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Understanding MSTP

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TitleUnderstanding MSTP
Objective
To understand the basics of MSTP and create a configuration 
Environment
  • S-Series 
  • K-Series
  • 7100-Series 
  • N-Series 
  • Securestack
  • MSTP
  • 802.1s
Procedure

IEEE 802.1s MSTP (Multiple Spanning Tree Protocol) makes it possible for VLAN switching devices to use multiple Spanning Trees, allowing traffic belonging to different VLANs to flow over potentially different paths within the LAN. It builds upon the advancements of RSTP with its decreased time for network re‐spans. MSTP’s principle objective is to increase bandwidth utilization by allowing:

  • One or more VLANs to be be mapped to a single Spanning Tree instance (MSTI).
  • Several Spanning Tree instances to 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 main benefits:
  • Previously blocked ports can now provide usable bandwidth.
  • 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.

N.B.  MSTP is the default Spanning Tree mode on all EOS switch devices

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.

Common and Internal Spanning Tree (CIST)

MSTP uses all Spanning Tree region information to create a single Common and Internal Spanning Tree (CIST) that represents the connectivity of the entire network. This is equivalent to the single Spanning Tree used for STP and RSTP.  The MSTP enabled network contains one CIST and a minimum of at least one MST region. A typical network may contain numerous MST regions as well as separate LAN segments running legacy STP and RSTP Spanning Tree protocols. The CIST contains a root bridge, which is the root of the Spanning Tree for the network. The CIST root is not necessarily located inside an MST region. Each region contains a CIST regional root, unless the CIST root is part of the region. Bridges in an MSTP topology compare their received BPDUs to calculate their shortest path to the CIST root, CIST regional root and MSTI regional root.

MST Region

An MST region is a group of devices that are configured together to form a logical region. The MST region presents itself to the rest of the network as a single switching device, which simplifies administration. Path cost is only incremented when traffic enters or leaves the region, regardless of the number of devices within the region. Each LAN can only be a member of one region. For a switching device to be considered as part of an MST region, it must be administratively configured with the same configuration identifier information as all other devices in the MST region.

The Configuration Identifier

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

This example is from a Matrix DFE, but all MST bridges 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 have the same FIDs mapped to the same instances and 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 MST bridge 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]
 
The following commands are very useful in checking the status of the MSTP configuration
 
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.
 

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.

Implementation issues

Most problems tend to stem from misconfiguration of vlan / sid on links and the fact that adding vlans to sids causes an interruption. A lot of people get around this by pre-conguring groups of vlans in advance that may be used in future.

Additional notes

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. 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 - filtering Database

  • The numeric representation of a Filtering Database / Source Address Table. VLANs can share a FID (SVL), or may be mapped to separate FIDs (IVL). 

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