Specifications

18 Copyright © 2009, Juniper Networks, Inc.
DESIGN GUIDE - Data Center LAN Connectivity Design Guide
(RSTP) was created to combat this, providing sub-second convergence but only on point-to-point links. The IEEE
802.1s Multiple Spanning Tree Protocol (MSTP) standard supports multiple instances of STP, but it also increases
configuration complexity.
Using Layer 2 versus Layer 3 at the Access Layer
Access switches can be configured to use Layer 2 STP bridging protocols or Layer 3 routing protocols.
Using Layer 2 at the access layer
Using Layer 2 at the access layer is a traditional configuration providing plug-and-play configuration and making the
deployment in smaller networks easier to implement and manage.
Since this option typically requires Spanning Tree with legacy solutions, troubleshooting can be more difficult in more
complex networks, and convergence in case of a switch or link failure often takes too long for larger highly-available
data center LANs.
Figure 8: Layer 2 versus Layer 3 at access layer
Using Layer 3 at the access layer
Routing is enabled on the switch when using Layer 3 at the access layer, but it still provides the ability to put data
flows into different VLANs. Routing at Layer 3 to the access layer eliminates the creation of layer 2 loops and the
need for spanning tree. Furthermore, Layer 3 routing is more deterministic. In this scenario STP can be disabled,
making it easier to troubleshoot, which is important in larger networks. Using OSPF or other open-standard
protocols for rapid convergence, delivers sub-second convergence. For larger or more complex networks, using
layer 3 routing to the access layer lowers maintenance and administrative costs in comparison to using Layer 2 at
the access layer.
Deploying layer 3 routing to the access layer is often more costly to deploy with legacy network equipment as it
usually requires the additional purchase of a layer 3 software license.
Unlike competitive products, the Juniper Networks data center solution provides the ability to implement either
Layer 2 or Layer 3 at the access layer without any added expense—Layer 3 features are built into the base license,
and no extra license required. Instead of STP, the Juniper data center solution also uses open-standard protocols
such as OSPF for rapid convergence. LAN designs using the EX4200 Ethernet Switches with Virtual Chassis
technology also benefit from Redundant Trunk Group (RTG) protocol as a built-in, optimized replacement to STP
for sub-second convergence and automatic load balancing. And, according to an independent 2007 Lake Partners3
study, time spent operating Juniper Networks solutions running JUNOS Software can be up to 25 percent lower
than competitive solutions. Since cost is not an issue, LAN size and complexity best determine when each solution
is most appropriate.
L2
Switch
L2/L3
Switch
Layer 2 at Access Layer 3 at Access
L2/L3
Switch
L2/L3
Switch
L2/L3
Switch
L2/L3
Switch
WAN EDGE
LAYER 3
LAYER 2
LAYER 2
LAYER 3
AGGREGATION LAYER
ACCESS LAYER