Ethernet interworking is also called bridged interworking. Ethernet frames are bridged across the pseudowire. The CE routers could be natively bridging Ethernet or could be routing using a bridged encapsulation model, such as Bridge Virtual Interface (BVI) or Routed Bridge Encapsulation (RBE). The PE routers operate in Ethernet like-to-like mode.

This mode is used to offer the following services:

• LAN services--An example is an enterprise that has several sites, where some sites have Ethernet connectivity to the service provider (SP) network and others have ATM connectivity. If the enterprise wants LAN connectivity to all its sites, traffic from the Ethernet or VLAN of one site can be sent through the IP/MPLS network and encapsulated as bridged traffic over an ATM VC of another site.
• Connectivity services--An example is an enterprise that has different sites that are running an Internal Gateway Protocol (IGP) routing protocol, which has incompatible procedures on broadcast and nonbroadcast links. The enterprise has several sites that are running an IGP, such as Open Shortest Path First (OSPF) or Intermediate System-to-Intermediate System (IS-IS), between the sites. In this scenario, some of the procedures (such as route advertisement or designated router) depend on the underlying L2 protocol and are different for a point-to-point ATM connection versus a broadcast Ethernet connection. Therefore, the bridged encapsulation over ATM can be used to achieve homogenous Ethernet connectivity between the CE routers running the IGP.

IP interworking is also called routed interworking. The CE routers encapsulate the IP on the link between the CE router and PE router. A new VC type is used to signal the IP pseudowire in MPLS. Translation between the L2 and IP encapsulations across the pseudowire is required. Special consideration needs to be given to the address resolution and routing protocol operation, because these are handled differently on different L2 encapsulations.

This mode is used to provide IP connectivity between sites, regardless of the L2 connectivity to these sites. It is different from a Layer 3 VPN because it is point-to-point in nature and the service provider does not maintain any customer routing information.

Address resolution is encapsulation dependent:

• Ethernet uses ARP
• ATM uses inverse ARP
• PPP uses IPCP

Therefore, address resolution must be terminated on the PE router. End-to-end address resolution is not supported. Routing protocols operate differently over broadcast and point-to-point media. For Ethernet, the CE routers must either use static routing or configure the routing protocols to treat the Ethernet side as a point-to-point network.

In routed interworking, IP packets that are extracted from the ACs are sent over the pseudowire. The pseudowire works in the IP Layer 2 transport (VC type 0x000B) like-to-like mode. The interworking function at network service provider's (NSP) end performs the required adaptation based on the AC technology. Non-IPv4 packets are dropped.

In routed interworking, the following considerations are to be kept in mind:

• Address resolution packets (ARP), inverse ARP, and IPCP are punted to the routing protocol. Therefore, NSP at the PE router must provide the following functionality for address resolution:
  • Ethernet--PE device acts as a proxy-ARP server to all ARP requests from the CE router. The PE router responds with the MAC address of its local interface.
  • ATM and Frame Relay point-to-point--By default, inverse ARP does not run in the point-to-point Frame Relay or ATM subinterfaces. The IP address and subnet mask define the connected prefix; therefore, configuration is not required in the CE devices.
• Interworking requires that the MTUs in both ACs match for the pseudowire to come up. The default MTU in one AC should match with the MTU of other AC. The table below lists the range of MTUs that can be configured for different ACs.

Note	The MTU configured on the AC should not exceed the MTU in the core network. This ensures that the traffic is not fragmented.

• The CE routers with Ethernet attachment VCs running OSPF must be configured with the ospfIfTypeoption so that the OSPF protocol treats the underlying physical broadcast link as a P2P link.

This interworking type provides interoperability between the ATM attachment VC and Ethernet attachment VC connected to different PE routers. Bridged encapsulation corresponding to the bridged (Ethernet) interworking mechanism is used.

The interworking function is performed at the PE router connected to the ATM attachment VC based on multiprotocol encapsulation over ATM AAL5 (see the figure below).

Figure 3

Network Topology for ATM-to-Ethernet AToM Bridged Interworking

The advantage of this architecture is that the Ethernet PE router (connected to the Ethernet segment) operates similarly to Ethernet like-to-like services.

On the PE router with interworking function, in the direction from the ATM segment to MPLS cloud, the bridged encapsulation (ATM/subnetwork access protocol (SNAP) header) is discarded and the Ethernet frame is encapsulated with the labels required to go through the pseudowire using the VC type 5 (Ethernet) (see the figure below).

In the opposite direction, after the label disposition from the MPLS cloud, Ethernet frames are encapsulated over AAL5 using bridged encapsulation.

The figure below shows the protocol stack for ATM-to-Ethernet AToM bridged interworking. The ATM side has an encapsulation type of aal5snap.

Figure 4

Protocol Stack for ATM-to-Ethernet AToM Bridged Interworking--without VLAN Header

This interworking type provides interoperability between the ATM attachment VC and Ethernet VLAN attachment VC connected to different PE routers. Bridged encapsulation corresponding to the bridged (Ethernet) interworking mechanism is used.

The interworking function is performed in the same way as for the ATM-to-Ethernet port case, implemented on the PE router connected to the ATM attachment VC. The implementation is based on multiprotocol encapsulation over ATM AAL5 (see the figure below).

For the PE router connected to the Ethernet side, one major difference exists due the existence of the VLAN header in the incoming packet. The PE router discards the VLAN header of the incoming frames from the VLAN CE router, and the PE router inserts a VLAN header into the Ethernet frames traveling from the MPLS cloud. The frames sent on the pseudowire (with VC type 5) are Ethernet frames without the VLAN header.

Encapsulation over ATM AAL5 is shown in the figure below.

Figure 5

Protocol Stack for ATM -to-VLAN AToM Bridged Interworking

To perform routed interworking, both the ATM PE router and Ethernet PE router must be configured. The figure below shows the routed interworking between ATM to Ethernet. The IP encapsulation over the pseudowire is performed on the ATM packets arriving from the ATM CE router.

The address resolution is done at the ATM PE router; it is required when the ATM CE router does an inverse ARP. It is not required when the ATM CE router is configured using Point-to-Point (P2P) subinterfaces or static maps.

When packets arrive from the Ethernet CE router, the Ethernet PE router removes the L2 frame tag, and then forwards the IP packet to the egress PE router, using IPoMPLS encapsulation over the pseudowire. The Ethernet PE router makes the forwarding decision based on the L2 circuit ID, the VLAN ID, or port ID, of the incoming L2 frame. At the ATM PE router, after label disposition, the IP packets are encapsulated over the AAL5 using routed encapsulation based on RFC 2684.

The address resolution at the Ethernet PE router can be done when the Ethernet CE router configures the static ARP, or by the proxy ARP on the Ethernet PE router. If the proxy ARP is used, the IP address of the remote CE router can be learned dynamically.

Routing protocols need to be configured to operate in the P2P mode on the Ethernet CE router.

Figure 6

Protocol Stack for ATM-to-Ethernet--Routed Interworking

This interworking type provides interoperability between the Frame Relay attachment VC and Ethernet attachment VC connected to different PE routers. Bridged encapsulation corresponding to the bridged (Ethernet) interworking mechanism is used.

For an FR-to-Ethernet port case, the interworking function is performed at the PE router connected to the FR attachment VC based on multiprotocol interconnect over Frame Relay (see the figure below). The interworking is implemented similar to an ATM-to-Ethernet case.

Figure 7

Network Topology for FR-to-Ethernet AToM Bridged Interworking

The advantage of this architecture is that the Ethernet PE router (connected to the Ethernet segment) operates similar to Ethernet like-to-like services: a pseudowire label is assigned to the Ethernet port and then the remote Label Distribution Protocol (LDP) session distributes the labels to its peer PE router. Ethernet frames are carried through the MPLS network using Ethernet over MPLS (EoMPLS).

On the PE router with interworking function, in the direction from the Frame Relay segment to the MPLS cloud, the bridged encapsulation (FR/SNAP header) is discarded and the Ethernet frame is encapsulated with the labels required to go through the pseudowire using the VC type 5 (Ethernet) (see the figure below).

In the opposite direction, after the label disposition from the MPLS cloud, Ethernet frames are encapsulated over Frame Relay using bridged encapsulation.

The following translations are supported:

• Ethernet without LAN FCS (0300800080C20007)
• Spanning tree (0300800080C2000E)

The PE router automatically supports translation of both Cisco and IETF Frame Relay encapsulation types coming from the CE, but translates only to IETF when sending to the CE router. This is not a problem for the Cisco CE router, because it can handle IETF encapsulation on receipt even if it is configured to send Cisco encapsulation.

The existing QoS functionality for Frame Relay is supported. The PVC status signaling works the same way as in the like-to-like case. The PE router reports the PVC status to the CE router, based on the availability of the pseudo wire.

The AC MTU must match when connected over MPLS. Only Frame Relay DLCI mode is supported; Frame Relay port mode is not supported in the bridged interworking.

The figure below shows the protocol stack for FR-to-Ethernet bridged interworking.

Figure 8

Protocol Stack for FR-to-Ethernet AToM Bridged Interworking--without VLAN Header

This interworking type provides interoperability between the Frame Relay attachment VC and Ethernet VLAN Attachment VC connected to different PE routers. The bridged encapsulation corresponding to the bridged (Ethernet) interworking mechanism is used.

The interworking function is performed in the same way as it is done for the Frame Relay to Ethernet port case; it is implemented on the PE router connected to the Frame Relay attachment VC, based upon a multiprotocol interconnect over Frame Relay (see the figure above).

As in the ATM-to-VLAN case, one difference exists on the Ethernet side due the existence of the VLAN header in the incoming packet. The PE router on the VLAN side discards the VLAN header of the incoming frames from the VLAN CE router, and the PE router inserts a VLAN header into the Ethernet frames traveling from the MPLS cloud. The frames sent on the pseudowire (with VC type 5) are Ethernet frames without the VLAN header.

The figure below shows the protocol stack for FR-to-VLAN AToM bridged interworking.

Figure 9

Protocol Stack for FR-to-VLAN AToM Bridged Interworking

This interworking type provides interoperability between the Frame Relay Attachment VC and Ethernet VLAN Attachment VC connected to different PE routers. The bridged encapsulation corresponding to bridged (Ethernet) interworking mechanism is used.

The interworking function is done in the same way as it is done for FR-to-Ethernet port case; it is implemented on the PE router connected to the Frame Relay attachment VC, based on RFC 2427(Multiprotocol Interconnect over Frame Relay).

When compared with Frame Relay DLCI-to-Ethernet port AToM, there is one major difference on the Ethernet access side, due the existence of the VLAN header in the incoming packet. The PE router on the VLAN side will discard the VLAN header of the incoming frames form the VLAN CE router, and it will insert a VLAN header into the Ethernet frames coming from the MPLS cloud. So the frames sent on the pseudo wire (with VC type 5) will be Ethernet frames without the VLAN header.

The following translations are supported on the Frame Relay PE router:

• Ethernet without LAN FCS (0300800080C20007)
• Spanning tree (0300800080C2000E)

Frame Relay encapsulation types supported for bridged interworking: Cisco and IETF for incoming traffic, IETF only for outgoing traffic towards CE router.

VC-to-VC local switching transports cells between two ATM attachment VCs on the same or different port on the PE router. The cells coming to the PE router can be AAL0 or AAL5 encapsulated ATM packets. ATM VC-to-VC local switching can be configured either on point-to-point interface or on multipoint interface.

There are two operation modes for managing OAM cells over ATM local switching interfaces:

• OAM transparent mode: In this mode, the PE router transports F5 OAM cells transparently across local switching interfaces.
• OAM local emulation mode: In this mode, the PE router does not transport OAM cells across local switching interfaces. Instead, the interfaces locally terminate and process F5 OAM cells.

In ATM single cell relay AAL0, the ATM virtual path identifier/virtual channel identifier (VPI/VCI) values of the ingress and egress ATM interfaces of a router must match. If L2 local switching is desired between two ATM VPIs and VCIs, which are on two different interfaces and have values that do not match, ATM AAL5 should be selected. However, if ATM AAL5 uses OAM transparent mode, the VPI and VCI values must match.

ATM OAM can be configured on ATM VC mode local switching AC using the oam-ac emulation-enableand oam-pvc manage commands. When emulation is enabled on the AC, all OAM cells going through the AC are punted to RP for local processing. The ATM common component processes OAM cells and forwards the cells towards the local CE router. This helps to detect the failures on the PE router by monitoring the response at the CE router end. When the oam-pvc manage command is enabled on the AC, the PVC generates end-to-end OAM loopback cells that verify connectivity on the VC.

The following example shows a sample configuration on the ATM PE router:

configure terminal
interface atm 4/0.50 multipoint
	no ip address
 	no atm enable-ilmi-trap
pvc 100/100 l2transport
	encapsulation aal5
	oam-ac emulation-enable
	oam-pvc manage
interface atm 5/0.100 multipoint
	no ip address
	no atm enable-ilmi-trap
	pvc 100/100 l2transport
		encapsulation aal5
		oam-ac emulation-enable
		oam-pvc manage
connect atm_ls atm 4/0 100/100 atm 5/0 100/100

VP-to-VP local switching transports cells between two VPs on the same port or different ports on the PE router. The cells coming to the PE router can be AAL0 encapsulated ATM packets only. ATM VP-to-VP local switching can be configured only on multipoint interfaces.

There are two operation modes for managing OAM cells over ATM local switching interfaces:

• OAM transparent mode: In this mode, the PE router transports F4 OAM cells transparently across local switching interfaces.
• OAM local emulation mode: In this mode, the PE router do not transport OAM cells across local switching interfaces. Instead, the interfaces locally terminate and process F4 OAM cells.

In ATM single cell relay AAL0, the ATM VPI values of the ingress and egress ATM interfaces on a router must match. If L2 switching is desired between two ATM VPIs which are on two different interfaces and have values that do not match, ATM AAL5 should be selected. If ATM AAL5 uses OAM transparent mode, the VPI value must match. Currently, the ATM VP-to-VP local switching supports only AAL0 encapsulation.

ATM OAM can be configured on the ATM VP mode local switching AC using the oam-ac emulation-enable command. When emulation is enabled on the AC, all OAM cells going through the AC are punted to RP for local processing. The ATM common component processes the OAM cells and forwards the cells towards the local CE router. This helps to detect failures on the PE router by monitoring the response at the CE router's end.

The following example shows a sample configuration on the ATM PE router:

configure terminal
interface atm 4/0.100 multipoint
	no ip address
 	no atm enable-ilmi-trap
atm pvp 100 l2transport
	encapsulation aal5
	oam-ac emulation-enable
interface atm 5/0.100 multipoint
	no ip address
	no atm enable-ilmi-trap
	atm pvp 100 l2transport
		encapsulation aal5
		oam-ac emulation-enable
connect atm_ls atm 4/0 100 atm 5/0 100

You can verify L2VPN configuration using the following steps:

• You can issue the show arp command between the CE routers to ensure that data is being sent:

Router# show arp
Protocol   Address       Age (min)   Hardware Addr    Type    Interface
Internet   10.1.1.5           134    0005.0032.0854   ARPA    FastEthernet0/0/0
Internet   10.1.1.7             -    0005.0032.0000   ARPA    FastEthernet0/0/0

• You can issue the ping command between the CE routers to ensure that data is being sent:

Router# ping 10.1.1.5
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.1.1.5, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms

• You can verify the AToM configuration by using the show mpls l2transport vc detail command.

1.    enable

2.    configure terminal

3.    mpls label protocol ldp

4.    interface type number

5.    ip address ip-address mask

6.    pseudowire-class [pw-class-name]

7.    encapsulation mpls

8.    interworking {ethernet | ip| vlan}

9.    interface atm slot / subslot / port . subinterface number

10.    pvc [name] vpi / vci 12transport

11.    encapsulation aal5snap

12.    xconnect ip-address vc-id pw-class pw-class-name

13.    end

1.    enable

2.    configure terminal

3.    mpls label protocol ldp

4.    interface type number

5.    ip address ip-address mask

6.    pseudowire-class [pw-class-name]

7.    encapsulation mpls

8.    interworking {ethernet | ip| vlan}

9.    interface type slot / subslot / port

10.    xconnect ip-address vc-id pw-class pw-class-name

11.    end

Note	When configuring bridged interworking, the PE2 router configuration does not include the interworking ethernet command because it is treated as like-to-like, and also because the AC is already an Ethernet port. However, when configuring routed interworking, the interworking ip command is required.

1.    enable

2.    configure terminal

3.    mpls label protocol ldp

4.    interface type number

5.    ip address ip-address mask

6.    pseudowire-class [pw-class-name]

7.    encapsulation mpls

8.    interworking {ethernet | ip| vlan}

9.    interface atm slot / subslot / port . subinterface number

10.    pvc [name] vpi / vci 12transport

11.    encapsulation aal5snap

12.    xconnect ip-address vc-id pw-class pw-class-name

13.    end

1.    enable

2.    configure terminal

3.    mpls label protocol ldp

4.    interface type number

5.    ip address ip-address mask

6.    pseudowire-class [pw-class-name]

7.    encapsulation mpls

8.    interworking {ethernet | ip| vlan}

9.    interface type slot / subslot / port . subinterface-number

10.    encapsulation dot1q vlan-id

11.    xconnect ip-address vc-id pw-class pw-class-name

12.    end

Note	In the case of ATM AAl5-to-VLAN, the PE2 router configuration includes the interworkingcommand for both bridged and routed interworking.

Note	To verify the L2VPN interworking status and check the statistics, refer to the Verifying L2VPN Interworking.

1.    enable

2.    configure terminal

3.    mpls label protocol ldp

4.    interface type number

5.    ip address ip-address mask

6.    pseudowire-class [pw-class-name]

7.    encapsulation mpls

8.    interworking ethernet

9.    interface type slot / subslot / port

10.    encapsulation frame-relay

11.    connect connection-name interface dlci {interface dlci | l2transport}

12.    xconnect ip-address vc-id pw-class pw-class-name

13.    end

1.    enable

2.    configure terminal

3.    mpls label protocol ldp

4.    interface type number

5.    ip address ip-address mask

6.    pseudowire-class [pw-class-name]

7.    encapsulation mpls

8.    interworking ethernet

9.    interface type slot / subslot / port

10.    xconnect ip-address vc-id pw-class pw-class-name

11.    end

Note

When configuring bridged interworking, the PE2 router configuration does not include the interworking ethernetcommand because it is treated as like-to-like, and also because the AC is already an Ethernet port. However, when configuring routed interworking, the PE2 router configuration does include the interworking ip command.

1.    enable

2.    configure terminal

3.    mpls label protocol ldp

4.    interface type number

5.    ip address ip-address mask

6.    pseudowire-class [pw-class-name]

7.    encapsulation mpls

8.    interworking {ethernet | ip| vlan}

9.    frame-relay switching

10.    interface type slot / subslot / port

11.    encapsulation frame-relay

12.    frame-relay intf-type [dce]

13.    connect connection-name interface dlci {interface dlci | l2transport}

14.    xconnect ip-address vc-id pw-class pw-class-name

15.    end

1.    enable

2.    configure terminal

3.    mpls label protocol ldp

4.    interface type number

5.    ip address ip-address mask

6.    pseudowire-class [pw-class-name]

7.    encapsulation mpls

8.    interworking {ethernet | ip| vlan}

9.    interface type slot / subslot / port . subinterface-number

10.    encapsulation dot1q vlan-id

11.    xconnect ip-address vc-id pw-class pw-class-name

12.    end

Note	In the case of an Frame Relay DLCI-to-VLAN, the PE2 router configuration includes the interworkingcommand for both bridged and routed interworking.

Note	To verify the L2VPN interworking status and check the statistics, refer to the Verifying L2VPN Interworking.