The switch block contains a balanced
implementation of scalable Layer 2 switching and Layer 3 services.
Although the current generation of LAN switches is replacing
shared-media concentrators, LAN switches are not replacements for
Layer 3 devices. Therefore, the switch block consists of both switch
and router functionality.
Layer 2 switches in the wiring
closets connect users to the network at the access layer and provide
dedicated bandwidth to each port. The access-layer devices merge
into one or more distribution-layer devices. The distribution device
provides Layer 2 connectivity between access switches and acts as a
central connection point for all the switches in the wiring closets.
The distribution layer also provides Layer 3 functionality, which
supports routing and networking services. The distribution layer
shields the switch block against failures in other parts of the
network as shown in the Figure.
The distribution device can be one of
the following:
- A switch and external router
combination
- A multilayer switch
If the switch block experiences a
broadcast storm, the router prevents the storm from propagating into
the core and across the rest of the network. Each block is protected
from the other blocks when failures occur. However, the switch
block, in which the broadcast storm occurs, still experiences
network problems until the device generating the broadcasts is found
and removed from the network.
A switch may support one or more
subnets. Remember that a subnet must reside within one broadcast
domain. This means that all stations residing in or ports configured
on the same VLAN are assigned network addresses within the same
subnet.
The broadcast-isolation feature of
VLANs is the characteristic that allows VLANs to be identified with
subnets. For example, the IP ARP propagates only within the VLAN of
the originating request. All subnets terminate on Layer 3 devices,
such as a router or a route switch module (RSM). To connect to
devices in other VLANs, the frame must traverse a Layer 3 device.
Using this model, VLANs should not extend beyond the distribution
switch.
Access devices have redundant
connections, or uplinks, to the distribution switch to maintain
resiliency. The Spanning-Tree Protocol allows these redundant links
to exist while preventing undesirable loops in the switch block. The
Spanning-Tree Protocol terminates at the boundary of the switch
block.
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