1-16 Cisco BPX 8600 Series Reference
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.
