Cisco Express Forwarding vs Fast Switching:

Cisco Express Forwarding vs Fast Switching:

 

Enabling CEF may be a method to alleviate the increased traffic load . CEF is supported in IOS releases 11.1( )CC, 12.0, and later on the Cisco 7200/7500/GSR platforms. Support for CEF on other platforms may be in IOS release 12.0 or later. You can investigate further with the Software Advisor tool on CCO.

 

It may not be possible to enable CEF on all routers for one of the following reasons:

 

--Insufficient memory

 

--Unsupported platform architectures

 

--Unsupported interface encapsulations
 
Fast Switching Behavior and Implications:
 
The following are implications of using fast switching:

 

--Traffic driven cache, or the cache is empty until the router switches packets and populates the cache.

 

--First packet is process switched (since cache is initially empty).

 

--Cache built at a granularity of the most specific Routing Information Base (RIB) entry part of a major net (if RIB has /24s for major net 131.108.0.0, the cache is built with /24s for this major network).

 

--/32 caching for load balancing - per destination load-balancing only (when load balancing, cache is built with /32s for that major net).

 

Note: These last two issues can potentially cause a huge cache that would consume all memory.

 

--With default route, caching is done at major network boundaries.

 

The Cache Ager runs every minute and checks 1/20th (5 percent) of the cache for unused entries under normal memory conditions, and 1/4th (25 percent) of the cache in a low memory condition (200k).
The command to change the above values is: ip cache-ager-interval X Y Z, where:

 

X is <0-2147483> number of seconds between ager runs, default = 60 seconds.

 

Y is <2-50> 1/(Y+1) of cache to age per run (low memory), default = 4.

 

Z is <3-100> 1/(Z+1) of cache to age per run (normal), default = 20.

 

CEF Advantages:

--The Forwarding Information Base (FIB) table is built based on the routing table, therefore forwarding information exists before the first packet is forwarded. The FIB also contains /32 entries for directly connected LAN hosts.

 

--The Adjacency (ADJ) table contains the Layer 2 rewrite information for next-hops and directly connected hosts (an ARP entry creates a CEF adjacency).

 

--There is no cache ager concept with CEF to spike CPU utilization. A FIB entry is deleted if a routing table entry is deleted.

 

--There is a more efficient lookup with CEF (maximum of 4 lookups).

 

Caution: Again, a default route pointing to a broadcast or multipoint interface means that the router will ARP for every new destination, potentially creating a huge adjacency table until the router runs out of memory. If CEF fails to allocate memory CEF/DECF will disable itself and will have to be manually re-enabled.