Cisco Catalyst Multilayer Switching – InterVLAN Routing

By default, although VLANs can span the entire Layer 2 switched network, hosts in one VLAN cannot communicate directly with hosts in another VLAN. In order to do so, traffic must be routed between the different VLANs. This is referred to as interVLAN routing. The three methods of implementing interVLAN routing in switched LANs, including their advantages and disadvantages, will be described in the following sections:

• InterVLAN Routing Using Physical Router Interfaces
• InterVLAN Routing Using Router Subinterfaces
• InterVLAN Routing Using Switched Virtual Interfaces

InterVLAN Routing Using Physical Router Interfaces

The first method of implementing interVLAN routing for communication entails using a router with multiple physical interfaces as the default gateway for each individually configured VLAN. The router can then route packets received from one VLAN to another using these physical LAN interfaces. This method is illustrated below in Figure 7-1:

Fig. 7-1.  InterVLAN Routing Using Multiple Physical Router Interfaces

Figure 7-1 illustrates a single LAN using two different VLANs, each with an assigned IP subnet. Although the network hosts depicted in Figure 7-1 are connected to the same physical switch, because they reside in different VLANs, packets between hosts in VLAN 10 and those in VLAN 20 must be routed, while packets within the same VLAN are simply switched.

The primary advantage of using this solution is that it is simple and easy to implement. The primary disadvantage, however, is that it is not scalable. For example, if 5, 10, or even 20 additional VLANs were configured on the switch, the same number of physical interfaces as VLANs would also be needed on the router. In most cases, this is technically not feasible.

When using multiple physical router interfaces, each switch link connected to the router is configured as an access link in the desired VLAN. The physical interfaces on the router are then configured with the appropriate IP addresses and the network hosts are either statically configured with IP addresses in the appropriate VLAN, using the physical router interface as the default gateway, or dynamically configured using DHCP. The configuration of the switch illustrated in Figure 7-1 is illustrated in the following output:

The router illustrated in Figure 7-1 is configured as shown in the following output:

InterVLAN Routing Using Router Subinterfaces

Implementing interVLAN routing using subinterfaces addresses the scalability issues that are possible when using multiple physical router interfaces. With subinterfaces, only a single physical interface is required on the router and subsequent subinterfaces are configured off that physical interface. This is illustrated below in Figure 7-2:

Fig. 7-2.  InterVLAN Routing Using Router Subinterfaces

Figure 7-2 depicts the same LAN illustrated in Figure 7-1. In Figure 7-2, however, only a single physical router interface is being used. In order to implement an interVLAN routing solution, subinterfaces are configured off the main physical router interface using the interface [name].[subinterface number] global configuration command. Each subinterface is associated with a particular VLAN using the encapsulation [isl|dot1Q] [vlan] subinterface configuration command. The final step is to configure the desired IP address on the interface.

On the switch, the single link connected to the router must be configured as a trunk link. If the trunk is configured as an 802.1Q trunk, a native VLAN must be defined, if a VLAN other than the default will be used as the native VLAN. This native VLAN must also be configured on the respective router subinterface using the encapsulation dot1Q [vlan] native subinterface configuration command. The following output  illustrates the configuration of interVLAN routing using a single physical interface (also referred to as ‘router-on-a-stick’). The two VLANs depicted in Figure 7-2 are illustrated in the following output, as is an additional VLAN used for Management; this VLAN will be configured as the native VLAN:

The router illustrated in Figure 7-2 is configured as shown in the following output:

The primary advantage of this solution is that only a single physical interface is required on the router. The primary disadvantage is that the bandwidth of the physical interface is shared between the various configured subinterfaces. Therefore, if there is a lot of interVLAN traffic, the router can quickly become a bottleneck in the network.

InterVLAN Routing Using Switched Virtual Interfaces

Multilayer switches support the configuration of IP addressing on physical interfaces. These interfaces, however, must be configured with the no switchport interface configuration command to allow administrators to configure IP addressing on them. In addition to using physical interfaces, Multilayer switches also support Switched Virtual Interfaces (SVIs).

SVIs are logical interfaces that represent a VLAN. Although the SVI represents a VLAN, it is not automatically configured when a Layer 2 VLAN is configured on the switch and must be manually configured by the administrator using the interface vlan [number] global configuration command. The Layer 3 configuration parameters, such as IP addressing, are then configured on the SVI in the same manner as they would be on a physical interface.

The following output illustrates the configuration of SVIs to allow interVLAN routing on a single switch. This output references the VLANs used in previous configuration outputs in this section:

When using Multilayer switches, SVIs are the recommended method for configuring and implementing an interVLAN routing solution.