In this example, the distribution layer
must be capable of providing the following capacity:
- Total load at the
distribution-layer switch is the number of access switches x 96
Mbps. In this scenario, there are ten access switches, or 10 x
96 Mbps, yielding a 960-Mbps aggregate bandwidth requirement at
the distribution layer.
- Eighty percent of the traffic is
local to the switch block and is not routed across the
core.
- Twenty percent of the traffic is
remote and is routed toward the core.
- Taking into consideration that only 20
percent of traffic is remote, we come up with 20 percent x 960
Mbps, yielding 192 Mbps of traffic that must be able to cross
the core.
This sample network supports a
redundant core; therefore, each core subnet would carry 50 percent
of the traffic load, or 96 Mbps of traffic. Given this amount of
traffic, the performance of the distribution switch must be capable
of switching 187,000 packets per second.
The Layer 3 module of the
distribution-layer switch will be responsible for routing the remote
traffic to the core. Therefore, a switch must be chosen that will
support this amount of traffic.
This topology presents no redundancy between the end user and the core. If the
link between an access switch and distribution device fails, 100
users lose connectivity. If the distribution device fails, the whole
building is disconnected from the network. One solution is to add a second distribution switch with backup links to
each access switch.
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