Abstract:
A method, network device, and computer readable medium are disclosed. In one embodiment of the method, a data packet is received at a node within a first segment routing enabled access network. The first access network is connected, via a first area edge node, to a first area of a subdivided segment routing enabled network, and a specified destination for the data packet is reachable via a second segment routing enabled access network connected to the subdivided network. The method embodiment further includes receiving from a centralized controller a remote segment identifier stack, where the remote segment identifier stack encodes a path extending from the first area edge node to the second access network. The embodiment of the method continues with encapsulating the data packet with a full segment identifier stack comprising the remote segment identifier stack, and forwarding the encapsulated data packet toward the first area edge node.

Description:
RELATED APPLICATIONS 
       [0001]    The present patent application is a continuation of U.S. patent application Ser. No. 15/234,794, filed on Aug. 11, 2016 and entitled “Seamless Segment Routing,” which is a continuation of U.S. patent application Ser. No. 14/210,837, filed on Mar. 14, 2014 and entitled “Seamless Segment Routing” (now U.S. Pat. No. 9,450,829 issued on Sep. 20, 2016), which claims the domestic benefit under Title 35 of the United States Code §119(e) of U.S. Provisional Patent Application Ser. No. 61/791,242, filed on Mar. 15, 2013 and entitled “Segment Routing.” All of these nonprovisional and provisional applications are hereby incorporated by reference in their entirety and for all purposes as if completely and fully set forth herein. 
     
    
     BACKGROUND 
       [0002]    Network nodes forward packets using forwarding tables. Network nodes may take form in one or more routers, one or more bridges, one or more switches, one or more servers, or any other suitable communications processing device. A packet is a formatted unit of data that typically contains control information and payload data. Control information may include: source and destination IP addresses, error detection codes like checksums, sequencing information, etc. Control information is typically found in packet headers and trailers, with payload data in between. 
         [0003]    Packet forwarding requires a decision process that, while simple in concept, can be complex. Since packet forwarding decisions are handled by nodes, the total time required for this can become a major limiting factor in overall network performance. 
         [0004]    Multiprotocol Label Switching (MPLS) is one packet forwarding mechanism. MPLS Nodes make packet forwarding decisions based on Label Distribution Protocol (LDP) labels attached to packets and LDP forwarding tables. LDP is a process in which network nodes capable of MPLS exchange LDP labels (hereinafter labels). Packet forwarding based on labels stands in stark contrast to traditional Internet Protocol (IP) routing in which packet forwarding decisions are made by nodes using IP addresses contained within the packet. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is a block diagram illustrating an example provider network. 
           [0006]      FIG. 2  is a block diagram illustrating an example provider network. 
           [0007]      FIG. 3  is a flow chart illustrating an example process employed by a node of the network shown in  FIG. 2 . 
           [0008]      FIG. 4  is a block diagram illustrating an example provider network. 
           [0009]      FIG. 5  is a block diagram illustrating an example provider network. 
           [0010]      FIG. 6  is a flow chart illustrating an example process employed by a node. 
           [0011]      FIG. 7  is a block diagram illustrating an example provider network coupled to example access networks. 
           [0012]      FIG. 8  is a block diagram illustrating certain components of an example node that can be employed in the networks of  FIG. 4, 5 , or  6 . 
       
    
    
     DETAILED DESCRIPTION 
     1. Overview 
       [0013]    An apparatus and method is disclosed for seamless segment routing (SR). In one embodiment, a packet and a segment ID stack is received at a node. The segment ID stack includes a plurality of segment IDs, one of which is a first area-segment ID that identifies a first area of a subdivided network. One of a plurality of forwarding tables at the node is selected based on the first area-segment ID. Thereafter, the packet is forwarded based on information contained in the selected forwarding table. 
       2. Packet Forwarding Mechanisms 
       [0014]    IP routing and MPLS are distinct packet forwarding mechanisms. IP routing uses IP addresses inside packet headers to make packet forwarding decisions. In contrast, MPLS implements packet forwarding decisions based on short path identifiers called labels attached to packets. Segment routing (SR) is yet another packet forwarding mechanism. SR is similar to MPLS in many regards. For example, packet forwarding decisions in SR can be based on short path identifiers called segment IDs attached to packets. However, substantial differences exist between SR and MPLS as will be more fully described below. 
         [0015]    2.1 IP Routing 
         [0016]    IP routing uses IP forwarding tables, which are created at nodes using routing information distributed between nodes via one or more protocols like the internal gateway protocol (IGP) and/or the border gateway protocol (BGP). In simple terms, IP forwarding tables map destination addresses to the next hops that packets take to reach their destinations. When a node receives a packet, the node can access a forwarding table using the destination address in the packet and lookup a corresponding egress interface for the next hop. The node then forwards the packet through the egress interface. The next hop that receives the packet performs its own forwarding table lookup using the same destination IP address, and so on. 
         [0017]    2.2 MPLS and LDP 
         [0018]    MPLS is commonly employed in provider networks. Packets enter an MPLS network via an ingress edge node, travel hop-by-hop along a label-switched path (LSP) that typically includes one or more core nodes, and exit via an egress edge node. 
         [0019]    Packets are forwarded along an LSP based on labels and LDP forwarding tables. Labels allow for the use of very fast and simple forwarding engines in the data plane of nodes. Another benefit of MPLS is the elimination of dependence on a particular Open Systems Interconnection (OSI) model data link layer technology to forward packets. 
         [0020]    A label is a short, fixed-length, locally significant identifier that can be associated with a forwarding equivalence class (FEC). Packets associated with the same FEC should follow the same LSP through the network. LSPs can be established for a variety of purposes, such as to guarantee a certain level of performance when transmitting packets, to forward packets around network congestion, to create tunnels for network-based virtual private networks, etc. In many ways, LSPs are no different than circuit-switched paths in ATM or Frame Relay networks, except that they are not dependent on a particular Layer  2  technology. 
         [0021]    LDP is employed in the control planes of nodes. Two nodes, called LDP peers, can bi-directionally exchange labels on a FEC by FEC basis. LDP can be used in a process of building and maintaining LDP forwarding tables that map labels and next hop egress interfaces. These forwarding tables can be used to forward packets through MPLS networks as more fully described below. 
         [0022]    When a packet is received by an ingress edge node of an MPLS network, the ingress node may determine a corresponding FEC. Characteristics for determining the FEC for a packet can vary, but typically the determination is based on the packet&#39;s destination IP address. Quality of Service for the packet or other information may also be used to determine the FEC. Once determined, the ingress edge node can access a table to select a label that is mapped to the FEC. The table may also map a next hop egress interface to the FEC. Before the ingress edge node forwards the packet to the next hop via, the ingress node attaches the label. 
         [0023]    When a node receives a packet with an attached label (i.e., the incoming label), the node accesses an LDP forwarding table to read a next hop egress interface and another label (i.e., an outgoing label), both which are mapped to the incoming label. Before the packet is forwarded via the egress interface, the node swaps the incoming label with the outgoing label. The next hop receives the packet with label and may perform the same process. This process is often called hop-by-hop forwarding along a non-explicit path. The penultimate node in the LSP may pop or remove the incoming label before forwarding the packet to an egress edge node in the network, which in turn may forward the packet towards its destination using the packet&#39;s destination address and an IP forwarding table. To illustrate MPLS aspects,  FIG. 1  shows a portion of an example MPLS network  100  that includes nodes  102 - 122  coupled together via communication links. An LSP from node  102  to node  122  can be created so that all packets of a stream associated with a particular FEC sent from node  102  to node  122  will travel through the same set of nodes. Each node maintains information for the LSP established through it in an LDP forwarding table. Thus, if node  110  knows that node  114  is the next hop along the LSP for all packets received from node  102  that are destined for node  122 , node  110  can forward the packets to node  114 . 
         [0024]    2.3 Segment Routing 
         [0025]    Segment routing (SR) is a mechanism in which nodes forward packets using SR forwarding tables and segment IDs. Like MPLS, SR enables very fast and simple forwarding engines in the data plane of nodes. SR is not dependent on a particular Open Systems Interconnection (OSI) model data link layer technology to forward packets. 
         [0026]    SR nodes (i.e., nodes employing SR) make packet forwarding decisions based on segment IDs as opposed to labels, and as a result SR nodes need not employ LDP in their control planes. Unless otherwise indicated, the SR nodes described below lack LDP in the control plane. 
         [0027]    Packets can enter an SR enabled network (i.e., a network of nodes that are SR enabled) via an ingress edge node, travel hop-by-hop along a segment path (SP) that includes one or more core nodes, and exit the network via an egress edge node. 
         [0028]    Like labels, segment IDs are short (relative to the length of an IP address or a FEC), fixed-length identifiers. Segment IDs may correspond to topological segments of a network, services provided by network nodes, etc. Topological segments represent one hop or multi hop paths to SR nodes. Topological segments act as sub-paths that can be combined to form an SP. Stacks of segment IDs can represent SPs, and SPs can be associated with FECs as will be more fully described below. 
         [0029]    There are several types of segment IDs including nodal-segment IDs, adjacency-segment IDs, etc. Nodal-segment IDs are assigned to nodes so that no two SR nodes belonging to a network domain are assigned the same nodal-segment ID. Nodal-segment IDs can be mapped to unique SR node identifiers such as node loopback IP addresses (hereinafter node loopbacks). In one embodiment, all assigned nodal-segment IDs are selected from a predefined ID range (e.g., [32, 5000]). A nodal-segment ID corresponds to a one-hop or a multi-hop, shortest path (SPT) to an SR node assigned the nodal-segment ID as will be more fully described below. 
         [0030]    An adjacency-segment ID represents a direct link between adjacent SR nodes in a network. Links can be uniquely identified. For purposes of explanation only, this disclosure will identify a link using the loopbacks of nodes between which the link is positioned. To illustrate, for a link between two nodes identified by node loopback X and node loopback Y, the link will be identified herein as link XY. Because loopbacks are unique, link IDs are unique. Link IDs should not be confused with adjacency-segment IDs; adjacency-segment IDs may not be unique within a network. This disclosure will presume that only one link exists between nodes in a network, it being understood the present disclosure should not be limited thereto. 
         [0031]    Each SR node can assign a distinct adjacency-segment ID for each of the node&#39;s links. Adjacency-segment IDs are locally significant; separate SR nodes may assign the same adjacency-segment ID, but that adjacency-segment ID represents distinct links. In one embodiment, adjacency-segment IDs are selected from a predefined range that is outside the predefined range for nodal-segment IDs. 
         [0032]    SR nodes can advertise routing information including nodal-segment IDs bound to loopbacks, adjacency-segment IDs mapped to link IDs, etc., using protocols such as IGP and/or BGP with SR extension. Nodes can use the routing information they receive to create or update SR forwarding tables. To illustrate, SR nodes may use the routing information they receive in order to create topology maps of the network, which in turn can be used to identify next hop egress interfaces of shortest paths (SPTs) to respective node loopbacks. The identified SPT or next hop egress interfaces are then mapped to respective nodal-segment IDs in an SR forwarding table. Nodes can also map their adjacency-segment IDs to egress interfaces for respective links in SR forwarding tables. Because adjacency-segment IDs are locally significant, however, adjacency-segment IDs should only be mapped in SR forwarding tables of the nodes that advertise the adjacency-segment IDs. In other words, an SR node that advertises an adjacency-segment ID should be the only node in the network area that has a SR forwarding table that maps the adjacency-segment ID to an egress interface. 
         [0033]    As noted above, SR enables segment paths (SPs), which can be used for transporting packets through a network. SPs can be associated with FECs, and can be established for a variety of purposes. Packets associated with the same FEC normally traverse the same SP towards their destination. Nodes in SPs make forwarding decisions based on segment IDs, not based on the contents (e.g., destination IP addresses) of packets. As such, packet forwarding in SPs is not dependent on a particular Layer  2  technology. 
         [0034]    Edge nodes and/or other devices (e.g., a centralized control plane server) of an SR network use routing information (nodal-segment IDs bound to loopbacks, adjacency-segment IDs mapped to link IDs, etc.) they receive in link advertisements to create ordered lists of segment IDs (i.e., segment ID stacks). Segment ID stacks correspond to respective SPs. Individual segment IDs in a stack may correspond to respective segments or sub paths of a corresponding SP. 
         [0035]    When an SR ingress edge node receives a packet, the node or a centralized control plane server in data communication with the node, can select an SP for the packet based on information contained in the packet. In one embodiment, a FEC may be calculated for the packet using the packet&#39;s destination address. The FEC is then used to select a segment ID stack mapped thereto. The ingress edge node can attach the selected segment ID stack to the packet via an SR header. The packet with attached stack is forwarded along and can traverse the segments of the SP in an order that corresponds to the list order of the segment IDs in the stack. A forwarding engine operating in the data plane of each SR node can use the top segment ID within the stack to look up the egress for next hop. As the packet and attached segment ID stack are forwarded along the SP in a hop-by-hop fashion, segment IDs can be popped off the top of the stack. In another embodiment, the attached stack of segment IDs remains unchanged as the packet is forwarded along the SP. In this embodiment, a pointer to an active segment ID in the stack can be advanced as the packet is forwarded along the SP. In contrast to MPLS, however, segment IDs are typically not swapped as the packet and attached segment ID stack are forwarded along the SP. 
         [0036]    To illustrate general concepts of SR,  FIG. 2  shows an example SR enabled provider network that is in data communication with access network nodes AE 1  and AE 2 . Network  202  consists of SR nodes  204 - 222 . Nodes  204 - 210  are assigned unique nodal-segment IDs  64 - 67 , respectively, nodes  212 - 218  are assigned unique nodal-segment IDs  68 - 71 , respectively, and node  222  is assigned unique nodal-segment ID of  72 . Each of the SR nodes  204 - 222  have interfaces that are identified as shown. For example, node  204  has three interfaces designated  1 - 3 , respectively. Each of the nodes  204 - 222  is assigned a unique loopback. Loopbacks A-D are assigned to nodes  204 - 210 , respectively, loopbacks M-P are assigned to nodes  212 - 218  respectively, and loopback Z is assigned to node  222 . These loopbacks are unique in the network and can be used for several purposes such as calculating the topology of network  202 , which in turn can be used to create SPs and/or to identify SPTs and thus next hop egress interfaces for SR forwarding tables. Nodes  204 - 222  can also assign locally significant adjacency-segment IDs. For example, node  208  can assign adjacency-segment IDs  9001 - 9003  to links CB, CD, and CO, respectively. 
         [0037]    Each of SR nodes  204 - 222  can advertise routing information to the other nodes in network  202  using IGP with SR extension. For example, node  208  can generate and send one or more link state advertisements that include adjacency-segment IDs  9001 - 9003  bound to link IDs CB, CD, and CO, respectively, and nodal-segment ID  66  bound to loopback C. One of ordinary skill understands that link state advertisements may contain additional information. Using the advertisements they receive, the control planes of nodes  204 - 222  can generate respective SR forwarding tables for use in the data planes. For example, node  208  can generate example SR forwarding table  240  that maps adjacency-segment IDs  9001 - 9003  to node interface IDs  1 - 3 , respectively, and nodal-segment IDs such as  64 ,  65 ,  67 ,  70 , and  72 , to node  208  interfaces 1, 1, 2, 3, and 2, respectively, which are the SPT next hop egress interfaces determined by node  208  for loopbacks A, B, D, O, and Z respectively. It is noted that in the embodiment shown, only SR forwarding table  240  maps adjacency-segment IDs  9001 - 9003  to interfaces; SR forwarding tables in the other nodes of network  202  should not map adjacency-segment IDs  9001 - 9003 . 
         [0038]    In addition to creating SR forwarding tables based on received segment ID advertisements, SR nodes or a centralized control plane server (not shown) can create segment ID stacks for respective SPs. For example, ingress edge node  204  creates example segment ID stack  224  for an SP between ingress edge node  204  and egress edge node  222 . Example segment stack  224  can be created for a particular FEC (e.g., FEC F). Example stack  224  includes three segment IDs: nodal-segment IDs  66  and  72  advertised by nodes  208  and  222 , respectively, and adjacency-segment ID  9003  advertised by node  208 . Stack  224  corresponds to an SP in which packets flow in order through nodes  204 ,  206 ,  208 ,  216 ,  218 , and  222 . 
         [0039]    In response to receiving a packet from access node AE 1 , SR node  204  or a centralized control plane server (not shown) can select a segment ID stack based on information contained in the packet. For example, node  204  can calculate FEC F for a received packet P based on the destination IP address in packet P. FEC F is mapped to example stack  224  in a table not shown. Node  204  attaches stack  224  to packet P. Example segment stack  224  lists segment IDs that correspond to one hop and multi hop segments that packets traverse to reach egress edge node  222 . The segments collectively form the SP corresponding to stack  224 . Once the segment stack  224  is attached to packet P, ingress SR enable node  204  may access a SR forwarding table (not shown) using the top segment ID (e.g., segment ID=66) to read egress interface identifier  2 , which is the next hop egress interface for the SPT to the SR node assigned nodal-segment ID  66 . 
         [0040]    With continuing reference to  FIG. 2 ,  FIG. 3  illustrates an example process of packet forwarding using segment IDs according to one embodiment. More particularly,  FIG. 3  illustrates an example method performed by an SR node, including an edge node, in a network like that shown in  FIG. 2 . In response to receiving a packet with an attached segment ID stack, or in response to attaching a segment ID stack to a packet, the SR node determines in step  304  whether the top segment ID of the stack matches the nodal-segment ID assigned to the SR node. If there is a match, the process proceeds to step  306  where the SR node pops the top segment ID, which may expose an underlying segment ID as the new top segment ID. If there is no new top segment ID (i.e., the segment popped in  306  was the last segment ID of the stack) the packet P has arrived at the egress edge node, and the process ends. If a new top segment ID is exposed, or if there is no match of segment IDs in step  304 , the SR node accesses its SR forwarding table in step  314  to read the egress interface that is mapped to the top segment ID. In step  316  the SR node determines whether the top segment ID is an adjacency-segment ID. This determination can be implemented by simply comparing the top segment ID with the designated range of adjacency-segment IDs that are available for assignment within the network. If the top segment ID is found to be within the designated range, the top segment ID is an adjacency-segment ID and it is popped. In step  322  the SR node forwards packet P and attached stack to the next node via the egress interface identified in step  314 . 
         [0041]    With continuing reference to  FIG. 3 ,  FIG. 2  shows packet P and attached stack  224  as it is forwarded by nodes. As shown, nodes  204  and  206  forward packet P and stack  224  without popping a segment ID. However, node  208  pops nodal-segment ID  66  and adjacency-segment ID  9003  in accordance with steps  306  and  320 , respectively, before the packet P and stack  224  are forwarded to node  216  in accordance with step  322 . Nodes  216  and  218  forward packet P and stack  224  without popping segment IDs. SR egress edge node  222  recognizes itself as the last hop of the SP. Eventually, node  222  may employ traditional IP routing and forward packet P to access node AE 2  based on routing table lookup using the destination IP address within packet P. 
       3. Inter Area Segment Routing 
       [0042]    A provider network may be subdivided into areas to simplify administration and optimize resource utilization. An area consists of nodes that have been administratively grouped together into an IGP domain. A backbone area typically forms the core of a subdivided network. Additional areas are connected to the backbone and referred to herein as aggregation areas. Nodes that are totally within an area are called internal nodes. All interfaces of internal nodes are directly connected to nodes within the area. Area border nodes have interfaces that are directly coupled to at least one node in the backbone area and at least one node in a connected aggregation area. Area border nodes can be viewed as being contained in more than one area. Inter-area packet forwarding happens via area border nodes. Edge nodes in aggregation areas may be connected to nodes (e.g., access network nodes) that are external to the subdivided provider network. 
         [0043]      FIG. 4  illustrates a portion of a subdivided provider network  400  employing SR. Network  400  includes backbone area  402  coupled between aggregation areas  404  and  406 . It is noted that additional aggregation areas (not shown) may be connected to backbone  402 . 
         [0044]    In one embodiment, nodal-segment IDs are unique in the areas in which their nodes are contained. As a result SR nodes in different areas may be assigned the same nodal-segment ID. For example, area border node  414  in area  404  is assigned nodal-segment ID  62 , and area border node  422  in area  406  is also assigned nodal-segment ID  62 . 
         [0045]    Each of the areas  402 - 406  can implement its own IGP with SR extension. Border area nodes implement a separate instance of IGP with SR extension for each area in which they are contained. Each node in an area advertises routing information including nodal-segment IDs bound to loopbacks to the other nodes in the area in the same or similar fashion that nodes in  FIG. 2  advertise their routing information. Nodes can use the routing information they receive to create SR forwarding tables for the area in which the nodes are contained in much the same fashion as described above. Area border nodes may create an SR forwarding table for each area in which they are connected. 
         [0046]    Internal BGP with SR extension can be employed to exchange routing information between area border nodes, edge nodes (e.g., nodes  410  and  442 ) and/or other devices like a centralized control plane server. The routing information exchanged via internal BGP can be used by the edge nodes and/or the dedicated devices to create segment ID stacks for edge-to-edge SPs. For example, area border node  422  may establish a session with area border node  414  in which area border node  422  advertises routing information to reach edge node  442 . This routing information may indicate that loopback Z (node  442 ), which is bound to nodal-segment ID  73 , can be reached via loopback J (node  422 ), which is bound to nodal-segment ID  62 . Area border node  414  in turn may establish a session with edge node  410  in which area border node  414  advertises routing information to reach edge node  442 . This routing information may indicate that loopback Z (node  442 ) bound to nodal-segment ID  73 , can be reached via node loopback J (node  422 ) bound to nodal-segment ID  62 , which in turn can be reached via loopback D (node  414 ) bound to nodal-segment ID  62 .  FIG. 4  provides visual illustration of these internal BGP session communications.  FIG. 4  also shows an example segment ID stack  450  created by edge node  410  for an SP to node  442  using the exchanged routing information. 
       4. Area-Segment IDs 
       [0047]    Area-segment IDs, an additional type of segment ID, can be assigned to each area in a subdivided network. Area-segment IDs are assigned so that no two areas in a subdivided network are assigned the same area-segment ID.  FIG. 5  illustrates the network of  FIG. 4  with area-segment IDs assigned to the areas. For example, backbone area  402  is assigned area-segment ID AS 2  while aggregation areas  404  and  406  are assigned area-segment IDs AS 1  and AS 3 , respectively. 
         [0048]    Each node in an area can be assigned the area-segment ID for the area in which the node is contained. Each node may bind its assigned area-segment ID to its loopback. The ranges for nodal-segment IDs and adjacency-segment IDs may be different from the predefined ranges for area-segment IDs. 
         [0049]    Area-segment IDs can be used for a variety of purposes. For example, area-segment IDs can be used to preclude forwarding errors that can arise from use of duplicate nodal-segment IDs in segment stacks. Stack  450  in  FIG. 4  has duplicate segment IDs. When node  414  receives and processes packet P with attached header  450  using the process shown in  FIG. 3 , the duplicate segment-IDs may create a forwarding error. For example, it is possible that border area node  414  may not know which SR forwarding table it should use in step  314 . 
         [0050]    Each node in an area can use IGP with SR extension to advertise its routing information including nodal-segment IDs and area-segment IDs bound to loopbacks to other nodes in the area in the same or similar fashion as described above with reference to  FIGS. 2 and 4 . Area border nodes may advertise additional backbone area related routing information into the aggregation areas in which they are contained. This additional routing information may include the area border node&#39;s loopback bound to AS 2 , the area-segment ID of the backbone area. 
         [0051]    Nodes in an area can use the routing information they receive to create SR forwarding tables for the area in which the nodes are contained in the same or similar fashion as described above with reference to  FIGS. 2 and 4 . Area border nodes may create separate SR forwarding tables: one for the aggregation area in which they are contained and one for the backbone area. Each SR forwarding table of an area border node may be linked to a respective area-segment ID. SR forwarding tables of edge and internal nodes of an aggregation area may map nodal-segment IDs and/or backbone area-segment IDs to next hop egress interfaces. 
         [0052]    Area-segment IDs can be included in routing information exchanged between area border nodes, edge nodes and/or a centralized control plane server during internal BGP with SR extension sessions. To illustrate with reference to  FIG. 5 , area border node  422  may establish a session with area border node  414  in which area border node  422  advertises routing information to reach edge node  442 . This routing information may indicate that loopback Z (node  442 ), which is bound to nodal-segment ID  73 , can be reached via loopback J (node  422 ) which is bound to nodal-segment ID  62  and area-segment ID AS 3 . Area border node  414  in turn may establish a session with edge node  410  in which area border node  414  advertises routing information to reach edge node  442 . This routing information may indicate that loopback Z (node  442 ), which is bound to nodal-segment ID  73 , can be reached via node loopback J (node  422 ), which is bound to nodal-segment ID  62  and area-segment ID AS 2 , which in turn can be reached via loopback D (node  414 ), which is bound to nodal-segment ID  62 .  FIG. 5  provides visual illustration of these internal BGP session communications. 
         [0053]    SR edge nodes and centralized control plane servers can use the routing information they receive via IGP and internal BGP sessions to create segment ID stacks for edge-to-edge SPs. These segment ID stacks may contain one or more area-segment IDs. For example, using the routing information it received during the session with area border node  414  described above, edge node  410  creates a segment ID stack  524  for an SP that extends from edge node  410  to edge node  442  via area border nodes  414  and  422 . 
         [0054]    In one embodiment, area-segment IDs within a stack can be used by area border nodes, such as area border nodes  414  and  422 , to select the appropriate SR forwarding table to use when making packet forwarding decisions. In another embodiment, area-segment IDs within a stack can be used by internal and/or edge nodes of an aggregation area to look up next hop egress interfaces for packets. To illustrate with continuing reference to  FIG. 5 ,  FIG. 6  illustrates an example process of packet forwarding using segment IDs including area-segment IDs according to one embodiment. The process shown in  FIG. 6  can be employed by nodes within the network  500 . When an SR node attaches a segment stack to packet P or when an SR node receives a packet P with attached segment stack, the node in step  604  determines whether the top segment ID of the stack matches the nodal-segment ID assigned to that SR node. If there is a match, the node pops off the top segment ID, and determines whether an underlying segment ID is exposed as a new top segment ID as shown in step  606 . If there is no new top segment ID, the packet P is presumably received by an egress edge node (e.g., node  442 ), and the packet is forwarded towards the access node based upon the packet&#39;s destination IP address and an IP forwarding table. However, if an underlying new top segment is exposed as a result of the pop operation in step  606 , the SR node determines whether the new top segment ID is an area-segment ID. This can be implemented by comparing the segment ID with the predefined range of segment IDs to see if the segment ID falls within the range. If the SR node determines that the top segment is an area-segment ID, which should occur when the SR node is an area border node, the area-segment ID is popped, which in turn will expose a new segment ID. The popped area-segment ID is used to access a corresponding SR forwarding table within the area border node, and the SR node reads the egress interface mapped to the new top segment ID as shown in step  616 . If it is determined in step  616  that the top segment ID is not an area-segment ID, then the SR node is presumably an internal node or an ingress edge node, and the process proceeds to step  622  where the SR node accesses its SR forwarding table to read the egress interface mapped to the top segment ID of the segment stack. Regardless of whether the access step  616  or access step  622  is performed, the packet P is forwarded with the attached segment stack the next hop via the egress interface read in step  622  or step  616  as shown in step  620 . In this fashion, packets can be forwarded seamlessly between aggregation and backbone area networks of a subdivided SR network. 
         [0055]    It is noted above that in an alternative embodiment, area border nodes advertise their loopbacks bound to the backbone area-segment ID into the aggregation. Edge and internal nodes within the aggregation area can map area-segment IDs to egress interfaces for the next hop toward the closest area border node. In this configuration, a more compact segment ID stack (i.e., less segment IDs) can be created and used for forwarding packets. To illustrate,  FIG. 5  shows an alternative example segment stack  526  employing this alternative. As seen, segment stack  526  lacks the nodal-segment ID  62  assigned to area border node  414 . The example forwarding process shown in  FIG. 6  can be employed by the SR nodes shown within  FIG. 5  using this alternative segment stack  526 . 
       5. Access to Aggregation SR 
       [0056]    The networks shown in  FIGS. 4 and 5  enable edge-to-edge SPs between aggregation area edge nodes. Aggregation area edge nodes (hereinafter aggregation edge nodes) may be connected to access networks that are also SR enabled. An access network connects subscribers to their immediate provider network. 
         [0057]      FIG. 7  illustrates the subdivided network of  FIG. 5  connected between a pair of SR enabled access networks AN 1  and AN 2 . In the embodiment shown, aggregation edge node  410  is connected to access network AN 1 , and aggregation edge node  442  is connected to access network AN 2 . It is noted additional access networks can be coupled to aggregation edge node nodes  410  and  442 . Moreover, additional access networks may be coupled to areas  404  and  406  within network  500  shown in  FIG. 7 . 
         [0058]    Each node is assigned a unique nodal-segment ID within an access network. These nodal-segment IDs are bound to respective loopbacks. Nodes  706 - 712  within an access network AN 1  are assigned nodal-segment IDs  80 - 82 , respectively, while nodes  714 - 718  in access network AN 2  are assigned nodal-segment IDs  83 - 85 , respectively. 
         [0059]    Each of the access networks AN 1  and AN 2  can implement its own IGP with SR extension. Aggregation edge nodes implement separate instances of IGP with SR extension, one for the access network to which they are connected and one for the aggregation area in which they are contained. Access network nodes and the connected aggregation edge node exchange routing information (e.g., nodal-segment IDs bound to loopbacks). Aggregation edge nodes can advertise additional routing information into their connected access networks. This additional routing information may include the aggregation edge node&#39;s loopback bound to its nodal-segment ID and its area-segment ID. Internal BGP sessions between end nodes (e.g., node  716 ), aggregation edge nodes, area border nodes, and centralized control plane servers can be used to exchange additional routing information. 
         [0060]    Nodes in an access network can use the routing information they receive to create SR forwarding tables for the access network in which they are contained in the same or similar fashion described above. Aggregation edge nodes may create separate SR forwarding tables: one for the access network they are connected to and one for the aggregation area they are contained in. The SR forwarding table for the aggregation area may be linked to the area-segment ID for that area. 
         [0061]    Nodes within a local access network may be incapable or otherwise not configured to create or store segment stacks for end-to-end SPs to nodes in remote access networks. For example, node  706  may lack enough memory needed to store thousands of segment stacks for SPs that extend between node  706  and destination nodes (e.g., node  716  within remote access network AN 2 ) in remote access networks. Thus, when an access network node (e.g., node  706 ) receives a packet P from a device (e.g., host  720 ), which is destined for a remote device (e.g., host  722 ), the access network node may need to rely on an alternative mechanism to provide a segment stack that can be used to transmit the packet P. 
         [0062]    In one embodiment, a centralized control plane server (not shown) can provide segment stacks to nodes of access networks. Access network nodes, such as node  706 , can generate and send a request for a segment stack. The request can be sent to the centralized control plane server that is capable of providing the stack. The request should contain SP related information that is needed by the centralized control plane server to create or select an appropriate segment stack. To that end the centralized control plane server may need either the destination address for packet P or the loopback of the node (i.e., destination node) in the remote access network that is closest to the destination address. In addition, the centralized control plane server may need the loopback of the aggregation edge node connected to the access network that contains the node requesting the segment stack. 
         [0063]    The centralized control plane server can create the needed segment stack using the SP related information provided in the request, or the centralized control plane server may access memory to read a segment stack that is mapped to the SP related information provided in the request. Either way, the centralized control plane server can reply to the request with a segment stack for an SP that extends between the aggregation edge node and the remote access network destination node identified in the request. 
         [0064]    The requesting access network node receives the reply and attaches the segment stack contained therein to packet P. Thereafter, the access node can add the nodal-segment ID and area-segment ID for the aggregation edge node to the top of the attached segment stack so that the nodal-segment ID is the top segment ID initially. Once the two segment IDs are added to the stack, the packet and attached stack can be forwarded along the SP toward its destination. The access network node that attaches the segment stack to packet P and each node in the SP that receives the packet with segment stack can make a forwarding decision employing the process shown within  FIG. 6 . The aggregation edge node can use the area-segment ID added to the segment stack to select the proper SR forwarding table when the aggregation edge node receives the packet P with attached segment stack in accordance with step  616 . In this fashion, packets can be forwarded seamlessly between access and aggregation area networks. 
         [0065]      FIG. 8  is a block diagram illustrating certain additional and/or alternative components of a node that can be employed in the networks described above. In this depiction, node  800  includes a number of line cards (line cards  802 ( 1 )-(N)) that are communicatively coupled to a forwarding engine or packet forwarder  810  and a processor  820  via a data bus  830  and a result bus  840 . Line cards  802 ( 1 )-(N) include a number of port processors  850 ( 1 , 1 )-(N,N) which are controlled by port processor controllers  860 ( 1 )-(N). It will also be noted that forwarding engine  810  and processor  820  are not only coupled to one another via data bus  830  and result bus  840 , but are also communicatively coupled to one another by a communications link  870 . 
         [0066]    The processors  850  and  860  of each line card  802  may be mounted on a single printed circuit board. When a packet or packet and header are received, the packet or packet and header with segment stack may be identified and analyzed by router  800  in the following manner. Upon receipt, a packet (or some or all of its control information) or packet and header is sent from the one of port processors  850 ( 1 , 1 )-(N,N) at which the packet or packet and header was received to one or more of those devices coupled to data bus  830  (e.g., others of port processors  850 ( 1 , 1 )-(N,N), forwarding engine  810  and/or processor  820 ). Handling of the packet or packet and header can be determined, for example, by forwarding engine  810 . For example, forwarding engine  810  may determine that the packet or packet and header should be forwarded to one or more of port processors  850 ( 1 , 1 )-(N,N). This can be accomplished by indicating to corresponding one(s) of port processor controllers  860 ( 1 )-(N) that the copy of the packet or packet and header held in the given one(s) of port processors  850 ( 1 , 1 )-(N,N) should be forwarded to the appropriate one of port processors  850 ( 1 , 1 )-(N,N). In addition, or alternatively, once a packet or packet and header has been identified for processing, forwarding engine  810 , processor  820  or the like can be used to process the packet or packet and header in some manner or add packet security information, in order to secure the packet. On a node sourcing such a packet or packet and header, this processing can include, for example, encryption of some or all of the packet&#39;s or packet and header&#39;s information, the addition of a digital signature or some other information or processing capable of securing the packet or packet and header. On a node receiving such a processed packet or packet and header, the corresponding process is performed to recover or validate the packet&#39;s or packet and header&#39;s information that has been thusly protected. 
         [0067]    Although the present disclosure has been described in connection with several embodiments, the disclosure is not intended to be limited to the specific forms set forth herein. On the contrary, it is intended to cover such alternatives, modifications, and equivalents as can be reasonably included within the scope of the disclosure as defined by the appended claims.