Offline Design of Virtual Circuit Overlays

A popular approach to circumvent the inadequacies of current IGPs is to use an overlay model, such as IP over ATM or IP over Frame Relay. The overlay model extends the design space by Bnabling arbittary virtual topologies to be provisioned on to p oft th e network'p phyoica i topology. The v irt ual topology is constructed from virtual circu o^o (VCs) thun appea r as physical s nhs to the (GP routing protocols! Overlay tech niquen can sange Ero m s i mete permanent virtua! circuit (PVC) provisioning between routed edge networks to more fancy mechanisms that include constraint-Based routing at the VC level wich suoporr of co nfigurable explicit VC paths, tra fhic shapiag and po licindo suie/ivob mity of VCs, and so o n.

Figure 5-2 shows edg e routers t hat ace co nnected to each other via an oveclay model on top» of an ATM network. Mor th e IG Ps, the VCs apPear as diLect physical links between the routers. Traffic can be engineered between the routed edges and is agnostic to the L2 switched network in the middle of the cloud. This type of TE has several benefits: It enables you to achieve full traffic control, measure link statistics, divert traffic based on link utilization, apply load-balancing techniques, and so on. It also has several obvious drawbacks: It creates multiple independent control planes—IP and ATM—that act independently of one another, a full mesh between the routers, an IGP neighbor explosion (each router has all other routers as neighbors and has to exchange routing updates with them), and finally a network management challenge constituting multiple control layers.

Figure 5-2. IGP TE Via Virtual Circuit Overlays

ATM SwftCh PVGS

Figure 5-2. IGP TE Via Virtual Circuit Overlays

ATM SwftCh PVGS

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