applications. CEF improves switching speed and avoids the overhead associated with a cache
that continually changes through the use of the FIB, which mirrors the entire contents of the IP
routing table.
such as the Cisco 7500. Distributed CEF (DCEF) provides each of the Cisco 7500 VIPs with
an identical on-card copy of the FIB database, allowing them to autonomously perform CEF
and therefore significantly increase aggregate throughput.
the other methods optimize the switching of a packet through a single router, tag switching
optimizes packet switching through a network of tag switches.
(MPLS). The idea behind MPLS is that by labeling or tagging the first packet in a flow of data,
subsequent packets can be expedited to the final destination. Tagging minimizes the processing
required of a router and thus significantly reduces delay on packet and cell-based networks that
include tag switches. In addition, packets can be tagged to travel along specified routes to
implement load balancing, QoS, and other optimization features. Tag information can be
carried in a packet as a small header inserted between the Layer 2 and Layer 3 headers, or as
part of the Layer 3 header if the Layer 3 protocol supports it. (In IPv6, tag information can be
included in the flow-label field.) Some Layer 2 implementations, such as ATM, support carrying
the tag directly in the Layer 2 header.
the following sections:
fails, the host loses connectivity to the network. To ensure that the host can maintain
connectivity if the default gateway fails, Hot Standby Router Protocol (HSRP) can be
implemented. With HSRP, two or more routers communicate on a virtual gateway address