VLAN Routing

VLAN Routing

1. ROAS(Router On A Stick)

   • Steps to configure 802.1Q trunking on a router: 1-Use the interface type number.subint command in global configuration mode to create a unique subinterface for each VLAN that needs to be routed. 2-Use the encapsulation dot1q vlan_id command in subinterface configuration mode to enable 802.1Q and associate one specific VLAN with the subinterface. 3-Use the ip address address mask command in subinterface configuration mode to configure IP settings (address and mask).
   • The two options to define a router interface for the native VLAN are -Configure the ip address command on the physical interface, but without an encapsulation command; the router considers this physical interface to be using the native VLAN. -Configure the ip address command on a subinterface, and use the encapsulation dot1q vlan-id native subcommand to tell the router both the VLAN ID and the fact that it is the native VLAN.
   • To check ROAS on the router, you need to start with the intended configuration, and ask questions about the configuration: 1-Is each non-native VLAN configured on the router with an encapsulation dot1q vlan-id command on a subinterface? 2-Do those same VLANs exist on the trunk on the neighboring switch (show interfaces trunk), and are they in the allowed list, not VTP pruned, and not STP blocked? 3-Does each router ROAS subinterface have an IP address/mask configured per the planned configuration? 4-If using the native VLAN, is it configured correctly on the router either on a subinterface (with an encapsulation dot1q vlan-id native command) or implied on the physical interface? 5-Is the same native VLAN configured on the neighboring switch's trunk? 6-Are the router physical or ROAS subinterfaces configured with a shutdown command?

2. Layer 3 Switch SVIs

   • The following steps show how to configure Layer 3 switching using SVIs. Note that on some switches, like the 2960 and 2960-XR switches used for the examples in this book, the ability to route IPv4 packets must be enabled first, with the reload of the switch required to enable the feature. The rest of the steps after Step 1 would apply to all models of Cisco switches that are capable of doing Layer 3 switching. 1- Enable IP routing on the switch, as needed: A. Use the sdm prefer lanbase-routing command (or similar) in global configuration mode to change the switch forwarding ASIC settings to make space for IPv4 routes at the next reload of the switch. B. Use the reload EXEC command in enable mode to reload (reboot) the switch to pick up the new sdm prefer command setting. C. Once reloaded, use the ip routing command in global configuration mode to enable the IPv4 routing function in IOS software and to enable key commands like show ip route. 2-Configure each SVI interface, one per VLAN for which routing should be done by this Layer 3 switch: A. Use the interface vlan vlan_id command in global configuration mode to create a VLAN interface, and to give the switch's routing logic a Layer 3 interface connected into the VLAN of the same number. B. Use the ip address address mask command in VLAN interface configuratino mode to configure an IP address and mask on the VLAN interface, enabling IPv4 routing on that VLAN interface. C. (As needed) Use the no shutdown command in interface configuration mode to enable the VLAN interface (if it is currently in a shutdown state).

3. Layer 3 Switch Routed Ports

   • no switchport
   • Once configured, the routed interface will show up differently in command output in the switch. In particular, for an interface configured as a routed port with and IP address: -show interfaces: Similar to the same command on a router, the output will display the IP address of the interface. (For switch ports, this command does not list an IP address.) -show interfaces status: Under the "VLAN" heading, instead of listing the access VLAN or the word "trunk", the output lists the word "routed", meaning that it is a routed port. -show ip route: Lists the routed port as an outgoing interface in routes. -show interfaces type number switchport: If a routed port, the output is short and confirms that the port is not a switch port. (If the port is a Layer 2 port, this command lists many configuration and status details.)