Design Guide

LAN Baseline Architecture Overview—Branch Office Network

OL-11333-01

Multilayered Branch Architecture

VLANs help to segregate the traffic from different endpoints. For example, voice, video, and data can

be segregated by putting the devices into different VLANs. VLANs are also widely used to segregate

different users. The proliferation of VLANs results in the various types of spanning tree protocols;

Spanning Tree Protocol (STP) prevents loops from being formed when switches or bridges are

interconnected by multiple links. Spanning tree protocols such as IEEE 802.1D spanning tree, which was

initially designed to protect from Layer 2 loops, has evolved. Multiple types of spanning tree protocols

can currently be deployed when using VLANs, including the following:

• Common Spanning Tree

• Per VLAN Spanning Tree (PVST)

• Per VLAN Spanning Tree Plus (PVST+)

• Multiple Instance Spanning Tree (MISTP/802.1S)

• Rapid Spanning Tree (RSTP/802.1W)

Note More information on STP can be found at the following URL:

http://www.cisco.com/en/US/partner/tech/tk389/tk621/tsd_technology_support_protocol_home.html

The limitations of 802.1D standard spanning tree protocol, such as slow convergence after a topology

change, are eliminated by RSTP. RSTP also supports Cisco specific features such as PortFast,

UplinkFast, and BackboneFast for faster network convergence. With RSTP, the convergence time is

reduced to a few hundred milliseconds as opposed to the standard 30 to 40 seconds with the 802.1D

standard. Cisco switches that support 802.1s/w spanning tree protocols should be deployed to achieve

quick convergence.

Voice and Data VLANs

Convergence of voice and data into a single infrastructure lowers the overall cost of ownership of a

network, and simplifies administration and maintenance through the elimination of separate voice and

data infrastructures.

Convergence also implies that to provide the reliability and quality for the voice and data applications,

the traffic type has to be identified at the edge of the network so that appropriate QoS parameters can be

applied to the traffic.

Cisco switches allows both the voice and data devices to be connected to a single physical port. On Cisco

switches, the concept of access port has been extended, and it is possible to configure a voice and data

VLAN. The switch can now receive traffic on two VLANs, as shown in

Figure 7.