HP 2424M - Manual (Page 11)

HP ProCurve Switch 4000M / 8000M / 2424M / 1600M Reviewer’s Guide
• Spanning Tree Protocol (802.1D) support, including “fast” mode to allow support of IPX client
environments (see the application note “HP Switches - Controlling Network Traffic” at
http://www.hp.com/rnd/products/solutions/techlib/techlib.htm for more details).
• Year 2000 compliant
2.2 Architecture
2.2.1 Store-and-Forward
There are two techniques by which switches move packets from the source to destination ports, 1)
store-and-forward or, 2) cut-through. In store-and-forward, the entire packet is received and stored in
memory by the switch before any decision is made as to the destination port for that packet. For cut-
through, the switch makes the destination decision and forwards the packet as soon as it has the
destination and source address of the packet. Since the destination address is contained at the
beginning of the packet, it is possible to have the packet being sent out the destination port before it is
entirely received on the incoming port. No CRC checking is done on the packet with cut-through
switching.
The HP ProCurve Switches 4000M/8000M/2424M/1600M use a variation of the store-and-forward
method of packet forwarding. The benefits of the store-and-forward approach over cut-through are:
• Allows multi-speed switching: Store-and-forward is the only way a switch can deal with
differing speeds on its various ports.
• Filters out bad packets: Packets that are too short (as well as collision fragments), too long, or
have bad checksums are not forwarded, preventing the proliferation of bad packets.
• Congestion management: All switches deal with packet congestion by using store-and-forward
methods.
• Priority Queuing: QoS priority queues work through a store-and-forward mechanism.
The usual concern with the store-and-forward method compared to cut-through is higher packet
latencies through the switch. The HP ProCurve Switches 4000M/8000M/2424M/1600M have a novel
backplane design that keeps packet latencies among the lowest in the industry for store-and-forward
switches. This is discussed in more detail in the Backplane Design section below.
2.2.2 Backplane Design
The HP ProCurve Switches 4000M/8000M/2424M/1600M has backplanes controlled by an HP-designed
programmable ASIC. Backplane speeds for the various switches are: 4000M/8000M: 3.8 gigabit/sec,
1600M: 3.5 Gbps, 2424M:
3.2 Gbps. The switches support
10,000 MAC addresses. Unlike
many other switches on the
market, data is stored on the
incoming port module and only
travels across the backplane
once on its way to the outgoing
port module. At a measured
latency of only 3.5-12
µsec (LIFO,
100Mbps speed), the
HP ProCurve
ASIC
Controller
3.8 Gbit switch bus
Slot
Module
Slot
Module
Slot
Module
Switches 4000M/8000M/2424M/1600M have low latency for a store-and-forward design. Latency is the
same for both inter and intra-module destinations.
For broadcast and multicast packets, LIFO latency is still at 4-21 µsec for all ports , and the variance in
broadcast and multicast latency port-to-port in these switches is ±1.2 µsec, which is very low. For
©1998, 1999, 2000 Hewlett-Packard Co Revision 3.2b – 1/15/2000 Page 11 of 36