Spanning tree interoperability between HP and Cisco switches

Introduction

This appendix explains and provides step-by-step configuration instructions for implementing multiple instance spanning-tree protocol (MSTP) and virtual router redundancy protocol (VRRP) on specific HP and Cisco Catalyst switches. By combining both MSTP and VRRP you create a highly available network with layer 2 and layer 3 redundancies and enables load-balance network traffic, optimizing network performance. All HP and Cisco switches are configured for MSTP. VRRP is configured on the HP 8200zl switches in the core.

Spanning Tree Protocol (STP) is a link management protocol that provides path redundancy while preventing undesirable loops in the network. For an ethernet network to function properly, only one active path can exist between two devices. Multiple Spanning Tree Protocol (MSTP) extends STP and the Rapid Spanning Tree Protocol (RSTP) and is backwards compatible with both versions.

With MSTP, each spanning tree instance can include one or more VLANs. It applies a separate, per-instance forwarding topology which uses RSTP for rapid convergence. When a port belongs to multiple VLANs, it may be blocked in one spanning tree instance, but forwarding in another instance. This provides multiple forwarding paths for data traffic and achieves load-balancing across the network. In contrast to PVST, MSTP reduces the switch’s CPU load to a moderate level by aggregating multiple VLANs in a single spanning tree instance. MSTP provides fast convergence in the case of a switch, port, or link failure.

The Virtual Router Redundancy Protocol (VRRP) is designed to eliminate the single point of failure inherent in an environment that uses statically configured default routes. VRRP specifies an election protocol that dynamically assigns responsibility for a virtual router to one of the VRRP routers on a LAN. The VRRP router controlling the IP addresses associated with a virtual router is called the Master, and forwards packets sent to these IP addresses. The election process provides dynamic fail-over in the forwarding responsibility if the Master become unavailable. Any of the virtual router's IP addresses on a LAN can then be used as the default first hop router by end-hosts. The advantage gained from using VRRP is a higher availability default path without requiring configuration of dynamic routing or router discovery protocols on every end-host.

When configured properly, this solution ensures that all redundant links are used and that the VRRP routers can share default gateway duties. Combining MSTP and VRRP protocols into your network design achieves a balanced, redundant, and highly available network solution.

HP to Cisco terminology

Description Cisco Switches HP Switches
A port that belongs to a unique VLAN and is untagged. access mode untagged
A port that carries multiple VLANs using 802.1q tags. trunk mode tagged
Aggregated links to support additional bandwidth and link redundancy. etherchannel or channel-group trunk-group
Configuration item Cisco Switches HP Switches
untagged VLAN traffic switchport mode access switchport access VLAN untagged
tagged VLAN traffic switchport trunk encapsulationswitchport mode trunkswitchport allowed VLAN tagged
aggregated links channel-group trunk (trk)

Path cost parameter values

Port Type RSTP and MSTP Path Cost

10Mbps

2,000,000

100 Mbps

200,000

1Gbps

20,000

10Gbps

2,000

Cisco switches reduce the path costs on aggregated links. For example on the 3750 switch, the path cost on an etherchannel group with two gigabit links is 10,000. One gigabit link is 20,000. HP Switches do not reduce path cost on aggregated links.

Equipment and software versions

Tables 37 and 38 list equipment and specified software version for each switch in this scenario. It’s a good practice to download the latest software for each switch that will participate in MSTP.

HP equipment S/W version

HP Equipment S/W Version
HP Switch 8200zl K.12.30 or greater
HP Switch 5400zl K.12.02 or greater
HP Switch 4202vl-48G L.10.24 or greater
HP Switch 2900-48G T.12.03 or greater
HP Switch 2810-48G N.10.09 or greater
HP Switch 3400cl-48G M.10.30 or greater

Cisco equipment S/W version

Cisco Equipment S/W Version
3550 IOS 12.2(25)SED
3750 IOS 12.2(25)SEC2

Configuration checklist

  • Upgrade all switches to the latest software version.

  • Ensure that the MST configuration on all switches match exactly. The name, revision, and instance VLAN mappings must be identical on all switches participating in MSTP.

  • Configure all trunks on Cisco switches as 802.1Q (dot1q) trunks.

  • Be sure to configure all edge ports. If you don’t, you will see a lot of topology changes. In this configuration no edge ports are configured.

  • All backbone ports (switch to switch links) must be members of all VLANs to insure your network supports all of the forwarding paths necessary for the desired connectivity. All ports connecting one switch to another within a region and one switch to another between regions should be configured as members of all VLANs configured in the region.

  • Never connect LAGs before they’re configured. This will create loops in your network and cause an unstable environment.

Network scenario with spanning tree configurations

MST Instance 1 shows which ports are forwarded and which ports are blocked for the following VLAN scenarios listed in VLAN scenarios:

VLAN scenarios

Location 8200A Instance 1 8200A VRRP Configuration 8200B VRRP Configuration 8200B Instance 2 8200A VRRP Configuration 8200B VRRP Configuration
IDF 1 3,4,5 Master Backup 7,8,9 Backup Master
IDF 2 23,24,25 Master Backup 27,28,29 Backup Master
IDF 3 33,34,35 Master Backup 37,38,39 Backup Master
IDF 4 43,44,45 Master Backup 47,48,49 Backup Master

The network design strategy is to configure VLANs based on location. This will help isolate problems quickly on the network.

MST Instance 1

MST Instance 1

MST Instance 1 Legend

MST Instance 1 Legend

VLANs that end with 3, 4, and 5 are blocked between 8200B and EAST IDFs. The ports connecting the 8200A and WEST IDFs are forwarding. Only if the active Trunk between the 8200A and WEST IDFs fail will the links between the 8200B and EAST IDFs become active.

MST Instance 2

MST Instance 2

MST Instance 2 Legend

MST Instance 2 Legend

VLANs that end with 7,8, and 9 are blocked between 8200A and WEST IDFs. The ports connecting the 8200B and EAST IDFs are forwarding. Only if the active Trunk between the 8200B and EAST IDFs fail will the links between the 8200A and WEST IDFs become active.

IST/CST

IST/CST

IST/CST Legend

IST/CST Legend

VLANs that end with 7,8, and 9 are blocked between 8200A and WEST IDFs. The ports connecting the 8200B and EAST IDFs are forwarding. Only if the active Trunk between the 8200B and EAST IDFs fail will the links between the 8200A and WEST IDFs become active.

For consistency purposes configure the IST/CST to Block on the same ports as Instance 1. This way you only have to remember two configurations.