Label Distribution Protocol (LDP) (described in Request For Comments (RFC) 5036, October 2007) is used to advertise mappings of Forward Equivalence Classes (FECs) to labels. IP (Internet Protocol) prefix FECs are used to setup Label Switched Paths (LSPs) along routed paths. LDP advertises label mappings for IP prefix FECs using routes in the routing table. As the number of FECs in the network increases the number of labels correspondingly increases. For example, in a label switched network of multiple Label Switched Routers (LSRs) that each run LDP, the egress LSR allocates a non-NULL label for each unique egress next-hop. If each prefix has a unique next-hop, a unique label will have to be allocated per prefix. In a wireless and/or wireline subscriber termination scenario with many subscribers (e.g., potentially thousands to millions), the number of labels that is allocated is large.
The LSRs that are not the egress for the prefix can know the association between the prefix (along with its associated label) and the egress LSR for that prefix using one of the following techniques: using the IP routing table to allocate labels for the FECs; running a link-state protocol (e.g., OSPF (Open Shortest Path First) described in RFC 2328, April 1998, IS-IS (Intermediate System to Intermediate System) described in RFC 1142, Feb. 1990), or running an additional protocol (e.g., BGP (Border Gateway Protocol) described in RFC 4271, January 2006).
However, the above techniques may in some cases not be possible or preferable. For example, in many cases there is a requirement that the edge LSRs run a simple routing protocol such as static or RIP (Routing Information Protocol) with BFD (Bidirectional Forwarding Detection) for redundancy. For example, an ingress LSR that is a base station network element in a wireless subscriber termination scenario may be required by the network provider to run a relatively simple non link-state routing protocol. These protocols do not give the association between the FEC and the egress LSR.
In addition, the convergence time for Interior Gateway Protocol (IGP) can be relatively high. For example, the time for IGP to converge is relatively high if there are a large number of prefixes and they are added and deleted to the routing table at a high rate (e.g., mobile subscribers that move between stations may cause the routing table to be modified relatively frequently). For example, a typical edge LSR in a wireless network (e.g., a 4G or LTE network) may support hundreds of thousands or millions of subscribers (each of which may have a unique prefix in IGP). IGP also has several dampening mechanisms that can increase the convergence time (e.g., LSA (link state advertisement) generation delay (e.g., through use of the MinLSlnterval described in RFC 2328); LSA pacing timer (e.g., use of the retransmission timer (Rxmtlnterval) described in RFC 2328); and SPF (Shortest Path First) dampening timer). Furthermore, when the LSDB (Link State Database) is large, SPF performs many memory accesses during a typical SPF run and also requires a significant amount of processing and/or memory accesses when downloading the routes to a common store (e.g., a RIB (routing information base)). These all increase the convergence time for IGP.
Running an additional protocol such as BGP is not preferable since it adds overhead (both in capital expenditures (CapEx) and operational expenditures (OpEx)). For example, the overhead in CapEx includes the development resources required to develop and/or support another protocol including features such as scalability, high availability, and/or redundancy; extra CPU and/or memory needed to run the additional protocol, which increases with redundancy; the requirement of a full mesh of connections (one connection between every pair of LSRs in the network) increases the amount of computational resources (CPU cycles, memory, etc.) to grow as the number of LSRs in the network increases; and if a full mesh is avoided it requires a separate route-reflector (RR), which is a separate type of network element. The overhead in OpEx includes the introduction of complexity in the network design since BGP has to be configured on each of the LSRs; if an RR is used (which is typically a separate network element), it needs to be maintained by an operator; and configuring and maintaining the BGP protocol requires the expertise of specialized and expensive personnel.