About MSTP / 802.1s Configuration

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Article ID: 5389 

Protocols/Features
MSTP
Multiple Spanning Tree Protocol 

Standards
802.1s 

Goals
Configure MSTP / 802.1s
Sample configuration 

Cause
IEEE 802.1s MSTP (Multiple Spanning Tree Protocol) is gaining in popularity due to an increase in support among manufacturers and the benefits it provides over Standard (STP) or Rapid (RSTP) Spanning Tree:
  • One or more VLANs can be mapped to a single Spanning Tree instance (MSTI).
  • Several Spanning Tree instances can be configured to allow the same link to block for some instances while forwarding for others - not possible with standard STP or RSTP, which are not VLAN-aware and use an "all or nothing" forwarding strategy on each port..
  • This has two key effects:
    1. Previously blocked ports can now provide useable bandwidth.
    2. Any data loop on a particular VLAN need not affect the stability or forwarding path of any other VLAN - depending on how the MSTIs are defined.
If MSTP is left unconfigured; then each switch represents its own MSTP Region, and all FIDS (loosely, VLANs) will be members of the default SID0 (Spanning Tree instance "0"). Thus, by default MSTP operates identically to 802.1w Rapid Spanning Tree Protocol except for the BPDU format. If an intermediate device is having issues with the MSTP BPDU, it is possible to swap the spanning tree version to RSTP ('set spantree version rstp'), though this is not generally necessary. 

Configuration and troubleshooting of MSTP requires some understanding of the protocol, as briefly described in this document. 

Solution 

The Configuration Identifier 

The central component for both configuration and troubleshooting of MSTP is the Configuration Identifier, as shown below. In order for MSTBs to join the same Region, all four components of the Configuration Identifier must match. 

This example is from a Matrix DFE, but all MSTBs have a similar capability. The same information can also be found in captured MSTP BPDUs. 

  DFE(su)->show spantree mstcfgid 
MST Configuration Identifier:
Format Selector: 0
Configuration Name: My region
Revision Level: 0
Configuration Digest: 6d:d7:93:10:91:c9:69:ff:48:f2:ef:bf:cd:8b:cc:de

Format selector
By default, has a value of 0. Reserved for future use. 

Configuration Name
By default, contains the bridge's MAC address, putting each MSTB into its own Region. To put all MSTBs into the same Region, change the Configuration Name of all MSTBs to a single common value. 

Revision Level
By default, has a value of 0. Reserved for future use, or can be manipulated to assign different regions for MSTBs which use the same Configuration Name. 

Configuration Digest
A 802.1s-standardized, read-only hash value reflecting the configured VLAN FID to MSTPI SID mapping. By default, every bridge maps all FIDs to SID0. 

In addition to having identical Configuration Identifiers, in order to join the same Region the MSTBs must not be interconnected by switches/routers running STP or RSTP. 

Configuration 

Shown below is an example command set to map FIDs 2 and 3 to one MSTI SID, using the same commands for each MSTB in the Region: 

  set vlan create 2-3                   [create VLANs]
set span version mstp [toggle from RSTP to MSTP mode]
set span mstcfgid cfgname "My region" [specify the Configuration Name]
set span msti sid 99 create [create a MSTP Instance]
set span mstmap 2 99 [map a FID to it]
set span mstmap 3 99 [map another FID to it]

show span mstmap [check the mapping]
show span mstcfgid [check the Configuration ID]

Recall that since a FID is already mapped to one or more VLANs, the overall effect is to map VLANs to MSTP Instances, with each MSTI being separately configurable (similar to STP/RSTP) with Bridge Priority / Path Cost / Port Priority parameters for unique spanning behavior. 

When possible it is best to configure all of the bridges to be in the same Region, since only within a Region (as opposed to the single active link between Regions) can multiple forwarding paths be utilized for different VLANs/FIDs/SIDs. 

5246 explains how to configure 802.1s to Load Share. 

Definitions 

STP - Spanning Tree Protocol
    For purposes of this document, as implemented by the IEEE 802.1d standard. STP recognizes the concept of a single Root Bridge, spanning out to a single flood path, blocking redundant paths to prevent data loops, and bringing blocked paths back into use in the event of primary link failure. A blocking condition on a port blocks all user traffic on the port.
RSTP - Rapid Spanning Tree Protocol
    For purposes of this document, as implemented by the IEEE 802.1w standard. RSTP is similar to STP, but is designed for more rapid failover, on the order of several seconds rather than up to 45 seconds to re-stabilization. A blocking condition on a port blocks all user traffic on the port.
MSTP - Multiple Spanning Tree Protocol
    For purposes of this document, as implemented by the IEEE 802.1s standard. MSTP is similar to RSTP, but is set up in Regions, each of which is a set of LANs and MSTP Bridges directly physically connected via ports on those MSTP Bridges, all having the same MSTP Configuration ID. A blocking condition on a port blocks only traffic which is egressed on behalf of one or more specific VLANs.
CST - Common Spanning Tree
    The single Spanning Tree calculated by STP and RSTP, and by MSTP to connect MSTP Regions. 

    MSTP provides for a single active forwarding path between any two Regions, which in the case of redundant links is determined by the currently prevailing settings relevant to SID0: Bridge Priority, Path Costs, and Port Priorities (4734). Once the forwarding link is chosen, then any VLANs which are permitted to egress onto that link will do so. As expected, frames which egress as VLAN-tagged retain their VLAN identity upon reception by the peer port; and otherwise are assigned a VLAN based upon VLAN Classification, PVID, etc. In turn they are associated with a FID based on locally (to the new Region) prevailing settings, and then with a SID. If the forwarding inter-Region CST path fails, then if possible a backup path will come active and this same process will repeat. It is customary to use the same VLAN assignments on all possible paths between any two Regions, though that is for purposes of behavioral consistency rather than being dictated by any requirement of MSTP.
SSTB - Single Spanning Tree Bridge
    A Bridge capable of supporting only the CST.
IST - Internal Spanning Tree
    The connectivity within a given MSTP Region.
CIST - Common and Internal Spanning Tree
    The single spanning tree calculated by STP and RSTP, together with the logical continuation of that connectivity through all MSTP Bridges and Regions. In other words, the entire Spanning Tree fabric.
MSTI - Multiple Spanning Tree Instance
    A sub-component of a MSTP Region, a MSTI is mapped to one or more VLANs (actually, FIDs) and operates independently of the other MSTIs. MSTP with a single Region and a single MSTI functionally equates to a CST.
MSTB - Multiple Spanning Tree Bridge
    A Bridge capable of supporting the CST, and one or more MSTIs, and of selectively mapping frames associated with any given VLAN to the CST or to its configured MSTI.
CST Root
    The one MSTB or CSTB in the entire network having the numerically lowest Bridge ID.
CIST Regional Root
    The MSTB in each Region having the lowest path cost to the CST Root.
MSTI Regional Root
    The MSTB in each MSTI having the numerically lowest Bridge ID.
SID - Spanning Tree Identifier
    The numeric representation of a MSTI.
FID - Spanning Tree Identifier
    The numeric representation of a Filtering Database / Source Address Table. VLANs can share a FID (SVL), or may be mapped to separate FIDs (IVL). See 4918 for background.

MSTP BPDU 

Shown below is an example MSTP BPDU reflecting the existence of MSTIs 1, 2, and 3, with 2 and 3 using the same MSTI Regional Root. The FID-to-SID mapping is indicated only by the Configuration Digest, so the specifics are not obvious without looking at the actual configuration. 

An MSTP BPDU represents the transmitting bridge's understanding of the entire IST for the local Region, with each Instance seeking the shortest path to its local MSTI Regional Root (there will be one or more of these, per Region). 

A STP/RSTP BPDU (not shown here) represents the transmitting bridge's understanding of the entire CST, seeking the shortest path to the CST Root. 

A MSTB will transmit both MSTP and STP/RSTP BPDUs, while a SSTB will transmit only STP/RSTP BPDUs. An MSTB transmits its BPDUs VLAN-untagged over VLAN 1, and this occurs regardless of user configuration of VLAN 1 egress

Frame 914 (167 bytes on wire, 167 bytes captured)
IEEE 802.3 Ethernet
Destination: 01:80:c2:00:00:00 (Spanning-tree-(for-bridges)_00)
Source: 00:01:f4:31:03:18 (Enterasys_31:03:18)
Length: 153
Logical-Link Control
DSAP: Spanning Tree BPDU (0x42)
IG Bit: Individual
SSAP: Spanning Tree BPDU (0x42)
CR Bit: Command
Control field: U, func = UI (0x03)
Spanning Tree Protocol
Protocol Identifier: Spanning Tree Protocol (0x0000)
Protocol Version Identifier: Multiple Spanning Tree (3)
BPDU Type: Rapid/Multiple Spanning Tree (0x02)
BPDU flags: 0x7c (Agreement, Forwarding, Learning, Port Role: Designated)
Root Identifier: 4096 / 00:01:f4:7e:a0:63
Port identifier: 0x8371
Message Age: 0
Max Age: 20
Hello Time: 2
Forward Delay: 15
Version 1 Length: 0
MST Extension, Length: 122
MST Config ID format selector: 0
MST Config name: a_sample
MST Config revision: 0
MST Config digest: D0A1A0221648D311DA8FFD31AF847AED
CIST Internal Root Pth Cost: 0
CIST Bridge Identifier: 4096 / 00:01:f4:7e:a0:63
CIST Remaining hops: 20
MSTID 1, Regional Root Identifier 4096 / 00:01:f4:7e:a0:63
MSTI flags: 0x7c (Agreement, Forwarding, Learning, Port Role: Designated)
MSTID 1, priority 4096 Rot Identifier 00:01:f4:7e:a0:63
Internal root path cost: 0
Bridge Identifier Priority: 1
Port identifier priority: 8
Remaining hops: 20
MSTID 2, Regional Root Identifier 4096 / 00:01:f4:7e:a0:C7
MSTI flags: 0x7c (Agreement, Forwarding, Learning, Port Role: Designated)
MSTID 2, priority 4096 Rot Identifier 00:01:f4:7e:a0:C7
Internal root path cost: 0
Bridge Identifier Priority: 2
Port identifier priority: 8
Remaining hops: 19
MSTID 3, Regional Root Identifier 4096 / 00:01:f4:7e:a0:C7
MSTI flags: 0x7c (Agreement, Forwarding, Learning, Port Role: Designated)
MSTID 3, priority 4096 Rot Identifier 00:01:f4:7e:a0:C7
Internal root path cost: 0
Bridge Identifier Priority: 2
Port identifier priority: 8
Remaining hops: 19

When interpreting Path Costs, it is useful to consider that in a traverse of a STP/RSTP BPDU through the CST of a MSTP Region, only the (Region-ingressing) CIST Regional Root adds to the path cost. STP/RSTP thus considers the CST of a particular MSTP Region to be a single bridge hop. 

4723 explains 802.1D Spanning Tree default path costing.
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