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Answers to Review Questions

1.B. The command to display label bindings in an MPLS environment is show mpls forwarding-table.

2.D. The MPLS label stack header is 32 bits in total size, or 4 octets.

3.B, C. The MPLS label stack is often referred to as a shim header because it resides between the Layer 2 header and Layer 3 payload.

4.C. The label portion of the MPLS label stack is 20 bits in length.

5.C. The command to display label bindings in a tag-switching environment is show tag forwarding-table.

6.B. The correct terminology for an MPLS-capable router/switch is that of a label switch router (LSR).

7.A. Network devices under control of the service provider and that only connect to other provider devices are called P devices.

8.B. Labels enter the service provider network as unlabeled IP. The PE, which is an edge-LSR, imposes a label.

9.C. To improve performance, the penultimate (next-to-last) router in the LSP pops the label and forwards it to the next hop router as an unlabeled packet.

10.B. The Experimental (EXP) field of the MPLS label stack is used for QoS. Packets enter the network as unlabeled IP. An edge-LSR applies the label and can set a value in the Experimental field that is used for QoS by other LSRs.

11.C. The major applications for MPLS are QoS, VPNs, and traffic engineering. An argument could be made that MPLS changes how routing protocols are used by service providers, but MPLS does not replace the need for them.

12.A. The ability to stack labels makes traffic engineering possible in MPLS networks. Label stacking also makes MPLS VPNs possible.

13.D. Cisco’s proprietary way of moving tagged packets through a network is called tag switching.

14.C. The proprietary protocol used by Cisco tag switching to exchange tags is Tag Distribution Protocol (TDP).

Answers to Review Questions 33

15.A. The protocol used by MPLS to exchange labels is Label Distribution Protocol (LDP).

16.D. Cisco Express Forwarding (CEF) creates an optimized, “cached” version of the routing table. CEF is a requirement for MPLS and tag switching.

17.B. A value of 1 in this field indicates the bottom, or last label, of the stack.

18.C. An FEC is a grouping of IP packets that are treated the same way. For unicast-based routing, an IP prefix is the equivalent of an FEC.

19.A. A label-switched path (LSP) is a unidirectional set of label switch routers (LSRs) that a labeled packet must flow through.

20.A. The proper term for an ATM switch that is MPLS-enabled is ATM-LSR.

Chapter 1, “An Introduction to MPLS,” introduced you to the basic operation of MPLS. You learned that with MPLS, packets are switched instead of routed. Unlabeled IP packets enter the service provider network at the edge, and a label is applied. Every label switch router (LSR) in the labelswitched path (LSP) uses that label to label-switch the packet.

This chapter will build on what you already know, adding a little more detail. This chapter starts with a review of traditional Layer 3 routing. To really understand MPLS, you need a solid understanding of Layer 3 routing.

After routing, this chapter takes you though frame-mode MPLS step by step in the “Frame-Mode MPLS Working Example” section. This section builds on the concepts introduced in the previous chapter and focuses on the interaction between MPLS and the routing protocols in the network. If you are not comfortable with LSPs, go back and re-read that section of Chapter 1. Labels and how they are bound to routes are described in greater detail in the “Label Distribution” section. Again, if there are any concepts that you are not totally comfortable with, make sure to re-read Chapter 1’s descrip-

tion of labels.

Finally, this chapter will explain troubleshooting and network verification using configurations and output from a simple network.

Routing Review

You might be thinking to yourself, “I don’t need to read this section on routing,” or “I already know all about routing.” Well, you might already know Layer 3 routing, but please read this section carefully anyway. If the ideas discussed here are somewhat new, take the time to really understand everything you’re reading. If your routing skills are rusty, you may have difficulty understanding the interaction of MPLS and routing protocols.

Routing Review 37

So, let’s do a quick and dirty review of routing. Figure 2.1 illustrates a simple Layer 3 routed network that you’ll use for this review.

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To begin this example, let’s say that Host A wants to send some packets to Host B. The first thing that Host A does is determine whether Host B is local (on the same subnet) or remote (on a different subnet). Host A, by comparing its network at 192.168.1.0 to that of Host B at 192.168.3.0, can see that the network portions of the IP addresses do not match, meaning that Host B is remote. Host A, now knowing that Host B is remote, puts a frame on the wire destined for the default gateway. Table 2.3 shows the Layer 2 and Layer 3 information as placed on the wire.

T A B L E 2 . 3 Layer 2 and Layer 3 Information from Host A to Router 1

From Host A to Router 1

As you look over this example, pay close attention to the source and destination IP addresses.

Router 1 knows that the frame is destined for it because it sees its own Ethernet0 MAC address in the destination field in the frame. Router 1 picks the frame up off the wire, discards the Layer 2 information, and looks in the destination part of the Layer 3 header. Router 1, knowing that the packet is destined for network 192.168.3.0, does a Layer 3 lookup and checks its routing table to see if it has an entry for 192.168.3.10. It finds a route to network 192.168.3.0/24 with a next hop of 192.168.2.2 via interface Ethernet1. The following output is the routing table as it exists on Router 1:

Router1#show ip route

R 192.168.3.0/24 [120/1] via 192.168.2.2, 00:00:01, Ethernet1 C 192.168.1.0/24 is directly connected, Ethernet0

C 192.168.2.0/24 is directly connected, Ethernet1

Routing Review 39

Router 1 knows that to get to network 192.168.3.0, it needs to send the packet out of Ethernet1 to 192.168.2.2. Router 1 programmatically moves the packet to the outbound Ethernet1 interface, creates a new frame, and places the new frame on the wire. Table 2.4 lists the Layer 2 and Layer 3 information as it is placed on the wire from Router 1 to Router 2.

T A B L E 2 . 4 Layer 2 and Layer 3 Information from Router 1 to Router 2

From Router 1 to Router 2

Notice in Table 2.4 that only the Layer 2 source and destination MAC addresses have changed. The Layer 3 information is unchanged.

Router 2 knows that the frame is destined for it because it sees its own Ethernet0 MAC address in the destination field in the frame. Router 2 picks the frame up off the wire, discards the Layer 2 information, and looks in the destination part of the Layer 3 header. Router 2, knowing that the packet is destined for 192.168.3.10, does a Layer 3 lookup and checks its routing table to see if it has an entry for 192.168.3.10. It finds a route to network 192.168.3.0/24 with a directly connected interface of Ethernet1. The following output is the routing table as it exists on Router 2:

Router2#show ip route

R 192.168.1.0/24 [120/1] via 192.168.2.1, 00:00:06, Ethernet0 C 192.168.2.0/24 is directly connected, Ethernet0

C 192.168.3.0/24 is directly connected, Ethernet1

Router 2 knows that to get to network 192.168.3.0, it needs to go out the directly connected interface Ethernet1. Router 2 programmatically moves

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the packet to the outbound Ethernet1 interface, creates a new frame, and places the new frame on the wire. Table 2.5 shows the Layer 2 and Layer 3 information as it is placed on the wire from Router 2 to Host B.

T A B L E 2 . 5 Layer 2 and Layer 3 Information from Router 2 to Host B

From Router 2 to Host B

Notice in Table 2.5 that the Layer 3 source and destination addresses remain unchanged.

Host B knows that the frame is destined for it because it sees its own MAC address in the destination field in the frame. Host B pulls the frame off the wire and processes the data it contains.

Let’s do that one more time just to be thorough. Suppose Host B needs to send something back to Host A. First, Host B determines whether Host A is local or remote. Host B, by comparing its network at 192.168.3.0 to that of Host A at 192.168.1.0, can see that the network portions of the IP addresses do not match, meaning that Host A is remote. Host B, now knowing that Host A is remote, puts a frame on the wire destined for the default gateway. Table 2.6 shows the Layer 2 and Layer 3 information as placed on the wire.

T A B L E 2 . 6 Layer 2 and Layer 3 Information from Host B to Router 2

From Host B to Router 2

Router 2 knows that the frame is destined for it because it sees its own Ethernet1 MAC address in the destination field in the frame. Router 2 picks the frame up off the wire, discards the Layer 2 information, and looks in the destination part of the Layer 3 header. Router 2, knowing that the packet is destined for network 192.168.1.10, does a Layer 3 lookup and checks its routing table to see if it has an entry for 192.168.1.10. It finds a route to network 192.168.1.0/24 with a next hop of 192.168.2 via interface Ethernet0. The following output is the routing table as it exists on Router 2:

Router2#show ip route

R 192.168.1.0/24 [120/1] via 192.168.2.1, 00:00:06, Ethernet0 C 192.168.2.0/24 is directly connected, Ethernet0

C 192.168.3.0/24 is directly connected, Ethernet1

Router 2 knows that to get to network 192.168.1.0, it needs to send the packet out of Ethernet0 to 192.168.2.1. Router 2 programmatically moves the packet to the outbound Ethernet0 interface, creates a new frame, and places the new frame on the wire. Table 2.7 shows the Layer 2 and Layer 3 information as it is placed on the wire from Router 1 to Router 2.

T A B L E 2 . 7 Layer 2 and Layer 3 Information from Router 2 to Router 1

From Router 2 to Router 1

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Router 1 knows that the frame is destined for it because it sees its own Ethernet1 MAC address in the destination field in the frame. Router 1 picks the frame up off the wire, discards the Layer 2 information, and looks in the destination part of the Layer 3 header. Router 1, knowing that the packet is destined for 192.168.1.10, does a Layer 3 lookup and checks its routing table to see if it has an entry for 192.168.1.10. It finds a route to network 192.168.1.0/24 with a directly connected interface of Ethernet0. The following output is the routing table as it exists on Router 1:

Router1#show ip route

R 192.168.3.0/24 [120/1] via 192.168.2.2, 00:00:01, Ethernet1 C 192.168.1.0/24 is directly connected, Ethernet0

C 192.168.2.0/24 is directly connected, Ethernet1

Router 1 knows that to get to network 192.168.3.0, it needs to go out the directly connected interface Ethernet0. Router 1 programmatically moves the packet to the outbound Ethernet0 interface, creates a new frame, and places the new frame on the wire. Table 2.8 shows the Layer 2 and Layer 3 information as it is placed on the wire from Router 1 to Host A.

T A B L E 2 . 8 Layer 2 and Layer 3 Information from Router 1 to Host A

From Router 1 to Host A

Host A knows that the frame is destined for it because it sees its MAC address in the destination field in the frame. Host A pulls the frame off the wire and processes the data it contains.

So now that we’re done with the review of Layer 3 routing, there were two very important things you should have noticed:

Throughout the entire process, the source IP address and destination IP address in the Layer 3 header never changes from source device to destination device. Only the Layer 2 information changes from hop to hop.