 | Copyright © 2008, Juniper Networks, Inc. All rights reserved. Trademark Notice. | |
After network link attributes and topology information are flooded by the IGP and placed in the TED, each ingress router uses the TED to calculate the paths for its own set of LSPs across the routing domain. The path for each LSP can be represented by either a strict or loose explicit route. An explicit route is a preconfigured sequence of routers that should be part of the physical path of the LSP. If the ingress router specifies all the routers in the LSP, the LSP is said to be identified by a strict explicit route. If the ingress router specifies only some of the routers in the LSP, the LSP is described as a loose explicit route. Support for strict and loose explicit routes allows the path selection process to be given broad latitude whenever possible, but to be constrained when necessary.
The ingress router determines the physical path for each LSP by applying a Constrained Shortest Path First (CSPF) algorithm to the information in the TED. CSPF is a shortest-path-first algorithm that has been modified to take into account specific restrictions when the shortest path across the network is calculated. Input into the CSPF algorithm includes:
As CSPF considers each candidate node and link for a new LSP, it either accepts or rejects a specific path component based on resource availability or whether selecting the component violates user policy constraints. The output of the CSPF calculation is an explicit route consisting of a sequence of router addresses that provides the shortest path through the network that meets the constraints. This explicit route is then passed to the signaling component, which establishes the forwarding state in the routers along the LSP.
Despite the reduced management effort resulting from online path calculation, an offline planning and analysis tool is still required to optimize traffic engineering globally. Online calculation takes resource constraints into account and calculates one LSP at a time. The challenge with this approach is that it is not deterministic. The order in which LSPs are calculated plays a critical role in determining each LSP’s physical path across the network. LSPs that are calculated early in the process have more resources available to them than LSPs calculated later in the process because previously calculated LSPs consume network resources. If the order in which the LSPs are calculated is changed, the resulting set of physical paths for the LSPs also can change.
An offline planning and analysis tool simultaneously examines each link’s resource constraints and the requirements of each LSP. Although the offline approach can take several hours to complete, it performs global calculations, compares the results of each calculation, and then selects the best solution for the network as a whole. The output of the offline calculation is a set of LSPs that optimizes utilization of network resources. After the offline calculation is completed, the LSPs can be established in any order because each is installed according to the rules for the globally optimized solution.
| Copyright © 2008, Juniper Networks, Inc. All rights reserved. Trademark Notice. | |