Design Guide

9
LAN Baseline Architecture Overview—Branch Office Network
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Multilayered Branch Architecture
Figure 6 Layer 3 at the Access Layer
Note It is quite possible that sub-second convergence under failover scenarios is achievable with both EIGRP
and OSPF routing protocols. The testing has not been done.
Layer 3 at the access is not recommended in the branch office designs because of the following reasons:
• Higher costs involved with deploying such a solution.
• The Layer 2 access solution provides a platform to seamlessly integrate all the various services
discussed in the previous sections.
• The distribution layer ties all the services together and is analogous to the campus core.
Adopting routing at the access layer creates a very thin Layer 2 domain on the switch. This Layer 2
domain provides the necessary VLANs for the end devices. With this solution, there are no Layer 2
loops, and spanning tree influence is diminished such that it can be disabled to make troubleshooting
easier. From a Layer 2 perspective, this is a low maintenance and quick convergence solution. Although
it might increase the cost in some situations because the low-end Layer 2 switches cannot be used in this
solution, it is a viable solution if cost is not a factor.
VLANs and Spanning Tree Protocol
VLANs are Layer 2 broadcast domains. The traffic in a VLAN is confined to the VLAN until it is routed
either into a different VLAN or into a traditional Layer 3 network. A VLAN consists of several end
systems, either hosts or network equipment (such as switches and routers), all of which are members of
a single logical broadcast domain. The network devices can be members of different VLANs as well.
Traffic between two switches that are members of different VLANs is carried on a common link between
the two switches while maintaining the broadcast domain. These links are called trunks. Several trunking
protocols form a trunk between two switches.
Layer 3
Layer 2
AccessPoint
Core
Or
Edge
Access
Distribution
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