Multiple Trunks from Core to Access Switches without Loop

[matthewbutt]

Hi there,

I hope this finds you well.

We have a Cisco 3750G Core switch which has physical connections, each configured as trunks to two HP Access switches. The client who uses these access switches would like to put a link between the two, but this would create a loop. I was wondering what is the best way to achieve this?

           ------- Core Switch -------

           |                                          |

           |                                |

        |                                |

Access Switch -------- Access Switch

                                ^

                         New Link

I hope the above makes sense, and thanks in advance for your time on this. As always if you have any questions of require any of the above clarifying then please don't hesitate to ask.

All the best,

Matthew

[NES IT]

Spanning tree will automatically shut one of the interfaces down, but it depends on what VLANs are running and where.

Can you tell me what VLANs are running between the three switches?

Dan

[NES IT]

also, the only benefit of placing a link between the two would be for redundancy (in case primary link fails). However, this would always be done by STP.

Is that their reason for adding the link?

[Giuseppe Larosa]

Hello Daniel,

yes it it is likely for redundancy reasons

Hope to help

Giuseppe

[NES IT]

yes, then there are few options on how to do this. I do not know if PVST will be needed as not sure how many VLANs, but hopefully Matt will reply soon.

[NES IT]

I believe you will need MST on the switches, as HP does not support PVST. Therefore, you will have one instance of STP per VLAN. This way, you can send traffic one way for one VLAN etc. This will need configuring on the Cisco and the HPs and you will need to tweak the priorities so that one of the interfaces on the new link is blocking.

Also, the core switch will need to be the root bridge for both VLANs.

Dan

[matthewbutt]

Hi Dan,

Thanks for getting back to me.

There are two vLAN's 305 and 104 running on the core (L3) which are specified to go down each trunk to the HP Access switches (L2).

All the best,

Matthew

[NES IT]

so one VLAN to one switch, one to another? Will you need both VLANs across the new link?

Dan

[matthewbutt]

Hi Dan,

Apologies, both vLAN's to both switches.

All the best,

Matthew

[NES IT]

Ok, so changing the Spanning Tree mode to MST on the Cisco, enabling STP on the HPs, then issuing a Spanning Tree Root command on the Cisco should do the trick.

Remember, one of the interfaces on the new link will not forward traffic.

Dan

[matthewbutt]

Hi Dan,

Apologies, I should have made the requirements clearer.

We would like one of the core switch trunks to be active, and the other standby with the link between the two HP's always connected.

This means one of the uplinks or one of the HP's can fail without taking out the network for this client.

I hope that makes sense,

Matthew

[NES IT · Accepted solution]

Ok, then you need to tweak your priorities so that the secondary uplink to the Cisco is non favoured in STP. That will allow for STP to make the primary uplink and the new link to be forwarding.

Dan

[Giuseppe Larosa]

Hello Matthew,

spanning tree protocol manages L2 redundancy and you need it.

Being a multi vendor context some care is required to verify what types of STP are supported on the HP access switches.

Cisco switch runs PVST by default, it can run Rapid PVST or MST 802.1s. All of these run multiple instances of STP, the first two modes one instance per Vlan the last one instance per set of Vlans.

The HP access layer switch may be able to run 802.1s  and they may be running mono instance rapid STP 802.1W as default.

MST 802.1s would be the better choice but it may be overkilling.

Interaction should happen in vlan 1 between PVST or Rapid PVST with the mono instance STP on access switches

see attached cookbook for HP cisco interoperability

Hope to help

Giuseppe