in brief overview , the present invention permits a pe device in a heterogeneous network to learn the address information for its locally - connected ce devices by monitoring the control messages sent by the ce devices . in one embodiment , the pe device then shares this learned address information with other pe devices . these remote pe devices may then provide their own locally - connected ce devices with the shared address information they have received . using this information , the pe devices transparently route communications between ce devices that are connected to their pe devices using different types of data links . as discussed above , one or more pe devices 116 , 120 , and 124 in the heterogeneous wan of fig1 participate in ip interworking by changing the layer 2 encapsulation of the ip protocol data units ( pdus ) flowing to and from the ce devices that are local to it . referring to fig2 , the pe devices gather information concerning their locally - connected ce devices by monitoring the control messages sent by the ce devices ( step 200 ). once this information is gathered , each pe device may share the information concerning its locally - connected ce devices with the other pe devices in the network using a pe protocol such as bgp , ldp , l2tp , etc . ( step 204 ). with the receipt of this shared information , a pe device in the vpn has address information concerning both its own locally - connected ce devices and those ce devices that are remote to it , but local to another pe device in the vpn . the pe device may now share the information concerning the remote ce devices with its locally - connected ce devices , either directly or indirectly ( step 208 ). using the exchanged address information , the pe device may now participate in interworking as described above ( step 212 ). in one embodiment , the control messages from the local ce devices ( step 200 ) are address resolution messages , as discussed in detail below . the information gathered from the control messages may include , but is not limited to , one or more of a local ce device &# 39 ; s ip address , circuit information , a virtual channel ( vc ) or vpn mpls label , or the ip address for the remote ce associated with that label . once gathered , this information may be stored as a tuple or other data structure in a volatile memory , such as a random - access memory ( ram ), and / or a nonvolatile memory , such as a hard disk . in a layer 2 interworking vpn there is typically one tuple for each circuit that the ce device has attached to its local pe device . although the process for gathering information ( step 200 ) varies among the various types of data links connecting ce devices to their local pe devices , it typically includes the receipt of a control message from a local ce device , the extraction of information concerning the local ce device from the control message and , optionally , the storage of this information in a volatile or a non - volatile memory . when the data link is a gigabit ethernet data link , e . g ., data link 128 , the pe device may learn the address of a local ce device ( step 200 ) on a given ethernet circuit using router discovery protocol ( rdp ), as set forth in ietf rfc 1256 and incorporated by reference as if set forth fully herein . if the pe device is connected to the local ce device using gigabit ethernet with virtual local - area network ( vlan ) tagging ( as described in ieee standard 802 . 1q and incorporated by reference as if set forth fully herein ), then the vlan tag may represent an ip subnet and the circuit information will then consist of ethernet interface information and the vlan tag . if the pe device does not use ieee 802 . 1q vlan tagging , then the entire ethernet port is treated as a single endpoint that is connected to one remote endpoint through a pair of pe devices , the ethernet interface is the ip subnet , and the circuit information only includes ethernet interface information . regardless of whether the pe device supports ieee 802 . 1q tagging , only one ce device — in this case the ethernet router end station — is presumed to participate within the ip interworking - based layer 2 vpn . when the data link is a frame relay data link , e . g ., data link 136 , a newly - attached ce device may generate an inverse address resolution protocol ( iarp ) request — defined in ietf rfc 2390 and incorporated by reference as if set forth in its entirety herein — to obtain the ip addresses of its neighboring devices when the data link connection identifier ( dlci ) associated with the ip interface becomes active . typically , the dlci will become active when the local pe device has learned cross - connect - related information from a remote pe device using bgp , presenting a stalemate problem that may be resolved as discussed below . once the local ce device issues the iarp request , the attached pe device may determine the local ce device &# 39 ; s ip address and dlci information from the iarp request ( step 200 ). when the data link uses point - to - point protocol ( ppp ), then the attached ce device participates in internet protocol control protocol ( ipcp , defined in ietf rfc 1332 and incorporated by reference as if set forth in its entirety herein ) to obtain the ip addresses for its neighbor devices . by examining the local ce device &# 39 ; s ipcp request , the pe device may determine the ce device &# 39 ; s ip address ( step 200 ). when the data link is an asynchronous transfer mode ( atm ) link , an attached ce device treats each virtual circuit ( vc ) as an ip subnet . the attached ce device may participate in inverse atm arp ( inatmarp , defined in ietf rfc 1577 and incorporated by reference as if set forth in its entirety herein ) to obtain the ip addresses for its neighbor devices . the pe device may learn the local ce device &# 39 ; s ip address from the local ce device &# 39 ; s inatmarp request ( step 200 ). the high - level data link control protocol ( hdlc , described in iso standard 3309 and incorporated by reference as if set forth in its entirety herein ) does not specify a protocol mechanism for obtaining the ip address of a neighboring device . instead , a device using hdlc receives ip data frames from a single remote endpoint and , therefore , implicitly assumes the presence of a single ip address . therefore , when the data link connecting the pe device to its local ce device is an hdlc data link , the pe device is manually configured with the ip address of the local ce device to permit the local pe device to distribute cross - connect information to remote pe devices ( step 200 ). having gathered information from the control messages , such as address resolution messages , sent by the local ce devices ( step 200 ), a pe device may now share information concerning its local ce devices with another pe device ( step 204 ). these cross - connect advertisements may include , for example , one or more of the local ce device &# 39 ; s ip address , circuit information , a virtual channel ( vc ) or vpn mpls label , or the ip address for the remote ce associated with that label . in various embodiments of the invention , the pe devices may communicate using border gateway protocol ( bgp ), label distribution protocol ( ldp ), layer 2 tunneling protocol ( l2tp ), etc . in one embodiment , the information may be sent to a pe device at the edge of the network cloud using multiprotocol bgp ( mp - bgp ) network layer reachability information ( nlri ) messages . a typical nlri message includes a label block offset , a label base , and a length of the circuit status vector sub - tlv . the nlri messages sent to the remote pe device provide a set of contiguous labels starting from the label base value that correspond to a set of remote ce identifiers starting from the label block offset . in accord with the present invention , the nlri message may also include a second sub - tlv of type tbd that contains an ip address . a pe device advertises this new ip address sub - tlv when the vpn &# 39 ; s encapsulation type is ip interworking . this second sub - tlv has the same length as the circuit status vector sub - tlv . the length field of the sub - tlv specifies the number of 4 - byte fields contained in the value field of the ip address sub - tlv , where each field is an ip address that has a one - to - one correspondence with the labels represented by the label base and length field . by iterating through each ip field value in order , a receiving pe device may determine the association between a label and an ip address . once a pe device has received these cross - connect advertisements , e . g ., an ip address - to - label association , the pe device may provide this information to its local ce devices ( step 208 ). although the process for sharing the information ( step 208 ) varies depending on the type of data link between the local ce devices and their pe devices , the process is typically either active , e . g ., the pe device transmits an control message to its local ce devices , or passive , e . g ., the pe device responds to its local ce device &# 39 ; s control message with the remote ce device &# 39 ; s address information . when the data link is a gigabit ethernet data link , the pe device may generate an address resolution protocol request ( arp , defined in ietf rfc 826 and incorporated by reference as if set forth in its entirety herein ) to present the ip address of the remote ce device as a neighbor to the local ce device ( step 208 ). if the pe device supports ieee 802 . 1q tagging , then the request must be generated within the vlan scope . the pe device may also respond to the local ce device &# 39 ; s arp request with its own mac address as the target hardware address when the wan ip address of the cross - connected remote ce device is known ( step 208 ). to make communications with the remote ce device transparent , the local pe device may optionally provide its mac address in the source hardware address field of the arp request and the target hardware address field of the arp reply . if two ce devices are locally attached to their pe device on gigabit ethernet data links , then the local pe device may propagate an arp request or an arp response with vlan tag translation directly to the second ce device . when the data link is a frame relay data link and the pe device has received cross - connect information from a remote pe device , then the corresponding dlci at the local pe device is inactive . the local pe device may initiate an iarp request providing the address information for the cross - connected remote ce device , or it may wait for the attached ce device to generate an iarp request and then respond with the address information ( step 208 ). when the data link uses ppp , the pe device may respond to ipcp requests from its local ce devices with the ip address information for the remote ce devices when that information becomes available ( step 208 ). if the ip address information and the cross - connect information are available , the pe device may also initiate an ipcp request to provide the local ce device with address information for the cross - connected remote ce devices ( step 208 ). when the data link is an atm data link , the pe device may generate an inatmarp message for its local ce devices when the address information for the remote cross - connected ce devices becomes available . if the local pe device has received the cross - connect information and the associated ip address information from a remote pe device , then the local pe device may optionally respond to an inatmarp request received from its local ce devices ( step 208 ). as discussed above , the address information for a remote ce device may not be available to a particular pe device when that pe device receives a control message , such as an address resolution message ; from its local ce device . in that event ; if the remote ce device &# 39 ; s information is not available , then the pe device may advertise the cross - connect information to its local ce devices without the remote ce device &# 39 ; s address by using a dummy value ( such as zero ) in the new ip address sub - tlv ( step 208 ). when the address information for the remote ce device becomes available , the pe device may generate a new advertisement to its local ce devices with an updated ip address field value ( step 208 ). this behavior prevents a stalemate situation , e . g ., in a frame relay data link , where the local ce device will not transmit a control message that will eventually be conveyed as cross - connect information until its attached pe device signals its readiness , and the attached pe device will not signal its readiness until it receives cross - connect information that originates with a remote ce device that is waiting for its own pe device to signal its readiness . many alterations and modifications may be made without departing from the spirit and scope of the invention . therefore , it is to be understood that these embodiments have been shown by way of example and should not be taken as limiting the invention , which is defined by the following claims . these claims are thus to be read as not only including literally what is set forth by the claims but also to include those equivalents which are insubstantially different , even though not identical in other respects to what is shown and described in the above illustrations .