Multilayer Switching: Bridging Performance and Policy in the Campus LAN

April 2016    11 min read

In the early 2000s, most enterprise networks were designed with a clear separation between Layer 2 switching and Layer 3 routing. But by 2016, that model had evolved. The need for lower latency, policy enforcement at scale, and high availability within the LAN led to widespread adoption of multilayer switching—also known as Layer 3 switching. In this post, we explore the architecture, configuration, and operational considerations of deploying multilayer switches in the campus environment.

What is Multilayer Switching?

Multilayer switching refers to network devices that combine the functionality of traditional routers and switches. They can make routing decisions based on IP addresses (Layer 3) while also performing fast Layer 2 switching—often using hardware-based forwarding for performance. This fusion enables fast inter-VLAN routing, granular access control, and better scalability in LAN designs.

Common Deployment Scenarios

Multilayer switches are typically deployed at the distribution or core layer in enterprise networks. They perform critical roles such as:

• Inter-VLAN routing between user access VLANs
• Applying Access Control Lists (ACLs) at the SVI level
• Enforcing QoS and policy routing
• Enabling HSRP/VRRP for gateway redundancy

In collapsed core architectures, multilayer switches may also replace traditional routers completely, consolidating infrastructure and reducing latency.

SVIs and Routed Interfaces

Switched Virtual Interfaces (SVIs) are logical Layer 3 interfaces tied to VLANs. Instead of relying on a physical router for inter-VLAN traffic, you define SVIs directly on the switch:

    interface vlan 10
     ip address 192.168.10.1 255.255.255.0
     no shutdown

    interface vlan 20
     ip address 192.168.20.1 255.255.255.0
     no shutdown
  

Each SVI acts as the default gateway for hosts in that VLAN. Multilayer switches route between these VLANs using internal ASICs, often at line rate.

Routing Configuration

Multilayer switches support both static and dynamic routing. You can enable routing globally and configure protocols like OSPF, EIGRP, or even BGP:

    ip routing

    router ospf 1
     network 192.168.10.0 0.0.0.255 area 0
     network 192.168.20.0 0.0.0.255 area 0
  

Unlike traditional routers, many Layer 3 switches offer protocol redistribution, route maps, and policy routing, but not all features are supported depending on platform and license.

Access Control and Policy Enforcement

With SVIs in place, it's easy to apply ACLs to control inter-VLAN traffic:

    ip access-list extended block_web
     deny tcp 192.168.10.0 0.0.0.255 any eq 80
     permit ip any any

    interface vlan 10
     ip access-group block_web in
  

This approach enforces segmentation and traffic policy closer to the access layer, reducing load on firewalls and upstream routers.

Redundancy and High Availability

Most enterprise-grade multilayer switches support gateway redundancy using HSRP, VRRP, or GLBP. By pairing two distribution switches, you can ensure seamless failover:

    interface vlan 10
     standby 10 ip 192.168.10.1
     standby 10 priority 110
     standby 10 preempt
  

Link aggregation (LACP or PAgP) is also widely used to create high-speed, redundant uplinks between access and distribution layers.

Performance Considerations

Because multilayer switches offload routing to hardware, performance is generally high. However, the following can impact throughput:

• ACL complexity and logging
• QoS shaping and classification rules
• CPU-based forwarding for control plane traffic

Understanding what is done in hardware (CEF) vs software (process switching) is key when troubleshooting performance issues.

Operational Tips

• Document all SVIs, VLANs, and ACLs with descriptions
• Use route summarization at distribution-to-core handoffs
• Keep control plane traffic isolated (e.g., use loopbacks for OSPF)
• Leverage SNMP, NetFlow, or IP SLA to monitor routing behavior

When Not to Use Multilayer Switching

While powerful, multilayer switches may not be ideal in all scenarios. Avoid using them for:

• Edge firewalls—use dedicated appliances instead
• Encrypted tunnels or VPN aggregation—offload to routers or firewalls
• Dynamic NAT—platforms may have limitations or lack features

Also, ensure licensing and feature sets support your design. Not all “Layer 3 switches” support full routing stacks.

Conclusion

Multilayer switching remains a cornerstone of modern LAN design. It merges performance and policy, reduces hop count, and supports simplified architectures. As networks grow more complex, the ability to enforce access controls, route intelligently, and scale horizontally makes multilayer switches a compelling tool in any enterprise arsenal.

Eduardo Wnorowski is a network infrastructure consultant and Director.
With over 21 years of experience in IT and consulting, he helps organizations build efficient, secure LAN architectures that scale with business needs.
Connect on Linkedin