Cisco IOS Wide-Area Networking Configuration Guide - Free Books

Local SwitchingDistributed SwitchingLocal switching (from one port to another port in the same router) is supported for both static anddynamic sessions. You must configure separate IP addresses for each xconnect statement.See the section “Configuration Examples for Layer 2 Tunnel Protocol Version 3” for an example of howto configure local port switching.Distributed CEF switching is supported for L2TP on the Cisco 7500 series routers.NoteFor the Cisco 7500 series, sequencing is supported, but all L2TP packets that require sequence numberprocessing are sent to the RSP.L2TPv3 Layer 2 FragmentationBecause the reassembly of fragmented packets is computationally expensive, it is desirable to avoidfragmentation issues in the service provider network. The easiest way to avoid fragmentation issues isto configure the CE routers with an path maximum transmission unit (MTU) value that is smaller thanthe pseudowire path MTU. However, in scenarios where this is not an option, fragmentation issues mustbe considered. L2TP initially supported only the following options for packet fragmentation when apacket is determined to exceed the L2TP path MTU:• Unconditionally drop the packet• Fragment the packet after L2TP/IP encapsulation• Drop the packet and send an Internet Control Message Protocol (ICMP) unreachable message backto the CE routerThe L2TPv3 Layer 2 Fragmentation feature introduces the ability to allow IP traffic from the CE routerto be fragmented before the data enters the pseudowire, forcing the computationally expensivereassembly to occur in the CE network rather than in the service-provider network. The number offragments that must be generated is determined based on the discovered pseudowire path MTU.To enable the discovery of the path MTU for Layer 2 traffic, enter the ip pmtu command in a pseudowireclass configuration (see “Configuring the L2TPv3 Pseudowire” section on page 58). On the PE router,the original Layer 2 header is then copied to each of the generated fragments, the L2TP/IP encapsulationis added, and the frames are forwarded through the L2TPv3 pseudowire.Because the Don’t Fragment (DF) bit in the Layer 2 encapsulation header is copied from the inner IPheader to the encapsulation header, fragmentation of IP packets is performed on any packets receivedfrom the CE network that have a DF bit set to 0 and that exceed the L2TP path MTU in size. To preventthe reassembly of fragmented packets on the decapsulation router, you can enter the ip dfbit setcommand in the pseudowire class configuration to enable the DF bit in the outer Layer 2 header.34