Cisco IOS Wide-Area Networking Configuration Guide - Free Books

Layer Frame Fragmentation RestrictionsLayer 2 frame fragmentation is not supported. Even if the Layer 2 frame recovered after the L2TPv3decapsulation exceeds the Layer 2 MTU on the CE-facing interface, the SIP-400 linecard still sends theentire Layer 2 frame to the CE device. The Layer 2 frame may be dropped on the CE device because ofMRU violations.Layer 2 Virtual Private Network Interworking RestrictionsThe SIP-400 linecard does not support Layer 2 VPN interworking (“like to like” is the only modesupported for L2TPv3 tunneling).Packet Sequencing RestrictionsThe initial release of L2TPv3 focuses on tunneling Ethernet and ATM traffic over L2TPv3. Because ofperformance issues, the SIP-400 linecard does not support L2TPv3 packet sequencing for Ethernet andATM traffic. As a result, the 4-byte Layer 2-specific sublayer control word is not supported for Ethernetpseudowires. Configuring sequencing on a pseudowire will cause L2VPN traffic corruption.By default, sequencing is disabled. However, you can configure sequencing in the pseudowire class,because the pseudowire class may be applied to pseudowires on other 7600 linecards that supportsequencing. You must keep sequencing disabled when the pseudowire is handled on the SIP-400linecard.Counters RestrictionsPer-session counters are provided by the linecard. Per-tunnel counters are not provided.Security and QoS ACLs RestrictionsThe security QoS ACLs are not supported on the Layer 2 interfaces facing customer device, which meansthat you cannot apply ACLs to Layer 2 VPN traffic. (The Security ACL and the QoS ACL can still beapplied to the IP interfaces at the core-facing side.)DF Bit Reflection from Inner IP to Outer IP RestrictionsYou cannot enable or disable DF bit reflection. The SIP-400 linecard makes DF bit reflection a defaultbehavior for traffic on Ethernet interfaces. When an Ethernet frame is received from the CE device, theSIP-400 linecard checks the IP header inside the frame. Once an IP header is found, the DF bit is copiedto the outer tunnel IP during L2TPv3 encapsulation. If no IP header is found inside the Layer 2 frame,the DF bit in the outer IP is set 0.Traffic on ATM interfaces may have a deep stack of Layer 2 encapsulations. For example, the IP packetmay be embedded first in Ethernet, then in SNAP and AAL5. There is no guarantee that the SIP-400linecard will find the IP packet inside the AAL5 envelope. Therefore, DF bit reflection from inner IP toouter IP is not the default behavior for traffic on ATM interfaces.Session CookieA cookie check is supported for data packets. Cookies (remote and local) can be part of the decapsulationtable indexed by session-id.ScalabilityUp to 8,000 pseudowires and 512 tunnels are supported.Set DF Bit in Outer IPWhen the ip dfbit set command is configured for the pseudowire, the SIP-400 linecard sets the DF bitin the outer IP header during L2TPv3 encapsulation. This DF bit handling is subject to ISIS packetfragmentation.7