Cisco IOS Wide-Area Networking Configuration Guide - Free Books

With ISIS packet fragmentation, ISIS packets are often padded to the maximum MTU size. L2TPv3encapsulation increases the packet size by 28 to 36 bytes. A Layer 2 frame with an ISIS packet embeddedmay exceed the tunnel path MTU after L2TPV3 encapsulation. Therefore, L3 fragmentation is oftenneeded. To support fragmentation, the SIP-400 linecard searches for ISIS packets in a Layer 2 Frame. Ifan ISIS packet is found during L2TPv3 encapsulation, the SIP-400 linecard clears the DF bit in the outerIP and sets IP precedence to 6. This allows the IP packet to be fragmented when traveling through theIP core.Ethernet Attachment CircuitsThe SIP-400 linecard supports Ethernet over L2TPv3 in compliance with RFC4719. Two types ofpseudowire are supported: Ethernet VLAN pseudowire type (0x0004) and Ethernet pseudowire type(0x0005). When xconnect is configured on an Ethernet main interface, Ethernet frames are tunneled overL2TPv3 using Ethernet port pseudowires (type 0x0005). In this mode, Ethernet frames received on theport (tagged or untagged) are delivered to the remote CE device unaltered.When xconnect is configured on a dot1q subinterface, the tagged Ethernet frames are tunneled using anEthernet VLAN pseudowire (type 0x0004). In this case, the pseudowire connects one Ethernet VLANto another Ethernet VLAN. Received Ethernet VLAN frames from the CE device are tunneled overL2TPv3 unchanged. When arriving on the destination PE device, the original VLAN tag is written to usethe destination VLAN ID. While doing so, the priority field in the VLAN tag is preserved.Ethernet OAM SupportThe SIP-400 linecard supports service-level OAM and link-level OAM features on Ethernet interfaces.Service-level OAM packets, also known as Connectivity Fault Management (CFM) packets, are sentusing SNAP header with type 0x0126. Link-level OAM packets, also known as Link Monitoring (LM)packets are sent on Ether-Type 0x8809.The SIP-400 linecard monitors the above two types of ingress OAM frames from the CE device. Whenthe OAM frames are found and OAM features are configured on the Ethernet interface, the OAM framesare intercepted and forwarded to the route processor. If there is no Ethernet OAM configuration, all OAMframes are tunneled in L2TPv3 as normal data frames.ATM Attachment CircuitsThe SIP-400 linecard supports ATM over L2TPv3 in compliance with RFC 4454 with minor deviation.RFC 4454 defines four types of ATM pseudowire:• ATM AAL5 SDU VCC transport (0x0002)• ATM Cell transport port mode (0x0003)• ATM cell transport VCC mode (0x0009)• ATM cell transport VPC mode (0x000A).ATM cell transport port mode is not supportedWhen xconnect is configured on a PVC with encapsulation AAL5, ATM AAL5 pseudowire (0x0002) isused to tunnel AAL5 frames between PE devices. The SIP-400 linecard supports Layer 2sublayer-specific control words for AAL5 pseudowire. This is the only type of pseudowire allowed tocarry control words.When xconnect is configured on PVC in AAL0 mode, an ATM cell transport VCC pseudowire (type0x0009) is used. When xconnect is configured on PVP in AAL0 mode, an ATM cell transport VPCpseudowire (type 0x000A) is used. In both types of pseudowire, each L2TPv3 packet carries one ATMcell. Cell packing is not supported.9