Process switching requires the CPU to be personally involved with every forwarding decision.
Fast switching still uses the CPU, but after a packet has been forwarded, information about how to reach the destination is stored in a fast-switching cache. This way, when another packet going to the same destination is seen, the next hop information can be re-used from the cache, so the processor doesn’t have to look up and assemble all the information again. If the information is not cached, (for example a first packet for a given destination network) the CPU will have a similar workload, for that packet, as if fast switching was not in use.
Cisco Express Forwarding (CEF), is the evolution of optimizing the router to make it be able to forward more packets faster. CEF cheats a little, by building a Forwarding Information Base (FIB), and an adjacency table. The FIB is accessed very quickly based on how they built it (it is Cisco proprietary), and contains pre-computed reverse lookups, next hop information for routes including the interface and L2 information to use. (All the stuff a router would have to consider when forwarding a packet).
Process switching is like doing math, long hand.
Fast switching, using the cache, is like doing a problem once long hand, and subsequent problems you remember the answer for, (from memory, or the cache).
CEF is like having programmed an excel spreadsheet, and when the numbers hit the cells, the answer is already calculated.
I have more question about this.
If CEF switching and Fast switching are both enabled
then which is priotized ?
CEF > Fast Switchig
Fast Switching > CEF
At least fast switching is necessary to use CEF swtching?
I think It can not configure only CEF switching without fast switching like below right?
no ip route-cache cef
no ip route-cache
R1(config-if)#ip route-cache cef
R1#sh ip int fa0
FastEthernet0 is up, line protocol is up
IP fast switching is enabled
IP CEF switching is enabled
Thanks and Regards,
indeed interesting; I disabled ip cef, this is what I got
R1# sh ip cef
%IPv4 CEF not running
R1# sh ip cache
IP routing cache 0 entries, 0 bytes
0 adds, 0 invalidates, 0 refcounts
Minimum invalidation interval 2 seconds, maximum interval 5 seconds,
quiet interval 3 seconds, threshold 0 requests
Invalidation rate 0 in last second, 0 in last 3 seconds
Prefix/Length Age Interface Next Hop
R1#sh ip int f0/0
FastEthernet0/0 is up, line protocol is up
Internet address is 10.1.1.1/24
Broadcast address is 255.255.255.255
Address determined by setup command
MTU is 1500 bytes
Helper address is not set
Directed broadcast forwarding is disabled
Outgoing access list is not set
Inbound access list is not set
Proxy ARP is enabled
Local Proxy ARP is disabled
IP fast switching is disabled
IP fast switching on the same interface is disabled
IP Flow switching is disabled
IP CEF switching is disabled
There is no Global command no ip route-cache on my router.
Can you disable fast switching on new router glabally ?
or it has to be under interface?
In process switching, each and every packet coming on to the router is looked up for in the routing table as to where it has to be forwarded....
In fast switching, Only the first packet coming on to the router is looked up for in the routing table, remaining are forwarded based on cached information.....This increases speed tremendously as compared to process swtching...
In CEF, this is fastest as compared to above 2 methods as here even the 1st packet lookup is eliminated, actually when CEF is enabled router does not wait for the 1st packet to arrive it does the lookup job beforehand....just imagine if there were many hosts and router has to do 1st packet lookup for packet coming from each host......