IP Routing in the LAN

This chapter covers the following exam topics:

• 1.0 Network Fundamentals

• 1.6 Configure and verify IPv4 addressing and subnetting

• 2.0 Network Access

• 2.4 Configure and verify (Layer 2/Layer 3) EtherChannel (LACP)

The preceding two chapters showed how to configure an IP address and mask on a router interface, making the router ready to route packets to/from the subnet implied by that address/mask combination. While true and useful, all the examples so far ignored the LAN switches and the possibility of VLANs. In fact, the examples so far show the simplest possible cases: the attached switches as Layer 2 switches, using only one VLAN, with the router configured with one ip address command on its physical interface. This chapter takes a detailed look at how to configure routers so that they route packets to/from the subnets that exist on each and every VLAN.

Because Layer 2 switches do not forward Layer 2 frames between VLANs, a network must use routers to route IP packets between subnets to allow those devices in different VLANs/subnets to communicate. To review, Ethernet defines the concept of a VLAN, while IP defines the concept of an IP subnet, so a VLAN is not equivalent to a subnet. However, the set of devices in one VLAN are typically also in one subnet. By the same reasoning, devices in two different VLANs are normally in two different subnets. For two devices in different VLANs to communicate with each other, routers must connect to the subnets that exist on each VLAN, and then the routers forward IP packets between the devices in those subnets.

This chapter discusses the configuration and verification steps related to three methods of routing between VLANs with three major sections:

• VLAN Routing with Router 802.1Q Trunks: The first section discusses how to configure a router to use VLAN trunking as connected to a Layer 2 switch. The router does the routing, with the switch creating the VLANs. The link between the router and switch use trunking so that the router has an interface connected to each VLAN/subnet. This feature is known as routing over a VLAN trunk and also known as router-on-a-stick (ROAS).

• VLAN Routing with Layer 3 Switch SVIs: The second section discusses using a LAN switch that supports both Layer 2 switching and Layer 3 routing (called a Layer 3 switch or multilayer switch). To route, the Layer 3 switch configuration uses interfaces called switched virtual interfaces (SVI), which are also called VLAN interfaces.

• VLAN Routing with Layer 3 Switch Routed Ports: The third major section of the chapter discusses an alternative to SVIs called routed ports, in which the physical switch ports are made to act like interfaces on a router. This third section also introduces the concept of an EtherChannel as used as a routed port in a feature called Layer 3 EtherChannel.

“Do I Know This Already?” Quiz

Take the quiz (either here or use the PTP software) if you want to use the score to help you decide how much time to spend on this chapter. The letter answers are listed at the bottom of the page following the quiz. Appendix C, found both at the end of the book as well as on the companion website, includes both the answers and explanations. You can also find both answers and explanations in the PTP testing software.

Table 17-1 “Do I Know This Already?” Foundation Topics Section-to-Question Mapping

• 1. Router 1 has a Fast Ethernet interface 0/0 with IP address 10.1.1.1. The interface is connected to a switch. This connection is then migrated to use 802.1Q trunking. Which of the following commands could be part of a valid configuration for Router 1’s Fa0/0 interface? (Choose two answers.)

  • a. interface fastethernet 0/0.4

  • b. dot1q enable

  • c. dot1q enable 4

  • d. trunking enable

  • e. trunking enable 4

  • f. encapsulation dot1q 4

• 2. Router R1 has a router-on-a-stick (ROAS) configuration with two subinterfaces of interface G0/1: G0/1.1 and G0/1.2. Physical interface G0/1 is currently in a down/down state. The network engineer then configures a shutdown command when in interface configuration mode for G0/1.1 and a no shutdown command when in interface configuration mode for G0/1.2. Which answers are correct about the interface state for the subinterfaces? (Choose two answers.)

  • a. G0/1.1 will be in a down/down state.

  • b. G0/1.2 will be in a down/down state.

  • c. G0/1.1 will be in an administratively down state.

  • d. G0/1.2 will be in an up/up state.

• 3. A Layer 3 switch has been configured to route IP packets between VLANs 1, 2, and 3 using SVIs, which connect to subnets 172.20.1.0/25, 172.20.2.0/25, and 172.20.3.0/25, respectively. The engineer issues a show ip route connected command on the Layer 3 switch, listing the connected routes. Which of the following answers lists a piece of information that should be in at least one of the routes?

  • a. Interface Gigabit Ethernet 0/0.3

  • b. Next-hop router 172.20.2.1

  • c. Interface VLAN 2

  • d. Mask 255.255.255.0

• 4. An engineer has successfully configured a Layer 3 switch with SVIs for VLANs 2 and 3. Hosts in the subnets using VLANs 2 and 3 can ping each other with the Layer 3 switch routing the packets. The next week, the network engineer receives a call that those same users can no longer ping each other. If the problem is with the Layer 3 switching function, which of the following could have caused the problem? (Choose two answers.)

  • a. Six (or more) out of 10 working VLAN 2 access ports failing due to physical problems

  • b. A shutdown command issued from interface VLAN 4 configuration mode

  • c. VTP on the switch removing VLAN 3 from the switch’s VLAN list

  • d. A shutdown command issued from VLAN 2 configuration mode

• 5. A LAN design uses a Layer 3 EtherChannel between two switches SW1 and SW2, with port-channel interface 1 used on both switches. SW1 uses ports G0/1, G0/2, and G0/3 in the channel. Which of the following are true about SW1’s configuration to make the channel be able to route IPv4 packets correctly? (Choose two answers.)

  • a. The ip address command must be on the port-channel 1 interface.

  • b. The ip address command must be on interface G0/1 (lowest numbered port).

  • c. The port-channel 1 interface must be configured with the no switchport command.

  • d. Interface G0/1 must be configured with the routedport command.

• 6. A LAN design uses a Layer 3 EtherChannel between two switches SW1 and SW2, with port-channel interface 1 used on both switches. SW1 uses ports G0/1 and G0/2 in the channel. However, only interface G0/1 is bundled into the channel and working. Think about the configuration settings on port G0/2 that could have existed before adding G0/2 to the EtherChannel. Which answers identify a setting that could prevent IOS from adding G0/2 to the Layer 3 EtherChannel? (Choose two answers.)

  • a. A different STP cost (spanning-tree cost value)

  • b. A different speed (speed value)

  • c. A default setting for switchport (switchport)

  • d. A different access VLAN (switchport access vlan vlan-id)

Answers to the “Do I Know This Already?” quiz:

• 1 A, F

• 2 B, C

• 3 C

• 4 C, D

• 5 A, C

• 6 B, C

Foundation Topics