Figure 57: Typical RSVP-Signaled LSP with EROs
Explicit Route Objects (EROs) limit LSP routing to a specified list of LSRs. By default, RSVP messages follow a path that is determined by the network IGP's shortest path. However, in the presence of a configured ERO, the RSVP messages follow the path specified.
EROs consist of two types of instructions: loose hops and strict hops. When a loose hop is configured, it identifies one or more transit LSRs through which the LSP must be routed. The network IGP determines the exact route from the inbound router to the first loose hop, or from one loose hop to the next. The loose hop specifies only that a particular LSR be included in the LSP.
When a strict hop is configured, it identifies an exact path through which the LSP must be routed. Strict-hop EROs specify the exact order of routers through which the RSVP messages are sent.
You can configure loose-hop and strict-hop EROs simultaneously. In this case, the IGP determines the route between loose hops, and the strict-hop configuration specifies the exact path for particular LSP path segments.
Figure 57 shows a typical RSVP-signaled LSP that uses EROs.
Figure 57: Typical RSVP-Signaled LSP with EROs
In the topology shown in Figure 57, traffic is routed from host C1 to host C2. The LSP can pass through router R4 or router R7. To force the LSP to use R4, you can set up either a loose-hop or strict-hop ERO that specifies R4 as a hop in the LSP. To force a specific path through routers R4, R3, and R6, configure a strict-hop ERO through the three LSRs.