configuration mode. To enable CEF operation on an interface after the CEF operation has been disabled, use the ip route-cache cef command in interface configuration mode. The ip route-cache cef command does not have any parameters. Figure shows the command syntax for the ip cef and ip route-cache cef commands. Figure describes considerations for using the ip cef [distributed] parameter. You can display entries in the FIB or display a summary of the FIB with the show ip cef command in user EXEC or privileged EXEC mode. Figure shows an example of this show command. The show ip cef command has several parameters as described in Figure . show ip cef [unresolved | summary]
show ip cef [network [mask [longer-prefixes]]] [detail]
show ip cef [type number] [detail]

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Content 4.3 Implementing Frame Mode MPLS 4.3.3 Configuring MPLS on a Frame Mode Interface MPLS is enabled on an interface as a step in implementing frame mode MPLS. Figure shows some of the considerations when enabling frame mode MPLS, including choosing between protocols.Cisco routers can use one or both of two protocols at this point:

• Tag Distribution Protocol (TDP): TDP is Cisco proprietary protocol. TDP is a two-party protocol that runs over a connection-oriented transport layer that has guaranteed sequential delivery. Tag-switching routers use this protocol to communicate tag binding information to their peers. Tag-switching routers create tag bindings and then distribute the tag binding information among other tag-switching routers. TDP provides the means for tag-switching routers to distribute, request, and release tag binding information for multiple network layer protocols. TDP also provides the means to open, monitor, and close TDP sessions and to indicate errors that occur during those sessions. TCP is used as the transport for TDP.
• Label Distribution Protocol (LDP): From a historical and functional standpoint, LDP is a superset of Cisco's proprietary TDP. LDP provides a standard methodology for hop-by-hop, or dynamic label, distribution in an MPLS network by assigning labels to routes that have been chosen by the underlying Interior Gateway Protocol (IGP) routing protocols. The resulting labeled paths, called label switched paths (LSPs), forward label traffic across an MPLS backbone to particular destinations. These capabilities enable Service Providers to implement Cisco's MPLS-based IP VPNs and IP+ATM services across multivendor MPLS networks. LDP provides the means for LSRs to request, distribute, and release label prefix binding information to peer routers in a network. LDP enables LSRs to discover potential peers and to establish LDP sessions with those peers for exchanging label binding information.

MPLS support is enabled by default in Cisco routers. However, if it becomes necessary to re-enable it, then the mpls ip command must be executed in global configuration mode. The mpls ip command can also be configured on an interface as shown in Figure . Note
Enabling MPLS forwarding of IP packets for the platform does not enable it for a platform interface. For a given interface to perform dynamic label switching, this switching function must be enabled for the interface and for the platform. MPLS can be disabled using the no mpls ip interface configuration command. You must configure MPLS by using the mpls ip command in interface configuration mode on every frame mode interface that will participate in MPLS. After enabling MPLS on the interface, you must select the label distribution protocol by using the mpls label protocol command in interface configuration mode. Depending on the Cisco IOS version, when issuing a show running-config command, the mpls ldp commands appear as tag-switching commands. The default MPLS label distribution protocol changes from TDP to LDP. If no protocol is explicitly configured by the mpls label protocol command, LDP is now the default label distribution protocol. You can save the LDP configuration commands by using the mpls ip form of the command rather than the tag-switching form. Commands were previously saved using the tag-switching form of the command for backward compatibility. To enable label switching of IP version 4 (IPv4) packets on an interface, use the mpls ip command in interface configuration mode. The mpls ip command does not have any parameters. This command starts LDP on all interfaces on a Cisco router. To select TDP, you must use the mpls label protocol tdp command, globally or for each interface. By default, label switching of IPv4 packets is disabled on an interface. To select which label distribution protocol is used on an interface, use the mpls label protocol command in interface configuration mode: mpls label protocol [tdp | ldp | both] Figure describes the parameters for the mpls label protocol command. LDP is the default protocol on Cisco IOS software Release 12.4(3) and later. In older releases, TDP was the default protocol. Note
For backward compatibility, the mpls syntax will be entered as tag-switching syntax in the configuration by the Cisco IOS software. Configuring MPLS on a Frame Mode Interface
Figure shows an example of enabling MPLS on an Edge LSR. The configuration includes an access control list (ACL) that denies any attempt to establish an LDP session from an interface that is not enabled for MPLS. In the example in the figure, Router A has “NoLDP” ACL on Serial 3/1 interface, which is not enabled for MPLS. You must globally enable CEF switching, which automatically enables CEF on all interfaces that support this form of switching. Note
CEF is not supported on logical interfaces, such as loopback interfaces. Non-backbone (non-MPLS) interfaces have an input ACL that denies TCP sessions on the well-known port number 711 (TDP uses TCP port 711). If you are using LDP, filter on UDP port 646 (LDP uses UDP port 646). Using this ACL is a precaution; without the mpls ip command on the interface, LDP cannot be established on Serial 3/1. Configuring TDP and LDP
Figure shows how to combine Cisco routers with equipment of other vendors that can require the use of both TDP and LDP to be used. TDP can be replaced by LDP on point-to-point interfaces. However, you can also use both protocols on shared media if some devices do not support TDP. Label switching is independent of the distribution protocol, so there should be no problem in mixing the two protocols. TDP and LDP function in a similar way, and both populate the LIB table.

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Content 4.3 Implementing Frame Mode MPLS 4.3.4 Configuring the MTU Size in Label Switching The MTU size is configured in label switching as a step in implementing frame mode MPLS as shown in Figure . This is an optional step that changes the maximum size of labeled packets. Because of the additional label header, the MTU on LAN interfaces should be increased in order to prevent IP fragmentation. The MPLS MTU size has to be increased on all routers that are attached to a LAN segment. The default MTU size on the LAN segments is 1500 bytes. The size of the MPLS MTU depends on the application you are running with MPLS. When you are using pure MPLS in the backbone, MTU size increases for one label header only to 1504 bytes. When you are implementing MPLS VPN, MTU size has to increase for two label headers to 1508 bytes. When you are implementing MPLS VPN with Traffic Engineering, the MTU size should increase for three label headers to 1512 bytes. One way of preventing labeled packets from exceeding the maximum size (and fragmenting as a result) is to increase the MTU size of labeled packets for all segments in the label switched path (LSP) tunnel. The problem of packets exceeding the maximum size will typically occur on LAN switches, where it is more likely that a device does not support oversized packets (also called jumbo