Which forwarding technology stores destination addresses in the cache?
A. MPLS
B. Cisco express forwarding
C. Process switching
D. Fast switching
Which forwarding technology stores destination addresses in the cache?
Which forwarding technology stores destination addresses in the cache?
A. MPLS
B. Cisco express forwarding
C. Process switching
D. Fast switching
Using the route cache is often called fast switching.
https://www.cisco.com/E-Learning/bulk/public/tac/cim/cib/using_cisco_ios_software/cmdrefs/ip_route-cache.htm
Answer: D
B IS RUGHT:
Cisco Router Fast Switching and CEF:
Historically speaking, Cisco has had three major variations of internal routing logic that
apply across the entire router product family. First, Cisco routers used internal logic called
process switching in the early days of Cisco routers, dating back to the late 1980s and early
1990s. Process switching works basically like the routing process, without any of the extra
optimizations.
Next, in the early 1990s, Cisco introduced alternative internal routing logic called fast
switching. Fast switching made a couple of optimizations compared to the older processswitching
logic. First, it kept another list in addition to the routing table, listing specific IP
addresses for recently forwarded packets. This fast-switching cache also kept a copy of the
new data link headers used when forwarding packets to each destination, so rather than
build a new data link header for each packet destined for a particular IP address, the router
saved a little effort by copying the old data link header.
Cisco improved over fast switching with the introduction of Cisco Express Forwarding (CEF)
later in the 1990s. Like fast switching, CEF uses additional tables for faster searches, and it
saves outgoing data link headers. However, CEF organizes its tables for all routing table destinations,
ahead of time, not just for some of the specific destination IP addresses. CEF also
uses much more sophisticated search algorithms and binary tree structures as compared to
fast switching. As a result, the CEF table lookups that replace the routing table matches take
even less time than with fast switching. And it caches the data link headers as well.
Fast switching was the initial Route caching service enabled on networking devices and was endorsed well by industry leaders due to its functionality of offloading the Route processor. Fast Switching is actually a reactive cache-based IP forwarding mechanism i.e. the address lookup uses a route cache to find the IP next hop, outgoing interface, and outbound layer-2 header. Destination addresses are stored in the high-speed cache (Fast Switching) to expedite forwarding. Routers offer better packet-transfer performance when Fast Switching is enabled.
When using Fast Switching, the first packet for a specific destination is forwarded to the route processor for a switching decision. When the processor completes its processing, it adds a forwarding entry for the destination to the fast cache. When the next packet for that specific destination enters device, the packet is forwarded using the information available in the fast cache without directly going to processor for routing decision.
Answer should be D:
Fast switching allows higher throughput by switching a packet using a cache created by the initial packet sent to a particular destination. Destination addresses are stored in the high-speed cache to expedite forwarding. Routers offer better packet-transfer performance when fast switching is enabled. Fast switching is enabled by default on all interfaces that support fast switching.
CEF offers the following benefits:
•Improved performance—CEF is less CPU-intensive than fast switching route caching. More CPU processing power can be dedicated to Layer 3 services such as quality of service (QoS) and encryption.
•Scalability—CEF offers full switching capacity at each line card when dCEF mode is active.
•Resilience—CEF offers an unprecedented level of switching consistency and stability in large dynamic networks. In dynamic networks, fast-switched cache entries are frequently invalidated due to 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 Forwarding Information Base (FIB) lookup table contains all known routes that exist in the routing table, it eliminates route cache maintenance and the fast-switch or process-switch forwarding scenario. CEF can switch traffic more efficiently than typical demand caching schemes.