Spanning Tree Algorithm Port Path Costs

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Article ID: 4723 

Protocols/Features
Spanning Tree 

Standards
802.1d
802.1t 

Goals
STA Port Path Costs 

Cause
With the advent of ever-faster port speeds, new methods have arisen for allocating Spanning Tree path costs to ports, based on their speed. 

This document contrasts the old and new methodologies, and details which Enterasys Networks products adhere to which of the costing methods. 

See 4832 for 802.1t-related Bridge Priority restrictions. 

Solution
There are three different algorithms which may be found on Enterasys Networks products:
  1. IEEE 802.1D-1990 standard:
    • Uses the computation 1 x 10 to the 9th power divided by port speed in bits per second
    • Path cost range is 1 - 65,535
    • Allocates unique path costs for port speeds up to 600 Mb/s
    • Most Spanning Tree products use this method.
     
  2. Extension of the IEEE 802.1D-1998 standard:
    • Generally adheres to the 1990 standard, but with variations (both IEEE and Enterasys originated) to correct for shortcomings
    • Path cost range is 1 - 65,535
    • Allocates unique path costs for port speeds up to 50 Gb/s
    • The X-Pedition product line uses this method, with firmware versions E8.2.1.1, and E8.3.0.1 and higher.
    • The SmartSwitch 2000/6000 2nd/3rd Generation switches use this method by default, with firmware version 5.04.09 and higher.
    • The Vertical Horizon L3 switches use this method exclusively.
    • The Vertical Horizon non-L3 switches use this method, as of unknown pre-2.05 firmware version.
     
  3. IEEE 802.1t-2001 standard:
    • Uses the computation 2 x 10 to the 13th power divided by port speed in bits per second
    • Path cost range is 1 - 200,000,000
    • Allocates unique path costs for port speeds up to 20 Tb/s
    • The SmartSwitch 2000/6000 2nd/3rd Generation switches optionally use this method, with firmware version 5.04.09 and higher.
    • The Matrix E1 switches optionally use this method.

    BPDUs are capable of carrying 32 bits of Path Cost information; however, IEEE Std. 802.1D, 1998 Edition and earlier revisions of this standard limited the range of the Path Cost parameter to a 16-bit unsigned integer value of 65535. The recommended values of the 802.1t standard make use of the full 32-bit range available in BPDUs in order to extend the range of link speeds supported by the protocol. Limiting the range of the Path Cost parameter to a 200,000,000 maximum value ensures that the accumulated Path Cost cannot exceed 32 bits over a concatenation of 20 hops.

    Link Speed ==> Path Cost

    802.1D-1990 802.1D-1998 802.1t-2001
    Link Speed Cost Cost Cost

    10 Kb/s 65,535 65,535 200,000,000[3]
    100 Kb/s 10,000 10,000 200,000,000[3]
    1 Mb/s 1000 1000 20,000,000[3]
    2 Mb/s 500 500 10,000,000[3]
    3 Mb/s 333 333 6,666,666[3]
    4 Mb/s 250 250[2] 5,000,000[3]
    5 Mb/s 200 200 4,000,000[3]
    6 Mb/s 166 166 3,333,333[3]
    7 Mb/s 142 142 2,857,142[3]
    8 Mb/s 125 125 2,500,000[3]
    9 Mb/s 111 111 2,222,222[3]
    10 Mb/s 100 99[2] 2,000,000[3]
    16 Mb/s 62 62[2] 1,250,000[3]
    20 Mb/s 50 90 1,000,000[3]
    30 Mb/s 33 81 666,666[3]
    40 Mb/s 25 72 500,000[3]
    50 Mb/s 20 63 400,000[3]
    60 Mb/s 16 54 333,333[3]
    70 Mb/s 14 45 285,714[3]
    80 Mb/s 12 36 250,000[3]
    90 Mb/s 11 27 222,222[3]
    100 Mb/s 10 19[2] 200,000[3]
    200 Mb/s 5 17 100,000[3]
    300 Mb/s 3 15 66,666[3]
    400 Mb/s 2 14 50,000
    500 Mb/s 2 12 40,000
    600 Mb/s 1 10 33,333
    700 Mb/s 1 9 28,571
    800 Mb/s 1 7 25,000
    900 Mb/s 1 5 22,222
    1 Gb/s 1 4[2] 20,000
    2 Gb/s 1 4 10,000
    3 Gb/s 1 4 6,666
    4 Gb/s 1 4 5,000
    5 Gb/s 1 3 4,000
    6 Gb/s 1 3 3,333
    7 Gb/s 1 3 2,857
    8 Gb/s 1 3 2,500
    9 Gb/s 1 2 2,222
    10 Gb/s 1 2[2] 2,000
    50 Gb/s 1 1 400
    100 Gb/s 1 1 200
    1 Tb/s 1 1 20
    10 Tb/s 1 1 2
    20 Tb/s 1 1 1

[2] The IEEE Std 802.1D, 1998 Edition standard lists only these designated entries. The remainder of entries in this column are Enterasys-originated extensions of the original suggested pattern. 

[3] Bridges conformant to IEEE Std 802.1D, 1998 Edition, i.e., that support only 16-bit values for Path Cost, should use 65535 as the Path Cost for these link speeds when used in conjunction with Bridges that support 32 bit Path Cost values. 

Note:
802.1D Spanning Tree uses the dot1dBridge=1.3.6.1.2.1.17 mib. 

Implementations of 802.1t will likely use the new ietf-bridge-bridgemib-smiv2 mib, which is an update to RFC1493; as well as supporting some measure of switch-level control over the port costs in use, by means of the ietf-bridge-rstpmib mib.
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