The router-on-a-stick implementation of inter-VLAN routing requires only one physical interface on a router and one interface on a switch, simplifying the cabling of the router. However, in other implementations of inter-VLAN routing, a dedicated router is not required.
Multilayer switches can perform Layer 2 and Layer 3 functions, replacing the need for dedicated routers to perform basic routing on a network. Multilayer switches support dynamic routing and inter-VLAN routing.
Click the Play button in the figure to see an animation of how switch-based inter-VLAN routing occurs.
As seen in the animation:
1. PC1 on VLAN 10 is communicating with PC3 on VLAN 30 through switch S1 using VLAN interfaces configured for each VLAN.
2. PC1 sends its unicast traffic to switch S2.
3. Switch S2 tags the unicast traffic as originating on VLAN 10 as it forwards the unicast traffic out its trunk link to switch S1.
4. Switch S1 removes the VLAN tag and forwards the unicast traffic to the VLAN 10 interface.
5. Switch S1 routes the unicast traffic to its VLAN 30 interface.
6. Switch S1 then retags the unicast traffic with VLAN 30 and forwards it out the trunk link back to switch S2.
7. Switch S2 removes the VLAN tag of the unicast frame and forwards the frame out to PC3 on port F0/6.
To enable a multilayer switch to perform routing functions, the multilayer switch must have IP routing enabled.
Multilayer switching is more scalable than any other inter-VLAN routing implementation. This is because routers have a limited number of available ports to connect to networks. Additionally, for interfaces that are configured as a trunk line, limited amounts of traffic can be accommodated on that line at one time.
With a multilayer switch, traffic is routed internal to the switch device, which means packets are not filtered down a single trunk line to obtain new VLAN-tagging information. A multilayer switch does not, however, completely replace the functionality of a router. Routers support a significant number of additional features, such as the ability to implement greater security controls. Rather, a multilayer switch can be thought of as a Layer 2 device that is upgraded to have some routing capabilities.
Note: In this course, configuring inter-VLAN routing on a switch is restricted to configuring static routes on a 2960 switch, which is the only routing functionality supported on the 2960 switches. The 2960 switch supports up to 16 static routes (including user-configured routes and the default route) and any directly connected routes and default routes for the management interface; the 2960 switch can have an IP address assigned to each switch virtual interface (SVI). For a full-featured, relatively inexpensive multilayer switch, the Cisco Catalyst 3560 Series switches support the EIGRP, OSPF, and BGP routing protocols.