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Type escape sequence to abort.

Tracing the route to 192.168.2.1

1 192.168.3.6 16 msec 16 msec 16 msec

2 204.134.83.10 44 msec 44 msec 44 msec

3 192.168.3.10 [AS 65002] 60 msec * 60 msec

What’s missing from the traceroute output without the no tagswitching propagate-ttl command? The Core router. To return the network to its original configuration, you need to use the tag-switching propagate-ttl command.

Summary

To keep packets from being dropped, a traditional router needs to know about all the destination networks or have a default route. If a packet arrives for a destination network that the router doesn’t know how to reach, the packet will be dropped. Not every router in an MPLS network needs full knowledge of every destination network. Typically, PE routers are running BGP and an IGP. P routers only run an IGP, thus freeing up resources on the P routers.

Customer devices do not need MPLS functionality, and packets are sent to a PE router as unlabeled IP. The PE router performs a label imposition and sends the newly labeled packet along its way. Each LSR in the LSP does not examine the Layer 3 portion of the packet, only the label. Actually, if the LSR examines the packet, the packet is dropped if the LSR does not have a corresponding route to the destination network. Before the labeled packet leaves the network, the next-to-last LSR in the path pops the label through a process called penultimate hop popping. The egress PE forwards the packet based on the Layer 3 header.

Frame-mode MPLS is called independent control with unsolicited downstream. Using this technique, LSPs are set up quickly. As soon as an IP prefix appears, a label is bound to it immediately. Neighbors are told of the new binding without having to request it.

Troubleshooting an MPLS or tag switching network is pretty straightforward. The best way to test to make sure that everything works is to do a ping from one CE device to another CE device. If it works, then MPLS or tag

Exam Essentials 85

switching is enabled and working properly. If the ping fails, it is likely that a core service provider router is not configured with MPLS or tag switching.

If you are sure that MPLS or tag switching is configured properly, you should perform basic troubleshooting such as verifying that the interface is properly attached, is up, or has the proper encapsulation to fix the connectivity problem.

Exam Essentials

Understand routing protocols in an MPLS network. With MPLS deployed in the core of the service provider network, it is not necessary to run BGP on every network device. In the core of the network, an IGP such as OSPF or IS-IS is used. On edge routers, an IGP and BGP is used.

Be able to identify packet flow in an MPLS network. Customer devices do not need MPLS functionality, and they send unlabeled IP packets to the PE router. The PE router, or edge-LSR, imposes a label and sends the packet to the next LSR in the LSP. Each LSR along the LSP does not examine the Layer 3 information of the packet. Packets are labelswitched using the MPLS label. The next-to-last router in the LSP pops the label and forwards it as unlabeled IP. The LSR receiving the unlabeled IP packet does a Layer 3 lookup and forwards the packet to its ultimate destination.

Be able to configure tag switching. Tag switching uses TDP to exchange tags with neighboring TSRs using well-known TCP port 711. For tag switching to work, CEF must be enabled. Once CEF has been enabled, tag switching is globally configured. Once tag switching has been globally configured, each appropriate interface needs to be configured as well. The commands to configure tag switching on a router are as follows:

P1#config t

P1(config)#ip cef

P1(config)#tag-switching advertise-tags

P1(config-if)#interface serial 0/0

P1(config-if)#tag-switching ip

Be able to configure MPLS. Configuring MPLS is very similar to configuring tag switching. Instead of TDP, MPLS uses LDP to exchange

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labels with neighboring LSRs using well-known TCP port 646. Just like tag switching, MPLS requires that CEF be enabled globally. Once CEF has been enabled, MPLS is configured globally. Each appropriate interface also needs to be configured for MPLS as well. The commands to configure MPLS are as follows:

P1#config t

P1(config)#ip cef

P1(config)#mpls ip

P1(config-if)#interface serial 0/0

P1(config-if)#mpls ip

Understand frame-mode label distribution. Frame-mode MPLS label distribution is called independent control with unsolicited downstream. When a new FEC appears on an LSR, a label is immediately bound to it.

This is called independent control. Once a new label is bound to the FEC, the LSR tells its neighbors about it without them having to ask. This is called unsolicited downstream.

Key Terms

Before you take the exam, be certain you are familiar with the following terms:

Review Questions 87

Review Questions

1.Which IOS command enables CEF in global configuration mode?

A.cef ip

B.cef enable

C.ip cef

D.ip cef enable

2.Which IOS command enables MPLS in global configuration mode?

A.ip mpls

B.mpls ip

C.mpls advertise labels

D.tag-switching advertise labels

3.Which IOS command enables MPLS on an interface?

A.ip mpls

B.mpls ip

C.mpls advertise labels

D.tag-switching advertise labels

4.Which IOS command enables tag switching globally on a router?

A.ip mpls

B.mpls ip

C.tag-switching advertise-tags

D.tag-switching advertise labels

5.Which IOS command enables tag switching on an interface?

A.ip mpls

B.mpls ip

C.ip tag-switching

D.tag-switching ip

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6.Which IOS command do you use to verify TDP neighbors?

A.show tag-switching neighbor

B.show tag-switching tdp neighbor

C.show tag-switching tdp-neighbor

D.tag-switching ip neighbor

7.What IOS command do you use to verify LDP neighbors?

A.show mpls neighbor

B.show mpls ldp neighbor

C.show mpls ldp-neighbor

D.mpls ip neighbor

8.Which protocol does Cisco’s tag switching use to exchange labels between neighbors?

A.CDP

B.LDP

C.TDP

D.MDP

9.Which protocol does MPLS use to exchange labels between neighbors?

A.CDP

B.LDP

C.TDP

D.MDP

10.Frame mode label distribution can be described as __________ with

__________?

A.Independent control; downstream-on-demand

B.Independent control; unsolicited downstream

C.Ordered control; downstream-on-demand

D.Ordered control; unsolicited downstream

Review Questions 89

11.Which MPLS-enabled device uses ordered control?

A.ATM-LSR

B.Edge-LSR

C.LSR

D.LSP

12.Which TCP port does TDP use to exchange tags with a peer?

A.17

B.1017

C.117

D.711

13.Which TCP port does LDP use to exchange labels with a peer?

A.46

B.646

C.1046

D.466

14.Which device in the network typically imposes labels on packets?

A.C

B.CE

C.P

D.PE

15.Which device does not need MPLS functionality?

A.CE

B.PE

C.P

D.None of the above

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16.Packets enter the network at the __________ and exit at the

__________.

A.Incoming router; exiting router

B.Ingress router; exiting router

C.Ingress router; egress router

17.When a labeled packet is received by an LSR, the LSR ___________.

A.Does a Layer 3 lookup

B.Label-switches the packet

C.None of the above

18.The LDP/TDP identifier is ____________. (Choose the best answer.)

A.The highest IP address of an active interface

B.The lowest IP address of the configured loopback interfaces

C.The lowest IP address of an active interface

D.The highest IP address of the configured loopback interfaces

19.MPLS in an ATM environment uses __________ with __________.

A.Independent control; downstream-on-demand

B.Ordered control; downstream-on-demand

C.Independent control; unsolicited downstream

D.Ordered control; unsolicited downstream

20.TDP and LDP are ___________.

A.Compatible

B.The same

C.Incompatible

D.None of the above