Cisco IOS Wide-Area Networking Configuration Guide - Free Books

General L2TPv3 Restrictions• CEF must be enabled for the L2TPv3 feature to function. The xconnect configuration mode isblocked until CEF is enabled. On distributed platforms, such as the Cisco 7500 series, if CEF isdisabled while a session is established, the session is torn down and remains down until CEF isreenabled. To enable CEF, use the ip cef or ip cef distributed command.• The IP local interface must be a loopback interface. Configuring any other interface with the ip localinterface command will result in a nonoperational setting.• The number of sessions on a PPP, High-Level Data Link Control (HDLC), Ethernet, or 802.1qVLAN port is limited by the number of interface descriptor blocks (IDBs) that the router cansupport. For PPP, HDLC, Ethernet, and 802.1q VLAN circuit types, an IDB is required for eachcircuit.When L2TPv3 is used to tunnel Frame Relay D channel data-link connection identifiers (DLCIs),an IDB is not required for each circuit. As a result, the memory requirements are much lower. Thescalability targets for the Engineering Field Test (EFT) program are 4000 L2TP session.• Frame Relay support includes only 10-bit DLCI addressing. The L2TPv3 feature does not supportFrame Relay extended addressing.• The interface keepalive feature is automatically disabled on the interface to which xconnect isapplied, except for Frame Relay encapsulation, which is required for Local Management Interface(LMI).• Static L2TPv3 sessions do not support Frame Relay LMI interworking.• Static L2TPv3 sessions do not interoperate with Universal Tunnel Interface (UTI) using keepalives.• The ip pmtu command used to configure the pseudowire class (see the section “Configuring theL2TPv3 Pseudowire”) is not supported for static L2TPv3 sessions. As a result, Layer 2fragmentation of IP packets and Intermediate System-to-Intermediate System (IS-IS) fragmentationthrough a static L2TPv3 session are not supported.• The L2TPv3 Layer 2 (IP packet) fragmentation feature (see “Configuring the L2TPv3 Pseudowire”)is not supported when the CE router is running special Layer 2 options such as Layer 2 sequencing,compression, or encryption. Examples of these options are Frame Relay compression andfragmentation or PPP compression. In these scenarios, the IP payload is not in a format that iscompatible with IP fragmentation.• The Stateful Switchover (SSO), Route Processor Redundancy (RPR) and RPR+ components of theHA functions are only supported at the coexistence level. If you attempt a switchover using SSO,RPR, or RPR+, the tunnels will fail and then eventually recover after an undetermined time duration.This includes both IPv4 and IPv6 traffic.Cisco 7200 Series-Specific Restrictions• ATM port mode cell relay is supported only on the PA-A3-T3, PA-A3-E3, and PA-A3-OC-3 ATMport adapters.• VPI or VPI/VCI rewrite is not supported for any ATM transport mode. Both pairs of PE to CE peerrouters must be configured with matching VPI and VCI values except in OAM local emulationmode. For example, if PE1 and CE1 are connected by PVC 10/100, PE2 and CE2 should also beconnected by PVC 10/100.4