Inter-VLAN Routing

There are three options for routing between VLANs:
1. Having a separate, physical router interface for each VLAN....ugh!

• Each router interface has the default gateway IP address for the subnet, and the adjacent switch port is set to mode access.

2. Router on a Stick: all VLAN's traverse and share one link to the router, however, there is more contention for bandwidth than using separate interfaces. This is accomplished by using Virtual Sub-Interfaces on the router.

• R1(config)# interface fa0/1
• R1(config-if)# no ip address
• R1(config-if)# no shut
• R1(config)#interface fa0/1.10     (creates the sub-interface)
• R1(config-if)# encapsulation dot1q 10   ('10' is VLAN number)
• R1(config-if)# ip address 10.10.10.1 255.255.255.0
• R1(config)#interface fa0/1.20
• R1(config-if)# encapsulation dot1q 20
• R1(config-if)# ip address 10.10.20.1 255.255.255.0
• R1(config)# ip router 0.0.0.0 0.0.0.0 203.0.113.2

• SW1(config)# interface fa0/1
• SW1(config-if)# switchport mode trunk

3. Layer 3 Switch  (probably not on the CCNA test)

• Traffic being routed within the campus is routed across the backplane of the switch and not through cabling to an external router.
• You must enable routing on this special layer 3 switch, and you will probably have a router for WAN connectivity and services.
• SVI - Switched Virtual Interfaces are used for routing.

• First, configure routing in the switch and the SVI
• SW1(config)#ip routing
• SW1(config)#interface vlan 10       ( SVI )
• SW1(config-if)# ip address 10.10.10.1 255.255.255.0
• SW1(config)#interface vlan 20       ( SVI )
• SW1(config-if)# ip address 10.10.20.1 255.255.255.0

• Second, config WAN routing on the switch
• SW1(config)#interface fa0/1  ↓ changing to a layer 3 interface ↓
• SW1(config-if)# no switchport  
• SW1(config-if)# ip address 10.10.100.1  255.255.255.0
• SW1(config)# ip route 0.0.0.0 0.0.0.0 10.10.100.2  (default gateway)

• Then matching configuration on the router
• R1(config)# interface fa0/1
• R1(config-if)# ip address 10.10.100.2  255.255.255.0
• R1(config)# interface fa0/2
• R1(config-if)# ip address 203.0.113.1  255.255.255.0
• R1(config)ip route 0.0.0.0 0.0.0.0  203.0.113.2  (default gateway)
• R1(config)ip route 10.10.0.0  255.255.0.0  10.10.100.1  (router for internal LAN traffic - this is a summary router for both the .10 and .20 network)