Router on a Stick is an uncommon configuration however you must understand the technology concepts to become a great network engineer. This lab will discuss and demonstrate the configuration and verification of inter-vlan routing known as Router on a Stick.
The only way to get off a layer two network segment is through a layer three device; commonly referred to as a Default Gateway for host machines. So let’s say for example you have a single switch with 5 different VLAN’s and machines on each VLAN; in which case each VLAN would require its own router to get out of that layer two network to a different layer two network. So does this mean that multiple routers are required or a router with 5 physical ethernet interfaces, one interface in each VLAN?
The answer is easier then you’d initially think. In this case you only need a SINGLE router and that’s it. Think back to the previous lab discussing Dot1q trunk interfaces. 802.1q trunk interfaces carry all VLAN traffic.
A single Router can utilize an 802.1q trunk link to place a sub-interface in each VLAN using a single physical link and technically have interfaces in all VLAN’s.
A Sub-Interface is a logical interface partitioned off from a physical interface. A sub-interface allows you to have multiple interface configurations on a single physical interface. In this lab you’ll use sub-interfaces to match the VLAN’s in the trunk to allow for interfaces in each VLAN and accomplish inter-vlan routing for the hosts as the router has an interface in each layer three network.
In this lab you’ll familiarize yourself with the following new commands;
|encapsulation dot1Q #||This command is executed from ethernet sub-interface configuration mode and binds the sub interface(s) to a particular 802.1q tagged vlan.|
|no ip routing||This command is executed from global configuration and disables the routers ability to be used as a router, effectively turning it into a test client machine like a windows box.|
|ip default-gateway 18.104.22.168||This command is executed in global configuration mode to configure a non-routing device to use the specific IP Address as a default-gateway.|
|show ip interface brief||This command can be executed from user or privileged mode to view the current IP addresses of all interfaces on the device.|
Step 1. – Configure a new Sub-Interface on R1 to match the VLAN 20 (Fa0/0.20) and configure the sub-interface to use 802.1q encapsulation and the Dot1q tag of 20. Configure the sub-interface to use the IP address 10.1.20.1/24.
To create a new sub-interface you’ll use the interface fa0/0.# command in global configuration mode. To enable the sub-interface to use 802.1q you’ll use the encapsulation dot1q # command whereas # is the dot1q VLAN tag as shown below;
R1 con0 is now available Press RETURN to get started. R1>enable R1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. R1(config)#interface fa0/0 R1(config-if)#no shut R1(config-if)#interface fa0/0.20 R1(config-subif)#encapsulation dot1q 20 R1(config-subif)#ip add 10.1.20.1 255.255.255.0 R1(config-subif)#exit R1(config)#
Step 2. – Configure a new Sub-Interface on R1 to match the VLAN 30 (Fa0/0.30) and configure the sub-interface to use 802.1q encapsulation and the Dot1q tag of 30. Configure the sub-interface to use the IP address 10.1.30.1/24. Verify your sub-interface configuration.
R1(config)#interface fa0/0.30 R1(config-subif)#encapsulation dot1q 30 R1(config-subif)#ip add 10.1.30.1 255.255.255.0 R1(config-subif)#end R1#sh run interface fa0/0.20 Building configuration... Current configuration : 96 bytes ! interface FastEthernet0/0.20 encapsulation dot1Q 20 ip address 10.1.20.1 255.255.255.0 end R1#sh run interface fa0/0.30 Building configuration... Current configuration : 96 bytes ! interface FastEthernet0/0.30 encapsulation dot1Q 30 ip address 10.1.30.1 255.255.255.0 end R1#show ip interface brief | inc FastEthernet0/0 Interface IP-Address OK? Method Status Protocol FastEthernet0/0 unassigned YES unset up up FastEthernet0/0.20 10.1.20.1 YES manual up up FastEthernet0/0.30 10.1.30.1 YES manual up up R1#
Step 3. – Disable IP Routing on R2 and R3 and configure the default gateway on R2 and R3 to use R1’s respected Sub-interface as the default gateway.
To disable IP Routing on R2 and R3 use the no ip routing command in global configuration. To specify a default gateway use the ip default-gateway x.x.x.x as shown below;
R2 con0 is now available Press RETURN to get started. R2>enable R2#configure terminal Enter configuration commands, one per line. End with CNTL/Z. R2(config)#no ip routing R2(config)#ip default-gateway 10.1.20.1 R2(config)#end R2#
R3 con0 is now available Press RETURN to get started. R3>enable R3#configure terminal Enter configuration commands, one per line. End with CNTL/Z. R3(config)#no ip routing R3(config)#ip default-gateway 10.1.30.1 R3(config)#end R3#
Step 4. – Verify that R2 can ping R3’s FastEthernet0/0 interface using R1 as the default-gateway as shown below;
R2#ping 10.1.30.3 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 10.1.30.3, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 8/23/52 ms R2#