Operational Differences
Although the underlying concepts of each of these features are basically the same
in the distributed versions, the exact implementation may differ significantly.To
maximize the efficiency of the router, the inner workings of a particular feature
may need revision when running in distributed mode. Consider Cisco express
forwarding as an example.With distributed CEF (DCEF), a forwarding information
base (FIB) is built on the RSP, which caches the outgoing interface corresponding
to a route.This FIB is shared by the RSP with each VIP by
downloading it directly to the VIPs. Changes are made to the FIB only when the
routing table changes.Any change to the FIB is again shared with all VIPs. Since
the RSP and each VIP use the same FIB, there is an efficiency gain by sharing it.
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262 Chapter 8 • Advanced QoS for AVVID Environments
This eliminates the per-flow overhead of route-cache maintenance.With DCEF,
switching decisions are handled by the VIP whenever possible. However, if DCEF
does not have the route cached, it will fall back to the next level of routing on
the RSP, usually fast switching or process switching.