Junos® OS For Dummies®, 2nd Edition

Placing Constraints on Packet Forwarding

When RSVP creates the paths for its LSPs, it uses information learned from the underlying IGP configuration. Basically, if OSPF has the shortest route from A to B, RSVP will establish an LSP across that path. At times, however, you may want to dictate your own path. For example, you may know that in terms of hops, one path is shorter, but when you talk about actual latency and link speed, a different path is preferable. In these instances, you want to put constraints on the path taken through your network.

Routing LSPs based on these constraints is called constraint-based routing. You may want to constrain the path that an LSP takes in a few different ways:

• Reserve bandwidth so that guaranteed bandwidth appears along the path.
• Specify a particular node in the network through which the LSP will pass.
• Identify the exact path for the LSP.

Reserving bandwidth on an LSP

One of the best parts about RSVP is that you can specify a minimum bandwidth that must be supported on each transit router in the LSP. This specification helps ensure that you have enough allocated bandwidth from ingress to egress, especially for traffic that is particularly sensitive to latency or drop.

For example, imagine that you're streaming video across your network that you're using to conduct simulations in multiple branch offices. For the simulation to work well, every site must receive the same feed with the same timing. You may want to ensure that the LSPs you're ...