Information Technology Reference
In-Depth Information
•
Improved performance
—CEF is less CPU-intensive than route caching. More CPU
processing power can then be dedicated to Layer 3 services, such as processing BGP
updates.
•
Resilience
—CEF offers better switching consistency and stability in large dynamic
networks. In such networks, fast switching cache entries are frequently invalidated
because of routing changes. These changes can cause traffic to be process-switched
using the routing table rather than fast-switched using the route cache. Because the
CEF lookup table contains all known routes that exist in the routing table, it eliminates
route cache maintenance and the fast switch/process switch forwarding scenario. CEF
can switch traffic more efficiently than typical demand caching schemes.
Because entries are maintained for all the routes in the IP RIB whether they are used or not,
more memory might be required by CEF than for other switching methods.
NOTE
CEF is a topology-driven switching mechanism whose forwarding table is tied to the
routing table. Whenever there are routing table changes, the CEF forwarding table is
updated. While entries are created, packets are switched in a slower switching path. CEF
splits the function of the route cache into two main components:
•
Forwarding information base (FIB)
•
Adjacency table
FIB
The
FIB
contains all IP prefixes from the routing table. If different routing tables are main-
tained, such as in an MPLS VPN environment, each VPN has its own FIB. The FIB is not
data-driven. Rather, it is created and updated by the routing table. The FIB subsystem is
responsible for ensuring that all recursive routes (routes are not associated with immediate
next hops) are resolved.
To increase consistency and decrease lookup time, FIB is organized in a multiway data
structure called mtrie. In an
mtrie
data structure
, the tree structure is used to locate the
desired data, but the data itself is stored elsewhere. In contrast, an
mtree
data structure
stores the actual data within the tree structure itself. For example, in the optimum switching
mtree cache, the MAC header data used to forward packets is actually stored inside the
mtree.
