Cisco 3750 RSTP and HP Procurve RSTP switch 2910al

[mbettolo]

Hi all,

Just a sanity check ...

HP 2910AL connected to Cisco 3750-A and Cisco 3750-B through 802.1q trunks (2 VLans : native VLAN 1 and tagged VLAN 2), this is a triangle :

Cisco 3750-A ----------- Cisco 3750-B

|                                               |

|                                               |

|                                               |

HP 2910AL--------------------------

All are configured to run RSTP with :

- 2 VLANS : native vlan 1 and tagged vlan 2

- Cisco 3750-A is set up to be the root for tagged Vlan 2

- HP 29210AL is set up to be the root for native Vlan 1 (so it will be the root for CST), and the 3750 will see the HP as the root for Vlan 1 as well

Question is : the link between HP 2910AL and Cisco 3750-B will be blocked or not ?

Thanks very much

[Reza Sharifi]

Yes, one link will be blocked by stp.

If the HP support LACP, you can stack the 3750s and then uplink the HP to both 3750s with a Portchannel using LACP.

this way, no link will be blocked.

HTH

[mbettolo]

Thanks Reza, but I made typo, my question was more about the link between the 2 Cisco 3750 : this link should not be blocked because the 3750-A is the root for VLAN 2 ... but what about the HP that is the root for CST ? Does this link between the two Cisco 3750 will be blocked or not ?

[Reza Sharifi]

I think, the simplest solution for this is to make the 3750s root and backup root for all vlans.  This way only the link from the hp to the backup root will be blocked and the link between the 3750s will not.

HTH

[Andras Dosztal]

Setting STP priorities will define which link will be blocked. By default, all of these switches' (including the HP) priority is 32768. IF you change them, the lowest priority becomes the root, the second lowest is the backup root. The link between the backup root and the 3rd switch will be blocked.

[mbettolo]

Ok, but can there be any issue with the fact that Cisco RSTP is per-VLAN based, whereas HP RSTP is for a set of VLAN (here, all links are trunks carrying VLAN 1 native and VLAN 2 tagged), so a trunk link could be blocked by the CST whereas VLAN 2 should not be blocked on that trunk link due to Cisco per-VLAN RSTP ? So my question is more about Cisco per-VLAN RSTP compatibilty with HP RSTP where the HP is root for CST and Cisco is root for a tagged VLAN

[mbettolo]

Reza, I agree with you, easiest way (other than moving to MSTP) is to have the same root for all VLANs, but just wondering what happening if this is not the case