LSPs on an Overloaded Router

An overloaded router is a router running IS-IS with its overload bit set in its IS-IS configuration. In this case, an MPLS LSP specifically refers to an RSVP -or LDP-signalled LSP. In the case of RSVP, it applies to both CSPF and non-CSPF LSPs.

You cannot establish transit LSPs through an overloaded router. However, you can configure ingress and egress LSPs through an overloaded router.

When you set the overload bit on an IS-IS router, all LSPs transiting through this router are recomputed and rerouted away from this node. If the recomputation fails, no additional attempt to reconfigure the LSP is made, and the affected LSPs are disconnected.

An example of when you might want to establish transit LSPs through an overloaded router is illustrated by Figure 3, which shows an aggregation router (Router A) dual-homed on two core routers (Router B and Router C). You want to include the aggregation router in the LSP mesh, but transit LSPs should not pass through it, because it is a less capable router with relatively low-bandwidth uplinks to the core. Certain failure and rerouting scenarios could make it impossible for the aggregation router to establish some of its LSPs. Consequently, you run the router in a steady state with the overload bit set, but you are still able to establish ingress and egress LSPs through it.

Figure 3: Aggregation Router A Dual-Homed on Core Routers B and C