Frame-Mode MPLS and ATM Configuration 97

an MPLS-capable router in the path from PE1 to PE2, a label would be imposed.

So, what do you need to remember about frame-mode MPLS? Well, part of the MPLS architecture is that control-plane information must be exchanged with plain old IP. ATM does not have interfaces supporting plain old IP. To implement MPLS in conjunction with a non-MPLS ATM core, PVCs must be set up between PE routers.

The problem with this method of MPLS deployment is scalability. To allow for maximum redundancy and optimum routing, a full mesh of VCs must be created between all the ATM edge-LSRs, resulting an full-mesh overlay topology. If you remember back to Chapter 1, “An Introduction to MPLS,” one of the reasons for going to MPLS was to avoid the scalability problems inherent in full-mesh overlay topologies.

However, if you don’t have MPLS-capable switches, this is how it is done. Before moving on to frame-mode MPLS and ATM configuration, let’s

talk about loop detection and prevention. Frame-mode MPLS uses standard IP TTL to detect routing loops. Without MPLS, an IP packet has its TTL decremented by 1 by each router it passes through. For MPLS, the TTL (Time- to-Live) field from the IP TTL is decremented by 1 by the ingress edge-LSR, and then copied into the MPLS label TTL field. Upon exiting the MPLS network, the MPLS label TTL value is copied back into the IP TTL field. If this field is set to 0, the packet is discarded.

To prevent loops, frame-mode MPLS relies on the routing protocol to ensure that the network is loop-free. By relying on the routing protocol

to prevent loops, an LSR uses the same loop-prevention mechanisms as nonMPLS routers.

Frame-Mode MPLS and ATM Configuration

The trick in configuring frame-mode MPLS does not have anything to do with which MPLS or tag switching commands are used. The trick is how the ATM sub-interface on the ATM edge-LSR router is set up.

There are two options available when setting up ATM sub-interfaces: mpls and point-to-point. If a sub-interface is set up as mpls, when MPLS is enabled, it will run in cell mode. When a sub-interface is set up as point- to-point, when MPLS is enabled, it will run in frame mode.

98 Chapter 3 MPLS and ATM

Let me illustrate. Chapter 2 introduced you to basic MPLS configuration. For example, the IOS commands to configure MPLS on a router are as follows:

PE1#config t

PE1(config)#ip cef

PE1(config)#mpls ip

PE1(config-if)#interface serial 0/0

PE1(config-if)#mpls ip

The IOS commands to configure tag switching on a router are as follows:

PE1#config t

PE1(config)#ip cef

PE1(config)#tag-switching advertise-tags

PE1(config-if)#interface serial 0/0

PE1(config-if)#tag-switching ip

In the preceding configuration commands, frame-mode MPLS is enabled because the interface in question is a serial interface. Frame-mode MPLS means that label distribution will be independent control with unsolicited downstream.

So, what does all this have to do with ATM? Let me show you. Earlier in this section, I said that the trick is in how the ATM sub-interface on the ATM edge-LSR router is set up. When an ATM sub-interface is configured with the point-to-point option, MPLS operates in frame mode. For example, in Figure 3.4, PE1 and PE2 are connected with an ATM PVC. The ATM subinterfaces on the PE routers are configured with the point-to-point command.

F I G U R E 3 . 4 ATM frame-mode interface configuration network

When MPLS is set up on a serial interface, it runs in frame mode. When MPLS is set up on a point-to-point ATM sub-interface, it runs in frame mode. The relevant configuration, assuming that the global configuration

Frame-Mode MPLS and ATM Configuration 99

tasks are complete, for MPLS on a router with an ATM interface is as follows:

interface ATM1/0 no ip address

!

interface ATM1/0.1 point-to-point

ip address 192.168.1.5 255.255.255.0 pvc 0/100

encapsulation aal5snap mpls ip

To verify your configuration, use the show mpls ldp neighbor command:

ATM_P1# show mpls ldp neighbor

Peer LDP Ident: 192.168.1.1:0; Local TDP Ident 192.168.1.2:0 TCP connection: 192.168.1.1:11033 - 192.168.1.2.647

State: Oper; PIEs sent/rcvd: 8/8; ; Downstream Up time: 00:02:15

LDP discovery sources: ATM1/0.1

Addresses bound to peer LDP Ident: 192.168.1.10 192.168.1.1

The configuration to enable tag switching on a router with an ATM interface, assuming that tag switching is globally enabled, is as follows:

interface ATM1/0 no ip address

!

interface ATM1/0.1 point-to-point

ip address 192.168.1.5 255.255.255.0 pvc 0/100

encapsulation aal5snap tag-switching ip

To verify your configuration, use the show tag-switching tdp neighbor command:

ATM_P1# show tag-switching tdp neighbor

Peer TDP Ident: 192.168.1.1:0; Local TDP Ident