Chapter 17 - IP Routing in the LAN

Question 1

True or False. Physical ports in an EtherChannel must all match speed and duplex.

Answer 1

True.

Question 2

What is ROAS?

Answer 2

Router On A Stick - When you have a router that trunks all VLANs to a switch over a single connection.

Question 3

What is an SVI?

Answer 3

• Switched Virtual Interface is a virtual interface that you can assign an IP address to on a layer 3 switch that corresponds to a VLAN.
• SVI functions at layer 3 whereas a VLAN functions at layer 2.
• They are used on layer 3 switches (except for management on layer 2 switches) to allow inter VLAN routing so that traffic that wants to go between VLANs doesn’t have to be sent back to a router to be routed since a PC in some VLAN can use the SVI as its gateway.
• They can also be setup on layer 2 switches for management.

Question 4

What is a Subinterface?

Answer 4

• A Subinterface is a virtual router interface that is a sub division of a physical router interface.
• This allows each VLAN to have its own subinterface that can have an IP address configured against it, thereby allowing the VLAN to have an IP configured against it.
• This is used when trunking VLANs to a switch (e.g. ROAS)

Question 5

True or False. You need to create a sub interface per VLAN if you want to use ROAS and also want to allow interVLAN routing.

Answer 5

True. However if you are using a layer-3 switch the IP address can be assigned directly to the VLAN interface (SVI) as they don’t necessarily need to be trunked anywhere else. In order to setup a sub interface you will:
- Use the ‘interface <type>' command to create the sub interface (e.g. fa0/1.10 for VLAN 10, fa0/1.20 for VLAN 20)
- Use the 'encapsulation dot1q <vlan_id>' command in subinterface configuration mode for the relevant subinterface to be associated with the relevant VLAN and enable 802.1Q
- Use the ip address command while in subinterface configuration mode for the relevant subinterface to assign a subnet to that subinterface, and by exension, the VLAN associated with that subinterface.</vlan_id></type>

Question 6

How would you set up a subinterface for a VLAN?

Answer 6

• Use the ‘interface <type>' command to create the sub interface (e.g. fa0/1.10 for VLAN 10, fa0/1.20 for VLAN 20)</type>
• Use the ‘encapsulation dot1q <vlan_id>' command in subinterface configuration mode for the relevant subinterface to be associated with the relevant VLAN and enable 802.1Q</vlan_id>
• Use the ip address command while in subinterface configuration mode for the relevant subinterface to assign a subnet to that subinterface, and by exension, the VLAN associated with that subinterface.

Question 7

True or False. If you have a subinterface number in use on one physical interface, you can’t then use that same subinterface number on another physical interface.

Answer 7

False. As long as the subinterface numbers are unique on the same physical interface this won’t cause problems. For example you could have fa0/1.10 and fa0/2.10 on the same router without a problem.

Question 8

What two sequences of commands could you use to allow the native VLAN to be configured on a physical interface that currently acts as an 802.1Q trunk?

Answer 8

• Simply configure the subnet of the native VLAN on the physical interface using the ‘ip address’ command in interface configuration mode. All traffic in this subnet will be untagged when going across the trunk.
• Configure the subnet of the native VLAN on a subinterface but use the ‘encapsulation dot1q <vlan_ID> native' command to tell the router that this subinterface is used for the native VLAN and to define the native VLAN ID.</vlan_ID>

Question 9

True or False. If an interface is down, any subinterfaces of that interface will also be down.

Answer 9

True. However you can change the state of a subinterface without it changing the state of the main interface. For example configuring shutdown on fa0/1.1 would set fa0/1.1 to administratively down but wouldn’t change the state of fa0/1.

Question 10

What commands could you use to verify that ROAS is functioning how you want it?

Answer 10

• ‘show vlans’ - This allows you to ensure that all VLANs are being trunked over the correct interface and that all VLANs have the correct networks configured against them.
• ‘show ip route connected’ - This allows you to ensure that all networks are connected on the correct interfaces and sub interfaces.

Question 11

If ROAS is not functioning how you want it to, what should you check?

Answer 11

• That each VLAN is configured on a subinterface on the router and these same VLANs exist on the device on the other side of the trunk.
• That the VLANs have not been pruned by VTP (VLAN Trunking Protocol) or blocked by STP (Spanning Tree Protocol).
• That each sub interface has the correct network configured against it.
• That the native VLAN (if used) is configured on a sub interface or implied on the physical interface on the router and is also configured on the device on the other side of the trunk.
• That none of the physical or sub interfaces are down or administratively down.

Question 12

How would you enable IP routing on a layer 3 switch?

Answer 12

• (Optional and dependent on the model of switch) Use the ‘sdm prefer lanbase-routing’ command in global config which changes the switch forwarding ASIC settings to allow memory for IPv4 routing.
• (Optional depending on the outcome of the first step) Use the ‘reload’ command to reboot the switch and allow it to pickup the new setting.
• Once the switch has rebooted, use the ‘ip routing’ command to enable ip routing.

Question 13

True or False. You require one SVI per VLAN that you would like to be routable at layer 3. (Decribe how to create an SVI)

Answer 13

True. To create the SVI you will:
- Use the ‘interface vlan <vlan_id>' command to create a VLAN interface and consequently give the switch's routing logic a virtual layer 2 interface that an SVI can be configured against.
- Use the 'ip address' command in VLAN interface configuration mode to configure an IP address on the SVI allowing the VLAN to be routable at layer 3.</vlan_id>

Question 14

What is another name for an SVI?

Answer 14

A VLAN interface.

Question 15

What could cause a VLAN interface to be down?

Answer 15

• The VLAN itsself is not defined on the local switch.
• The VLAN must have at least one up/up interface, this can be an access interface or a trunk interface for which the VLAN has not been pruned by VTP or blocked by STP.
• The VLAN itsself nor the VLAN interface cannot not be administratively disabled.

Question 16

What is a routed port?

Answer 16

A routed port is a physical interface on a layer 3 switch that acts as a layer 3 interface (e.g. making routing decisions, de-encapsulating and re-encapsulating packets into frames) and not layer 2. These can be used instead of SVIs in such situations where you only need one interface to become routable rather than using a VLAN and SVI for just a single interface.

Question 17

How would you turn a switch port into a routed port?

Answer 17

Use the ‘no switchport’ command in interface configuration mode. Once it is a routed port it can be treated as if it was a port on a router, as in it can have a network configured against it.

Question 18

When would you use an SVI over a routed port?

Answer 18

If you had multiple interfaces that needed to be part of a particular subnet, you would use an SVI. If you had just a single interface that needed to access a subnet then you would use a routed port (e.g. in a P2P link).

Question 19

What is a benefit of using a Layer 3 EtherChannel?

Answer 19

If you have multiple routed ports to a single device, this will create multiple (physical) possible routes for the neighbor to send traffic over. With a Layer 3 EtherChannel it creates a way of having multiple physical routes to the same subnet while also utilising efficient load balancing.

Question 20

How would you configure a Layer 3 EtherChannel?

Answer 20

In interface configuration mode you would:
- Enter interface/range configuration for the interfaces you want to be part of the EtherChannel.
- Use the ‘no switchport’ command on each interface to make them routed interfaces.
- Use the ‘channel-group <number> mode on' command. This needs to be the same on the local device for all interfaces you want to add into a port channel but can be different on any neighboring devices.
- Move to port channel configuration mode by entering the 'interface port-channel <number>' command.
- Use the 'no switchport' command to make the port-channel itsself a routed port (may already be routed).
- Use the 'ip address' command to give the port-channel an address</number></number>

Question 21

True or False. Layer 3 EtherChannels can be treated as P2P links like single interfaces.

Answer 21

True. This means the link can have it’s own subnet.

Question 22

How could you find out what interfaces are part of a Layer 3 EtherChannel and whether the interfaces are routed or switched?

Answer 22

By using the ‘show etherchannel summary’ command.

Separately, ‘show int status’ will show ports that are routed ports in the VLAN collumn.

Question 23

What parameters must match for a port to be allowed into a Layer 3 EtherChannel?

Answer 23

• ‘no switchport’ must be configured on all participating interfaces
• The speed must match all other interfaces
• The duplex must match all other interfaces

Question 24

True or False. An SVI is configured separately to a VLAN.

Answer 24

True. A VLAN is configured with the ‘interface vlan <vlan-id> command and an SVI is configured by simply configuring an IP address against this VLAN.</vlan-id>

Question 25

True or False. SVIs are shutdown by default.

Answer 25

False. Layer 1 state is up and layer 2 state is down (unless there is an up/up access/trunk port)

Question 26

What are the conditions that an SVI needs to meet for it to appear as up/up?

Answer 26

• The VLAN must exist on the switch. It won’t be created by default just because the SVI has been created.
• There must be at least one access port in the VLAN of the corresponding SVI that is in an up/up state, or one trunk port that allows the VLAN in an up/up state.
• The VLAN itsself must not be shutdown.
• The SVI itsself must not be shutdown.

Question 27

True or False. When configuring ROAS, VLANs don’t need to be explicitly configured on the router, only the sub interfaces do.

Answer 27

True.