Subnet Bandwidth Management

Subnet bandwidth management (SBM) specifies a signaling method and protocol for LAN-based CAC for RSVP flows. SBM allows RSVP-enabled Layer 2 and Layer 3 devices to support reservation of LAN resources for RSVP-enabled data flows.

SBM uses the concept of a designated entity elected to control the reservations for devices on the managed LAN. The elected candidate is called the designated subnetwork bandwidth manager (DSBM). It is the DSBM's responsibility to exercise admission control over requests for resource reservations on a managed segment. A managed segment includes a Layer 2 physical segment shared by one or more senders, such as a shared Ethernet network. The presence of a DSBM makes the segment a managed one. One or more SBMs may exist on a managed segment, but there can be only one DSBM on each managed segment. A router interface can be configured to participate in the DSBM election process. The contender configured with the highest priority becomes the DSBM for the managed segment.

Figure 8-34 shows a managed segment in a Layer 2 domain that interconnects a group of routers.

Figure 8-34 DSBMManagedSubnet

DSBM Client DSBM DSBM Client

DSBM Client DSBM DSBM Client

DSBM Client DSBM Client

When a DSBM client sends or forwards an RSVP path message over an interface attached to a managed segment, it sends the path message to the DSBM of the segment rather than to the RSVP session destination address, as is done in conventional RSVP processing. As part of its message-processing procedure, the DSBM builds and maintains a path state for the session and notes the previous Layer 2 or Layer 3 hop from which it received the path message. After processing the path message, the DSBM forwards it toward its destination address.

The DSBM receives the RSVP resv message and processes it in a manner similar to how RSVP handles reservation request processing, basing the outcome on available bandwidth. The procedure is as follows:

• If it cannot grant the request because of lack of resources, the DSBM returns a resverror message to the requester.

• If sufficient resources are available and the DSBM can grant the reservation request, it forwards the resv message toward the previous hops using the local path state for the session.

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