Inter-VLAN routing is the process of forwarding network traffic from one VLAN to another VLAN using a router. VLANs divide broadcast domains in a LAN environment. Whenever hosts in one VLAN need to communicate with hosts in another VLAN, the traffic must be routed between them. This is known as inter-VLAN routing. On Catalyst switches it is accomplished by creating Layer 3 interfaces (Switch virtual interfaces (SVI)).

1. Inter-VLAN Routing
Inter-VLAN routing is the process of forwarding network traffic from one VLAN to another VLAN using a
router.
VLANs divide broadcast domains in a LAN environment. Whenever hosts in one VLAN need to
communicate with hosts in another VLAN, the traffic must be routed between them. This is known as
inter-VLAN routing. On Catalyst switches it is accomplished by creating Layer 3 interfaces (Switch virtual
interfaces (SVI)).
A host can communicate with only those hosts that are members of the same VLAN. In order to change
this default behaviour and allow communication between different VLANs, you need a router or a layer
3 switch.
The router has to support ISL or 802.1Q trunking on a FastEthernet or GigabitEthernet interface in order
to perform routing between different VLANs. The router’s interface is divided into logical interfaces
called subinterfaces, one for each VLAN. From a FastEthernet or GigabitEthernet interface on the router,
you can set the interface to perform trunking with the encapsulation command:
R1(config)#interface FastEthernet0/0.10
R1(config-subif)#encapsulation ?
dot1Q IEEE 802.1Q Virtual LAN
R1(config-subif)#encapsulation dot1Q ?
<1-4094> IEEE 802.1Q VLAN ID
R1(config-subif)#encapsulation dot1Q 10
Figure 1 Router-based Inter-VLAN routing is a process for
forwarding network traffic from on e VLAN to another VLAN
using a router

2. Inter-VLAN Routing
All Catalyst multilayer switches support the following types of layer 3 interfaces:
1. Routed port- a pure layer 3 port similar to that on a router.
2. Switch virtual interface (SVI)- virtual routed VLAN interface for inter-VLAN routing.
3. Bridge virtual interface (BVI)- a layer 3 bridging interface.
Figure 2 The router supports one Vlan per interface
Figure 3 A single ISL link can support multiple VLANs

3. Inter-VLAN Routing
Router Interface and Subinterface Comparison
a. Port Limits
b. Performance
c. Access ports and Trunk ports
d. Cost
e. Complexity
Physical Interface Subinterface
One Physical interface per VLAN One Physical interface for many VLAN
No bandwidth contention Bandwidth contention
Connected to access mode switch port Connected to trunk mode switch port
More expensive Less expensive
Less complex connection configuration More complex connection configuration
There are 3 inter-VLAN routing device options
1. Layer 3 multilayer Catalyst switch
2. External router that allows trunking (router-on-a-stick)
3. External router with enough interfaces for every VLAN (this doesn’t scale and is very expensive)
Inter-VLAN Routing Types
Figure 4 Inter VLAN Implementation

4. Inter-VLAN Routing
 External Router (Router-on-a-Stick)
A layer two switch can be connected to a single router to allow inter-VLAN communication either
using a single physical link as a trunk with multiple sub-interfaces (a.k.a. router-on-a-stick) or
using seperate physical links between the switch and router for each individual VLAN.
Configuring Router-on-a-Stick
a. Enable trunking on the switch port
b. Enable the router interface with the no shut command
c. Create the subinterfaces on the router for each VLAN
d. Configure IPs and encapsulation on each subinterface as they relate to their VLANs
Switch (conf-subif)# encapsulation [dot1q | isl] vlan-id {native}
Switch (conf-subif)# ip address x.x.x.x x.x.x.x
Example router interface configuration
Router(config)# interface FastEthernet0/0
Router(config-if)#no shutdown
Router(config)# interface FastEthernet 0/0.1
Router(config-subif) description VLAN 1
Router(config-subif)# encapsulation dot1Q 1 native
Router(config-subif)# ip address 10.1.1.1 255.255.255.0
Router(config-subif)# exit
Router(config)# interface FastEthernet 0/0.2
Router(config-subif)# description VLAN 2
Router(config-subif)# encapsulation dot1Q 2
Router(config-subif)# ip address 10.2.2.1 255.255.255.0
Router(config-subif)# exit
Router(config)# end
Example switch trunk interface configuration (connected to router’s Fa 0/0)
switch(config)# interface FastEthernet 4/2
switch(config-if)# switchport trunk encapsulation dot1q
switch(config-if)# switchport mode trunk
Advantages
1. Works with almost all switches because the switches do not have to support layer 3, just
VLANs and trunking.
2. Simple configuration (one switch port, one router interface).

5. Inter-VLAN Routing
Disadvantages
1. Router is a single point of failure.
2. If the trunk becomes congested, it can affect every VLAN.
3. Slightly higher latency because-
-traffic must leave and re-enter the switch.
-the router makes the traffic decisions in software (which is slower than hardware).
Switch Virtual Interfaces
SVIs are virtual VLAN interfaces on multilayer switches; one SVI is created for each VLAN to be routed
and it performs the process for all the packets associated with that VLAN.
Configuring SVIs
a. Enable IP routing
b. Create the VLANs
c. Create the SVI
d. Assign an IP address to each SVI
e. Enable the interface
f. Optional – Enable an IP routing protocol
Example Configuration
Switch# configure terminal
Switch(config)# ip routing
Switch(config)# vlan 10
Switch(config)# interface vlan 10
Switch(config-if)# ip address 10.10.1.1 255.0.0.0
Switch(config-if)# no shutdown
Switch(config)# router rip
Switch(config-router)# network 10.0.0.0
SVIs are commonly used for:
1. Default gateways for users within the VLAN
2. Virtual route between VLANs
3. Provides an IP address for connectivity to the switch itself
4. Can be used as an interface for routing protocols
An SVI is considered “up” when at least one interface in its associated VLAN is active and forwarding
traffic. If all interfaces within that VLAN are down, the SVI goes down to prevent creating a routing loop.

6. Inter-VLAN Routing
Advantages
1. Fast because all performed in hardware
2. No need for external links for routing
3. Low latency (doesn’t need to leave the switch)
Disadvantages
1. May require a more expensive switch.
 Routed Ports
Routed Ports are physical ports on the switch that act much like a router interface with an IP
address configured. Routed Ports are not associated with an particular VLAN and do not run
layer 2 protocols like STP or VTP.
(Note: Routed interfaces also do not support subinterfaces.)
Routed ports are point-to-point links that usually connect core switches to other core switches or
distribution layer switches (if the distribution layer is running layer 3). They can also be used
when a switch has only a single switch port per VLAN or subnet.
Make sure when configuring a routed port that you use the no switchport command to make
sure the interface is configured to operate at layer 3. Also make sure to assign an IP addresses
and any other layer 3 information required. Lastly, check that the appropriate routing protocols
are configured.
A multilayer switch can have both SVIs and routed ports configured. Multilayer switches forward
all layer 2 and 3 traffic in hardware, so it is very fast.
Configuring Inter-VLAN Routing with Routed Ports
1. Select the interface
2. Convert to layer 3 port (no switchport command)
3. Add an IP address
4. Enable the interface (no shut command)
Example Configuration
Core(config)# interface GigabitEthernet 1/1
Core(config-if)# no switchport
Core(config-if)# ip address 10.10.1.1 255.255.255.252
Core(config-if)# exit

7. Inter-VLAN Routing
Multilayer Switching
A Multilayer switch can perform both layer two switching as well as inter-VLAN routing. While I spend a
considerable amount of time walking through the low-level details here, Cisco thinks it is really
important. It’s also easy for Cisco to ask SWITCH exam questions on (like the order of operations), so
take your time and make sure you understand the process. Knowing the order of events within the
switch will help you understand how the many forwarding and filtering options interact.
 Cisco Express Forwarding
Multilayer Switching, or MLS, is a fairly general term used to describe features that enable very
efficient routing of traffic between VLANs and routed ports. Cisco Express Forwarding, or CEF, is
the specific implementation of MLS Cisco uses on their multilayer switches.
Layer 2 Forwarding Process
Input Output
1. Receive frame Apply outbound VLAN ACL
2. Verify integrity Apply outbound QoS ACL
3. Apply inbound VLAN ACL Select outbound port
4. Lookup destination MAC Place in port queue
5. Rewrite
6. Forward frame
Layer 3 Forwarding Process
Input ROUTING Output
1. Receive frame Apply input ACL Apply outbound VLAN
ACL
2. Verify integrity Switch if entry is in CEF cache Apply outbound QoS ACL
3. Apply inbound VLAN ACL Identify exit interface and next
hop address using routing table
Select outbound port
4. Lookup destination MAC Apply outbound ACL Place in port queue
5. Rewrite
6. Forward frame