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default. To verify that the ATM-LSR has the VC merge capability enabled, use the show mpls atm-ldp capability command. If VC merge is not enabled, use the mpls ldp atm vc-merge command to re-enable it. For troubleshooting cell-mode MPLS, use standard commands such as show mpls interfaces for verification.

Loop Prevention

Before moving into cell-mode MPLS configuration, let’s talk just a bit about loop detection and prevention. Cell-mode loop detection and prevention is a little different than in frame-mode MPLS because ATM cells do not have a TTL field. Cell-mode MPLS devices rely, just as their frame-mode counterparts do, on the routing protocol to ensure that the network is loop-free and to prevent loops from occurring.

Instead of the TTL value, Cisco uses the LDP hop-count object Type- Length-Value (TLV). When an ATM-LSR receives a Label Request message with the LDP hop-count object TLV, it increments the hop-count value by 1. Frame-mode MPLS decrements the TTL, but cell-mode MPLS increments the TLV. The default Cisco hop count is 254, and that value is configurable. When the maximum TLV value is reached, a loop-detected message is sent back to the device that originated the label request.

Cell-Mode MPLS Configuration

In frame-mode MPLS, a VC is set up between PE (ATM edge-LSR) routers. To configure the PE LC-ATM interface, a sub-interface is configured with the point-to-point command option. The IOS commands for frame-mode MPLS configuration on a PE ATM edge-LSR are as follows:

interface ATM1/0 no ip address

!

interface ATM1/0.1 point-to-point

To configure the PE LC-ATM interface for cell-mode MPLS, the subinterface is configured with the mpls command option. The IOS commands for cell-mode MPLS configuration on a PE ATM edge-LSR are as follows:

interface ATM1/0 no ip address

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!

interface ATM1/0.1 mpls

To configure the PE LC-ATM interface for cell-mode tag switching, the sub-interface is configured with the tag-switching command option. The IOS commands for cell-mode tag switching configuration on a PE ATM edge-LSR are as follows:

interface ATM1/0 no ip address

!

interface ATM1/0.1 tag-switching

The configuration in this section is only for cell-mode MPLS on routers with LC-ATM interfaces. IP addresses and MPLS, or tag switching, must still be enabled on the LC-ATM interface.

To configure cell-mode MPLS on an ATM-LSR, the mpls ip command is used under the interface. (There are no sub-interfaces on an ATM-LSR.) The IOS commands for cell-mode MPLS configuration on an ATM-LSR are as follows:

!

interface ATM1/0/0 mpls ip

The IOS commands for cell-mode tag switching configuration on an

ATM-LSR are as follows:

!

interface ATM1/0/0 tag-switching ip

It looks just like frame-mode router configuration, doesn’t it? (The answer is Yes. If you think No, go back and reread Chapter 2.)

Summary

In this chapter, you learned about frame-mode MPLS in ATM networks and cell-mode MPLS. When ATM switches do not support MPLS, PE routers with ATM interfaces, acting as ATM edge-LSRs, are connected

Exam Essentials 109

together with a standard ATM PVC. To configure the ATM edge-LSRs, the ATM sub-interface is configured with the point-to-point option, which indicates frame-mode MPLS.

For cell-mode MPLS, an ATM switch functions as an ATM-LSR. Cellmode MPLS is different from frame-mode MPLS in that with cell-mode MPLS, an unlabeled packet enters the network and the ingress ATM edgeLSR and the VPI/VCI mappings are used as the label. Each ATM-LSR in the LSP through the service provider network switches the packet based solely on the VPI/VCI values. In cell-mode MPLS, both the ATM switch, now functioning as an ATM-LSR, and the edge routers with ATM interfaces functioning as ATM edge-LSRs need to be configured for cell-mode MPLS. On the ATM edge-LSR, you need to specify either mpls or tag-switching as an option when configuring the ATM sub-interface. On the ATM-LSR, an interface is configured for MPLS or tag switching using either the mpls ip or the tag-switching ip options. To save labels and fix cell interleaving, Cisco uses VC merge, which is enabled on ATM-LSRs by default.

For loop prevention, both cell-mode and frame-mode MPLS make use of the routing protocol to prevent a loop. Frame-mode MPLS decrements the TTL value, and cell-mode increments the TLV.

Exam Essentials

Understand frame-mode MPLS in an ATM environment. In an ATM environment, frame-mode MPLS indicates that the ATM switches do not support MPLS. ATM edge-LSRs are connected together with PVCs through the non-MPLS ATM network.

Understand cell-mode MPLS. ATM switches operate by switching cells based on VPI/VCI values. ATM switches can’t examine labels; they must switch traffic based only on VPI/VCI values. MPLS works in an ATM network by mapping the label to the VPI/VCI value. In essence, the label replaces the VPI/VCI value. The process of an unlabeled IP packet entering the network, having a label imposed, and then being label-switched has not changed. The only thing that’s different is that in ATM, the label is the VPI/VCI value.

Be able to configure cell-mode MPLS on an ATM-LSR. For cell-mode

MPLS, both the ATM-LSR and ATM edge-LSRs need to be configured.

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On a PE ATM edge-LSR such as a router, the relevant IOS commands for cell-mode MPLS configuration are as follows:

interface ATM1/0 no ip address

!

interface ATM1/0.1 mpls

Remember that on the ATM edge-LSR, it is the mpls or tag-switching command option on the sub-interface configuration that configures it as cell-mode MPLS.

The relevant IOS commands for cell-mode tag switching configuration on a PE ATM edge-LSR are as follows:

interface ATM1/0 no ip address

!

interface ATM1/0.1 tag-switching

To configure cell-mode MPLS on an ATM-LSR, the mpls ip command is used under the interface. (There are no sub-interfaces on an ATM-LSR.) The relevant IOS commands for cell-mode MPLS configuration on an ATM-LSR are as follows:

!

interface ATM1/0/0 mpls ip

To configure cell-mode tag switching on an ATM-LSR, the tagswitching ip command is used under the interface. (There are no subinterfaces on an ATM-LSR.) The relevant IOS commands for cell-mode tag switching configuration on an ATM-LSR are as follows:

!

interface ATM1/0/0 tag-switching ip

Be able to configure frame-mode MPLS on an ATM edge-LSR. For frame-mode MPLS, only the ATM edge-LSRs need to be configured. There are no ATM switches supporting MPLS-only routers with ATM interfaces connected through a standard ATM PVC. On the ATM edge-LSR such as a router, the relevant IOS commands for frame-mode