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Cisco CCNA And CCENT Certification Exam Training :

Configuring The Router In "Router On A Stick"

By Chris Bryant, CCIE #12933

In the first part of this CCNA 640-802 certification exam tutorial on when to configure router-on-a-stick, we talked about the reasons why we'll use ROAS. Now we'll begin the actual configuration, starting with the router. Here's the network we're using:

Router On A Stick CCNA 640-802 Lab

Let's move up to the router and begin the ROAS configuration. The next important detail is that the Fast Ethernet port on the router will be using subinterfaces, and we'll use two commands on each subinterface:

the encapsulation command, matching the encap type set on the connecting switch's trunk port
an appropriate IP address for the VLAN indicated by the encapsulation command

Sounds complicated, but it's not. However, you have to use the encapsulation command first. If you try to apply an IP address onto the subinterface first, here's the message you get.

R1(config)#int fast 0/0.2
R1(config-subif)#ip address 172.12.2.1 255.255.255.0

% Configuring IP routing on a LAN subinterface is only allowed if that
subinterface is already configured as part of an IEEE 802.10, IEEE 802.1Q, or ISL vLAN.

So let's do put the subinterface into the correct ISL VLAN, and then apply the IP address.

R1(config-subif)#encapsulation isl 2
R1(config-subif)#ip address 172.12.2.1 255.255.255.0

Let's use IOS Help to look at the options we had for the encapsulation command, and just why I have that number "2" in bold.

R1(config)#int fast 0/0.2
R1(config-subif)#encapsulation ?
dot1Q IEEE 802.1Q Virtual LAN
isl Inter Switch Link - Virtual LAN encapsulation

R1(config-subif)#encapsulation isl ?
<1-1000> Virtual LAN Identifier.

R1(config-subif)#encapsulation isl 2

R1(config-subif)#ip address 172.12.2.1 255.255.255.0

The IP address must come from the address space of the VLAN we indicate with the encapsulation command. This interface will be part of VLAN 2, so we had to put an IP address from the 172.12.2.0 /24 subnet. Where did I get that IP range? Check the IP address of the host that's already in VLAN 2.

CCNA Router On A Stick Lab

Make sure you have the right VLAN ID associated with the appropriate IP address on the router subinterfaces!

Now let's configure a subinterface to be part of VLAN 4. That subinterface will need an IP address from the 172.12.4.0 /24 subnet.

R1(config-subif)#int fast 0/0.4
R1(config-subif)#encap isl 4
R1(config-subif)#ip address 172.12.4.1 255.255.255.0

When you're done, don't forget to open the physical interface!

R1(config-subif)#int fast 0/0
R1(config-if)#no shut
R1(config-if)#^Z

*Nov 27 04:35:53.675: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up

*Nov 27 04:35:54.871: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to up

I'll run show interface fast 0/0.2 to see what we can see!

R1#show interface fast 0/0.2
FastEthernet0/0.2 is up, line protocol is up
Hardware is AmdFE, address is 000a.4164.31c1 (bia 000a.4164.31c1)
Internet address is 172.12.2.1/24
MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ISL Virtual LAN, Color 2.
ARP type: ARPA, ARP Timeout 04:00:00
Last clearing of "show interface" counters never

The encapsulation type is listed along with the VLAN number - that's behind the term "Color". We'll verify fast 0/0.4 as well.

R1#show interface fast 0/0.4
FastEthernet0/0.4 is up, line protocol is up
Hardware is AmdFE, address is 000a.4164.31c1 (bia 000a.4164.31c1)
Internet address is 172.12.4.1/24
MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ISL Virtual LAN, Color 4.
ARP type: ARPA, ARP Timeout 04:00:00
Last clearing of "show interface" counters never

So far, so good! There's work left to be done, though, and it's not all on the router. We'll continue our ROAS configuration and do a little troubleshooting as well in the next installment of my exclusive Cisco CCENT / CCNA certification exam tutorial series!

Five Minutes From Now, You Can Be Studying For Exam Success Just As Thousands Of Other Candidates Around The World Have - With Chris Bryant, CCIE #12933 With You Every Step Of The Way - With The Ultimate CCNA Exam Study Package!

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