~clay

A Proposed Extension to the IEEE 802.1D Spanning Tree Protocol

Network administrators who maintain complicated Layer 2 networks should be very familiar with the operation of the Spanning Tree Protocol. One common point of confusion among administrators is the use of (essentially) arbitrary numbers to identify the current root bridge. While the root bridge identifier is deterministic, comprised of bridge priority and MAC address, it is not information that network administrators can use without consulting a reference that maps MAC addresses to human-friendly names.

Bridges and switches are often given hostnames for administrative purposes. The extension to Spanning Tree Protocol described below adds this user-friendly hostname information to the BPDU. The hostname could then be used by equipment vendors for diagnostic purposes, i.e., Cisco’s “show spantree” command.

The extensions described below are relative to ANSI/IEEE Std 802.1D, 1998 Edition. I believe these changes will not affect existing implementations of Spanning Tree Protocol, since the standard indicates that “any octets beyond Octet 35 are ignored”, and that “the Protocol Version Identifier is not checked on receipt, in order to allow the possibility of future specification of extensions to the Spanning Tree Protocol”. The extensions below would be detected by a compliant bridge implementation by checking the Protocol Version Identifier field of the BPDU.

This proposal was sent to the IEEE 802.1 committee on August 22, 2001. Initial response has been somewhat discouraging. The standards committee does not accept proposals from non-members, however becoming a member requires attending periodic meetings in Washington, DC. Without the means to attend these meetings, it is unlikely that this proposal will ever be considered. Perhaps an existing member of the committee will volunteer to represent this proposal in my stead…

Proposal

9.2.X Encoding of Bridge Names
[New section between "9.2.5 Encoding of Bridge Identifiers" and "9.2.6
Encoding of Root Path Cost"]

A Bridge Name shall be encoded as sixteen octets, taken to represent a
string of printable ASCII characters (octal 040 through 0177 [decimal 32
through 127]).  The string shall contain the first 16 characters of the
bridge's hostname, if configured, or any other string which might
uniquely identify the bridge.  This parameter is intended for human
consumption only.

The most significant octet is the first character of the string.  If the
intended ASCII string is less than sixteen characters in length, the
unused octets in the BPDU will be set to ASCII NUL (octal 0).  If the
intended string is greater than sixteen characters in length, any
additional characters will be truncated so that only the first sixteen
characters will appear in the BPDU.

9.3.1 Configuration BPDUs

Figure 9-1 will have the following data structure appended to the
diagram:

+-------------+
|             | 36
|             | 37
|             | 38
|             | 39
|             | 40
|             | 41
|             | 42
|   Bridge    | 43
|    Name     | 44
|             | 45
|             | 46
|             | 47
|             | 48
|             | 49
|             | 50
|             | 51
+-------------+

b) The Protocol Version Identifier is encoded in Octet 3 of the BPDU. It
takes the value 0000 0001.