Microsoft KB Archive/228830

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Description of Subnet Bandwidth Manager (SBM) in Windows 2000

Article ID: 228830

Article Last Modified on 10/30/2006


  • Microsoft Windows 2000 Server

This article was previously published under Q228830


Quality of Service (QoS) Subnet Bandwidth Manager (SBM) is a signaling technique used to manage network resources (bandwidth) on legacy and newer local area network (LAN) topologies. SBM manages network resources and uses Admission Control Services (ACS) to make traffic-flow decisions.


SBM is a signaling protocol for Resource Reservation Protocol-based (RSVP-based) admission control over legacy and new implementations of 802-style networks. In present 802-style LANs, there is no admission control, only traffic control. Using SBM, best-effort traffic generated from present day LANs relies on admission control and bandwidth management from SBM server to meet QoS specifications.

The SBM server contains the limitations of available resources that SBM clients or RSVP-aware clients can reserve on the managed subnet. SBM limits only traffic imposed by RSVP-based flows. Therefore, SBM admission control focuses solely on the QoS traffic. Reservations are made and kept track of. Because TCP/IP best-effort traffic is rate-adaptive, it uses the Slow Start algorithm during periods of congestion and adapts to stay within the LAN's bandwidth. If the client is not QoS-aware (RSVP), the client should use the SBM client software for admission control, and then keep the traffic control defined by IEEE 802 (802.1p). If the client is RSVP enabled, SBM expects traffic control to stay within the boundaries previously implemented in the RSVP reservation. In Windows 2000, SBM communicates with the ACS service for traffic flow boundaries that may have been set on reservation requests by LAN administrators.

A designated SBM (DSBM) manages resources on the subnet on which it resides. If a subnet exists with two SBMs, an election must take place to decide which will become the DSBM. During this process, a DSBM candidate sends out an advertisement to the AllSBMaddress. Included in the advertisements are DSBM_WILLING and I_AM_DSBM messages. Once the I_AM_DSBM message is detected, the DSBMaddress is recorded by DSBM clients. Layer 2 devices have a greater chance of becoming the DSBM for a segment with a priority range from 128 to 255, with 255 being the highest precedence. The priority order for nodes is:

Hosts (Windows 2000) - 1 to 63
Layer 3 devices - 64 to 127
Layer 2 devices - 128 to 255

The default priority for SBM is 1. A value of zero indicates that a node is not eligible to become a DSBM.

Keywords: kbenv kbinfo kbnetwork KB228830