A Layer 2 Virtual Private Network (VPN) emulates a physical sub-network in an IP or MPLS network, by creating private connections between two points. Building a L2VPN network requires coordination between the service provider and customer. The service provider establishes Layer 2 connectivity. The customer builds a network by using the data link resources obtained from the service provider. In a L2VPN service, the service provider does not require information about the customer's network topology and other information. This helps maintain customer privacy, while using the service provider resources to establish the network.

The service provider requires Provider Edge (PE) routers with the following capabilities:

• Encapsulation of L2 protocol data units (PDU) into Layer 3 (L3) packets.
• Interconnection of any-to-any L2 transports.
• Support for MPLS tunneling mechanism.
• Process databases that include all information related to circuits and their connections.

This section introduces Layer 2 Virtual Private Networks (VPNs) and the corresponding Gigabit Ethernet services.

A L2VPN network enables service providers (SPs) to provide L2 services to geographically disparate customer sites. Typically, a SP uses an access network to connect the customer to the core network. This access network may use a mixture of L2 technologies, such as Ethernet and Frame Relay. The connection between the customer site and the nearby SP edge router is known as an attachment circuit (AC). Traffic from the customer travels over this link to the edge of the SP core network. The traffic then tunnels through a pseudowire over the SP core network to another edge router. The edge router sends the traffic down another AC to the customer's remote site.

The L2VPN feature enables the connection between different types of L2 attachment circuits and pseudowires, allowing users to implement different types of end-to-end services.

Note	BOOTP traffic (dst UDP 68) over any type of pseudowire is unsupported.

Cisco IOS XR software supports a point-to-point end-to-end service, where two Ethernet circuits are connected together. An L2VPN Ethernet port can operate in one of two modes:

• Port Mode—In this mode, all packets reaching the port are sent over the pseudowire, regardless of any VLAN tags that are present on the packets. In Port mode, the configuration is performed under the l2transport configuration mode.

• VLAN Mode—Each VLAN on a CE (customer edge) or access network to PE (provider edge) link can be configured as a separate L2VPN connection (using either VC type 4 or VC type 5). To configure L2VPN on VLANs, see The Carrier Ethernet Model chapter in this manual. In VLAN mode, the configuration is performed under the individual sub-interface.

Switching can take place in the following ways:

• AC-to-PW—Traffic reaching the PE is tunneled over a PW (pseudowire) (and conversely, traffic arriving over the PW is sent out over the AC). This is the most common scenario.

• Local switching—Traffic arriving on one AC is immediately sent out of another AC without passing through a pseudowire.

• PW stitching—Traffic arriving on a PW is not sent to an AC, but is sent back into the core over another PW.

Note

If your network requires that packets are transported transparently, you may need to modify the packet’s destination MAC (Media Access Control) address at the edge of the Service Provider (SP) network. This prevents the packet from being consumed by the devices in the SP network. The encapsulation dot1ad vlan-id and encapsulation dot1ad vlan-id dot1q any commands cannot co-exist on the same physical interface or bundle interface. Similarly, the encapsulation dot1q vlan-id and encap dot1q vlan-id second-dot1q any commands cannot co-exist on the same physical interface or bundle interface. If there is a need to co-exist, it is recommended to use the exact keyword in the single tag encapsulation. For example, encap dot1ad vlan-id exact or encap dot1q vlan-id exact. In an interface which already has QinQ configuration, you cannot configure the QinQ Range sub-interface where outer VLAN range of QinQ Range overlaps with outer VLAN of QinQ. Attempting this configuration results in the splitting of the existing QinQ and QinQ Range interfaces. However, the system can be recovered by deleting a recently configured QinQ Range interface. In an interface which already has QinQ Range configuration, you cannot configure the QinQ Range sub-interface where outer VLAN range of QinQ Range overlaps with inner VLAN of QinQ Range. Attempting this configuration results in the splitting of the existing QinQ and QinQ Range interfaces. However, the system can be recovered by deleting a recently configured QinQ Range interface.

You can use the show interfaces command to display AC and pseudowire information.

Link Loss Forwarding (LLF) is supported on Cisco router. The LLF is used to avoid any packet loss and trigger the network convergence through alternate links.

LLF sends signals across the PW to the neighbouring device to bring the PW and far-end AC down if the local AC goes down. The LLF feature supports the l2transport propagate remote-status command used to propagate Layer 2 transport events.

LLF is supported for TenGigE and GigE interfaces and not supported on the Bundle interfaces.

/* Configuring propagation remote-status */
  interface TenGigE 0/0/0/5
    l2transport
      propagate remote-status
     !
     !
 

The Ethernet Data Plane Loopback function allows you to run loopback tests to test the connectivity and quality of connections through a Layer 2 cloud. You can run this test on:

• main interface or sub-interfaces

• bundle or its sub-interfaces

• multiple hops through the underlying network

You can use this feature to test the throughput of an ethernet port remotely. You can verify the maximum rate of frame transmission with no frame loss.

This feature allows for bidirectional or unidirectional throughput measurement, and on-demand or out-of-service (intrusive) operation during service turn-up.

Two types of Ethernet loopback are supported:

• External loopback - Traffic loopback occurs at the Ingress interface. Traffic does not flow into the router for loopback.

• Internal loopback - Traffic loopback occurs at the Egress interface. Traffic loopback occurs after the traffic flows into the router to the other interface.

Ethernet data traffic can be looped back on per port basis. This feature supports a maximum of 100 concurrent Ethernet data plane loopback sessions per system. Filters based on frame header can be used for initiating the loopback session. This ensures that only a subset of traffic that is received on an interface is looped back. You can use Source MAC, Destination MAC, and VLAN Priority (COS bits) as filters.

The Cisco NCS 5500 Series Router supports the Ethernet Local Management Interface (E-LMI) protocol as defined by the Metro Ethernet Forum, Technical Specification MEF 16, Ethernet Local Management Interface (E-LMI), January 2006 standard.

E-LMI runs on the link between the customer-edge (CE) device and the provider-edge (PE) device, or User Network Interface (UNI), and provides a way for the CE device to auto-configure or monitor the services offered by the PE device (see this figure).

E-LMI is an asymmetric protocol whose basic operation involves the User-facing PE (uPE) device providing connectivity status and configuration parameters to the CE using STATUS messages in response to STATUS ENQUIRY messages sent by the CE to the uPE.

The E-LMI protocol as defined by the MEF 16 standard, defines the use of only two message types—STATUS ENQUIRY and STATUS.

These E-LMI messages consist of required and optional fields called information elements, and all information elements are associated with assigned identifiers. All messages contain the Protocol Version, Message Type, and Report Type information elements, followed by optional information elements and sub-information elements.

E-LMI messages are encapsulated in 46- to 1500-byte Ethernet frames, which are based on the IEEE 802.3 untagged MAC-frame format. E-LMI frames consist of the following fields:

• Destination address (6 bytes)—Uses a standard MAC address of 01:80:C2:00:00:07.

• Source address (6 bytes)—MAC address of the sending device or port.

• E-LMI Ethertype (2 bytes)—Uses 88-EE.

• E-LMI PDU (46–1500 bytes)—Data plus 0x00 padding as needed to fulfill minimum 46-byte length.

• CRC (4 bytes)—Cyclic Redundancy Check for error detection.

For more details about E-LMI messages and their supported information elements, refer to the Metro Ethernet Forum, Technical Specification MEF 16, Ethernet Local Management Interface (E-LMI), January 2006.

The basic operation of E-LMI consists of a CE device sending periodic STATUS ENQUIRY messages to the PE device, followed by mandatory STATUS message responses by the PE device that contain the requested information. Sequence numbers are used to correlate STATUS ENQUIRY and STATUS messages between the CE and PE.

The CE sends the following two forms of STATUS ENQUIRY messages called Report Types:

• E-LMI Check—Verifies a Data Instance (DI) number with the PE to confirm that the CE has the latest E-LMI information.

• Full Status—Requests information from the PE about the UNI and all EVCs.

The CE device uses a polling timer to track sending of STATUS ENQUIRY messages, while the PE device can optionally use a Polling Verification Timer (PVT), which specifies the allowable time between transmission of the PE’s STATUS message and receipt of a STATUS ENQUIRY from the CE device before recording an error.

In addition to the periodic STATUS ENQUIRY/STATUS message sequence for the exchange of E-LMI information, the PE device also can send asynchronous STATUS messages to the CE device to communicate changes in EVC status as soon as they occur and without any prompt by the CE device to send that information.

Both the CE and PE devices use a status counter (N393) to determine the local operational status of E-LMI by tracking consecutive errors received before declaring a change in E-LMI protocol status.

Before you configure E-LMI on the router, be sure that you complete the following requirements:

• Identify the local and remote UNIs in your network where you want to run E-LMI, and define a naming convention for them.

• Enable E-LMI on the corresponding CE interface link on a device that supports E-LMI CE operation.

E-LMI is not supported on physical sub-interfaces and bundle main and sub- interfaces. E-LMI is configurable on Ethernet physical interfaces only.

In order to ensure the correct interaction between the CE and the PE, each device has two configurable parameters. The CE uses a Polling Timer (PT) and a Polling Counter; the PE uses a Polling Verification Timer (PVT) and a Status Counter.

/* Configure EVCs for E-LMI/

RP/0/RSP0/CPU0:router# configure 
RP/0/RSP0/CPU0:router(config)# interface TenGigE0/3/0/9/1.1 l2transport 
RP/0/RSP0/CPU0:router(config-subif)# encapsulation dot1q 1
RP/0/RSP0/CPU0:router(config-subif)# xconnect group evpn
RP/0/RSP0/CPU0:router(config)# l2vpn
RP/0/RSP0/CPU0:router(config-l2vpn)# xconnect group evpn
RP/0/RSP0/CPU0:router(config-l2vpn-xc)# p2p p1
RP/0/RSP0/CPU0:router(config-l2vpn-xc-p2p)# interface TenGigE0/3/0/9/1.1
RP/0/RSP0/CPU0:router(config-l2vpn-xc-p2p)# neighbor evpn evi 1 target 3001 source 1
RP/0/RSP0/CPU0:router(config-l2vpn-xc-p2p)#commit


/* Configure Ethernet CFM for E-LMI */

RP/0/RSP0/CPU0:router# configure
RP/0/RSP0/CPU0:router(config)#interface TenGigE0/3/0/9/1.1 l2transport
RP/0/RSP0/CPU0:router(config-subif)# encapsulation dot1q 1
RP/0/RSP0/CPU0:router(config-subif)# ethernet cfm
RP/0/RSP0/CPU0:router(config-if-cfm)#  mep domain irf_evpn_up service up_mep_evpn_1 mep-id 3001
RP/0/RSP0/CPU0:router(config-if-cfm-mep)#exit
RP/0/RSP0/CPU0:router(config)#ethernet cfm
RP/0/RSP0/CPU0:router(config-cfm)# domain irf_evpn_up level 3 id null
RP/0/RSP0/CPU0:router(config-cfm-dmn)#service up_mep_evpn_1 xconnect group evpn p2p p1 id number 1
RP/0/RSP0/CPU0:router(config-cfm-dmn-svc)# mip auto-create all ccm-learning
RP/0/RSP0/CPU0:router(config-cfm-dmn-svc)# continuity-check interval 1m loss-threshold 3
RP/0/RSP0/CPU0:router(config-cfm-dmn-svc)#continuity-check archive hold-time 10
RP/0/RSP0/CPU0:router(config-cfm-dmn-svc)#mep crosscheck
RP/0/RSP0/CPU0:router(config-cfm-xcheck)# mep-id 1
RP/0/RSP0/CPU0:router(config-cfm-xcheck)#ais transmission interval 1m cos 6
RP/0/RSP0/CPU0:router(config-cfm-dmn-svc)#log ais
RP/0/RSP0/CPU0:router(config-cfm-dmn-svc)#log continuity-check errors
RP/0/RSP0/CPU0:router(config-cfm-dmn-svc)#log crosscheck errors
RP/0/RSP0/CPU0:router(config-cfm-dmn-svc)#log continuity-check mep changes
RP/0/RSP0/CPU0:router(config-cfm-dmn-svc)#commit

/* Enable E-LMI on the Physical Interface */

RP/0/RSP0/CPU0:router# configure
RP/0/RSP0/CPU0:router(config)#interface TenGigE0/3/0/9/1
RP/0/RSP0/CPU0:router(config-if)# ethernet lmi
RP/0/RSP0/CPU0:router(config-if-elmi)#commit


/* Configure the Polling Verification Timer */

The MEF T392 Polling Verification Timer (PVT) specifies the allowable time between transmission of a STATUS message and receipt of a STATUS ENQUIRY from the UNI-C before recording an error. The default value is 15 seconds.

RP/0/RSP0/CPU0:router# configure
RP/0/RSP0/CPU0:router(config)#interface gigabitethernet 0/0/0/0
RP/0/RSP0/CPU0:router(config-if)# ethernet lmi
RP/0/RSP0/CPU0:router(config-if-elmi)#polling-verification-timer 30
RP/0/RSP0/CPU0:router(config-if-elmi)#commit

/* Configure the Status Counter */

The MEF N393 Status Counter value is used to determine E-LMI operational status by tracking receipt of consecutive good packets or successive expiration of the PVT on packets. The default counter is four, which means that while the E-LMI protocol is in Down state, four good packets must be received consecutively to change the protocol state to Up, or while the E-LMI protocol is in Up state, four consecutive PVT expirations must occur before the state of the E-LMI protocol is changed to Down on the interface.

RP/0/RSP0/CPU0:router# configure
RP/0/RSP0/CPU0:router(config)#interface gigabitethernet 0/0/0/0
RP/0/RSP0/CPU0:router(config-if)# ethernet lmi
RP/0/RSP0/CPU0:router(config-if-elmi)#status-counter 5
RP/0/RSP0/CPU0:router(config-if-elmi)#commit