Cisco Systems 8600 Series manual Congestion Management, VS/VD, Congestion Management, ForeSight

Traffic and Congestion Management

PNNI

The Private Network to Network Interface (PNNI) protocol provides a standards-based dynamic routing protocol for ATM and frame relay SVCs. PNNI is an ATM-Forum-defined interface and routing protocol which is responsive to changes in network resources, availability, and will scale to large networks. PNNI is available on the BPX switch when an ESP is installed. For further information about PNNI and the ESP, refer to the Cisco WAN Service Node Series Extended Services Processor Installation and Operation publication.

Congestion Management, VS/VD

The BPX/IGX/IPX switch networks provide a choice of two dynamic rate based congestion control methods, ABR with VS/VD and ForeSight. This section describes Standard ABR with VSVD.

Note ABR with VSVD is an optional feature that must be purchased and enabled on a single node for the entire network.

When an ATM connection is configured between BXM cards for Standard ABR with VSVD per ATM Forum TM 4.0, Resource Management (RM) cells are used to carry congestion control feedback information back to the connection’s source from the connection’s destination.

The ABR sources periodically interleave RM cells into the data they are transmitting. These RM cells are called forward RM cells because they travel in the same direction as the data. At the destination these cells are turned around and sent back to the source as backward RM cells.

The RM cells contain fields to increase or decrease the rate (the CI and NI fields) or set it at a particular value (the explicit rate ER field). The intervening switches may adjust these fields according to network conditions. When the source receives an RM cell, it must adjust its rate in response to the setting of these fields.

When spare capacity exists with the network, ABR with VSVD permits the extra bandwidth to be allocated to active virtual circuits.

Congestion Management, ForeSight

The BPX/IGX/IPX switch networks provide a choice of two dynamic rate based congestion control methods, ABR with VS/VD and ForeSight. This section describes ForeSight.

Note ForeSight is an optional feature that must be purchased and enabled on a single node for the entire network.

ForeSight may be used for congestion control across BPX/IGX/IPX switches for connections that have one or both end points terminating on other than BXM cards, for example ASI cards. The ForeSight feature is a dynamic closed-loop, rate-based, congestion management feature that yields bandwidth savings compared to non-ForeSight equipped trunks when transmitting bursty data across cell-based networks.

ForeSight permits users to burst above their committed information rate for extended periods of time when there is unused network bandwidth available. This enables users to maximize the use of network bandwidth while offering superior congestion avoidance by actively monitoring the state of shared trunks carrying frame relay traffic within the network.

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