3Com 4007, Implementation Manual, Page: 229 / 684

Key Guidelines for Implementation
229
Trunk Capacity
Guidelines
■
The device-to-device burst-transmission rate across a trunk is limited
to the speed of just
one of the port-to-port links within the trunk. For
example, the maximum burst rate over a 400-Mbps pipeline with four
trunked Fast Ethernet links is 100 Mbps. This limitation preserves
frame ordering between devices, usually by moving all traffic between
two specific MAC addresses across
only one port-to-port link .
Therefore, trunking provides no direct benefit for some one-way
applications, such as server-to-server backups. This limit exists for most
vendor implementations.
■
The total throughput of a trunk is typically less than the bandwidth
obtained by adding the theoretical capacity of its individual links. For
example, four 1000-Mbps links do not yield a 4000-Mbps trunk. This
is true with all vendor implementations.
■
A trunked Fast Ethernet pipeline may seem to offer comparable
bandwidth to a single Gigabit Ethernet link, and trunked Fast Ethernet
may seem like a good way to buy some time before you upgrade
connections to Gigabit Ethernet. Table 47 shows that trunking Fast
Ethernet may not be an effective strategy.
If you cannot upgrade to Gigabit Ethernet, then trunking Fast
Ethernet in switch-to-switch or switch-to-server links can help you
fine-tune or expand network capacity. After Gigabit Ethernet is in
place, you can use trunking to further expand switch-to-switch or
server-to-switch links.
Table 47 Comparing Gigabit Ethernet with Trunked Fast Ethernet
Comparison Point Gigabit Ethernet Trunked Fast Ethernet
Max burst rate 1000 Mbps 100 Mbps
Max aggregate rate 1000 Mbps
(2000 Mbps full duplex) Multilayer modules:
800 Mbps (over 8 links)
(1600 Mbps full duplex)
Layer 2 modules:
600 Mbps (over 6 links)
(1200 Mbps full duplex)
Standards compliance IEEE 802.3z In progress