Design Guide

6
LAN Baseline Architecture Overview—Branch Office Network
OL-11333-01
Multilayered Branch Architecture
• Network Admission Control (NAC) to protect against viruses
With many of these services provided at the access layer, the best design practice should integrate all
these services seamlessly either at Layer 2 or Layer 3 access. The following sections provide more
details of the considerations that go into the design of an access layer and the various elements of the
access layer.
Layer 2 versus Layer 3 at Access Layer
There are two options for the switches in the access layer. The first option is to use Layer 2 at the access
layer, and the second option is to enable routing and to use VLANs to place users in different groups at
the access layer. These two options are shown in
Figure 3.
Figure 3 Layer 2 versus Layer 3 at the Access Layer
Layer 2 Access
Traditionally, the switches deployed at the access layer operate at Layer 2, which can result in the
following two spanning tree issues for some customers:
• Troubleshooting is more difficult
• Convergence in high availability designs can take longer in case of switch or link failure
These problems arise in a traditional, highly-available architecture. In a traditional design, two
distribution switches and an access switch are involved with a Layer 2 loop, as shown in
Figure 4.
29xx or 35xx
Access Switches
Access
Distribution
Layer 3
Layer 3
Layer 2 at Access
Layer 3 at Access
Core
Or
Edge
3560 and
above
Core
Or
Edge
AccessPoint
AccessPoint
180056