The 7705 SAR provides MPLS technology using static LSPs, RSVP-TE for traffic-engineered signaled routing of LSPs and LDP for non-traffic-engineered signaled routing of LSPs. A network operator may choose to use any combination of static LSPs, RSVP-TE, and LDP to establish paths for services. RSVP-TE and LDP are considered to be Layer 2.5 protocols.
The 7705 SAR can be used as an ingress and egress Label Edge Router (iLER and eLER), and as a transit router. A transit router is also referred to as a Label Switch Router (LSR).
OSPF and IS-IS are the interior gateway protocols with traffic engineering extensions (IGP-TE) available to the 7705 SAR. These are the Layer 3 protocols. Typically, one or the other of these gateway protocols will be in use in the network. Whichever protocol is the chosen gateway protocol, it must be working in order for LDP or RSVP-TE to function. These Layer 3 protocols identify the next hop, which is information needed by the Layer 2.5 protocols (LDP or RSVP-TE) in order to assign labels.
In addition, the 7705 SAR provides link and node redundancy protection through LSP redundancy and Fast Reroute (FRR) features.
The LSP redundancy and FRR features have the ability to take shared risk link groups (SRLGs) into consideration when the Constrained Shortest Path First (CSPF) algorithm is used to determine an alternate LSP. The selection of a route is determined by the IGP-TE protocol. The added constraints imposed by SRLGs and CSPF ensure that the redundant route selected will be unique from the principal route (route being protected); that is, it will use physical equipment that is different from the equipment that carries the principal route. CSPF will constrain the alternate route to be the shortest possible alternative route. There may be more than one alternative route.