bpdu.

Warning:

According to at least one blog I found, hybrid mode must be configured before any switchport command; this means you might need to unbundle Port-channel 2 and add it again with portmode hybrid. Sadly, I can't confirm this myself since I don't have a Force10.

Also note that the Force10 needs manual configuration for each Vlan which gets tagged; I'm assuming you've already tagged your Force10 Vlans.

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