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PowerConnect 5324 MSTP switch fabric selecting ports for MSTI that do not support the MSTI VLANs
I am running Multiple Spanning-Tree Protocol (MSTP) on a small PowerConnect 5324 switch fabric supporting 2 VLANs in addition to default VLAN 1. Each VLAN belongs to a unique Multiple Spanning-Tree Instance (MSTI); VLAN 1 in MSTI 0; VLAN 2 in MSTI 2; VLAN 3 in MSTI 3.
As I understand it, MSTI 0 serves as the Internal Spanning-Tree (IST) and is responsible for distributing the spanning-tree information for all the instances in the MSTP region. The MSTI 0 root bridge generates BPDUs with M-Records that identify the other instances. The other instances are responsible for load balancing traffic for their member VLANs and do not generate BPDUs.
The issue I am seeing involves MSTI 2 showing alternate and designated ports that are not configured to support VLAN 2. Likewise, MSTI 3 has alternate and designated ports that do not support VLAN 3.
I have employed port costs to establish the proper root port for each spanning-tree instance. However, a loss of an MSTI root port results in an alternate port taking over that is unable to carry the MSTI VLAN. This often results in root port flapping between the root and alternate.
Any ideas?
Thanks,
Paul
bh1633
909 Posts
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November 17th, 2010 14:00
post the output of "show running-config" and "show spanning-tree" and any additional information you think is relevant.
Paul Hauck
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December 7th, 2010 10:00
BH1633,
I appreciate your reply and apologize for my slow response. Hopefully this forum will find the results of my investigation useful.
Unlike Per-VLAN Spanning Tree (PVST) that creates a unique spanning-tree instance for each VLAN, Multiple Spanning Tree Protocol (MSTP) maintains an Internal Spanning Tree (IST) known as Multiple Spanning Tree Instance 0 (MSTI0). MSTI0 is responsible for carrying all STP-related information for the MSTP region; including Rapid Spanning Tree Protocol (RSTP)-like information for the IST itself, along with information supporting each additional spanning-tree instance within the region. All VLANs belong to MSTI0 until assigned to another spanning-tree instance.
A key MSTP caveat concerns VLAN paths versus STP paths. MSTI0 creates and maintains the IST topology for an MSTP region based upon the actual physical topology of that region's switch fabric. However, VLANs assigned to other spanning-tree instances may be unable to utilize every physical path identified by the IST.
In my case I have assigned each VLAN to its own MSTP instance. I have configured path costs within each instance to direct its VLAN traffic across interswitch links that support the assigned VLAN.
Therein lies the rub. All VLAN traffic flows per the path costs assigned to its spanning-tree instance; but only as long as all links in the IST topology remain available. For example, when a root port for a given instance fails the alternate root port may or may not be configured to allow the VLAN traffic belonging to that MSTP instance.
For this reason it is recommended that one not use access ports or VLAN-pruned trunk ports to connect MSTP switches. In other words, one avoids a situation where VLAN traffic can no longer reach the root bridge when an alternate port becomes root but does not allow that VLAN's traffic.
For a more in-depth understanding you may wish to read the following CCIE blog http://blog.ine.com/2010/02/22/understanding-mstp/.
Cheers,
Paul