a cef is born...

route one switch many...

in the beginning multi-layer switching began as a marriage between a route processor and a switch engine and it was good.  the rp gets the first packet, determines a route and forwards the packet to the correct interface.  the se  participates by learning this path from the rp, and if it is determined that the switch engine can switch both directions, the se cuts out the routing process.  this is also called route cache switching or netflow switching...

cisco express forwarding evolved from netflow switching and is more efficient because it uses built in hardware and dynamic lookups... cef's first functional block is: the fib, forwarding information base...

the FIB is not a lie
the fib block is the layer 3 engine, the router.  to create the fib, the router builds a  table and orders the most specific route to the destination first, the longer the destination prefix, the closer to first place.  the fib contains the next hop entry for each destination.  the fib also contains a host entry, or the subnet mask of the address in all 1's or 255.255.255.255. after a topology change the fib is updated by the router, and if an arp entry ages out, the fib will again be updated to reflect that change.


sw3550_01#sh arp
Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet  192.168.1.111           0   0009.b752.d780  ARPA   Vlan10
Internet  192.168.1.100           -   000f.8ffe.0980  ARPA   Vlan10
Internet  192.168.1.120           0   0014.6ad0.da00  ARPA   Vlan10
Internet  192.168.1.112           0   0009.b73f.ce80  ARPA   Vlan10
Internet  192.168.1.50            0   000f.2394.6c40  ARPA   Vlan10
Internet  192.168.1.1             0   c03f.0eab.d1ec  ARPA   Vlan10
Internet  192.168.69.69           -   000f.8ffe.0980  ARPA   Vlan69
Internet  172.16.50.2             0   000d.ed22.eb44  ARPA   Vlan50
Internet  172.16.50.50            0   000f.2394.6c40  ARPA   Vlan50
Internet  172.16.50.100           -   000f.8ffe.0980  ARPA   Vlan50
Internet  192.168.1.130           0   0011.5c2d.b8c0  ARPA   Vlan10

of course the command is not show ip fib, rather...

sw3550_01#sh ip cef
Prefix               Next Hop             Interface
0.0.0.0/0            192.168.1.1          Vlan10
0.0.0.0/32           receive
1.1.1.0/24           172.16.50.50         Vlan50


omitted
192.168.1.130/32     attached             Vlan10
192.168.1.255/32     receive              Vlan10
192.168.69.0/24      attached             Vlan69
192.168.69.0/32      receive              Vlan69
192.168.69.69/32     receive              Vlan69
192.168.69.255/32    receive              Vlan69


and after the vlan information has been eliminated:
no more arp for 192.168.69.69...

sw3550_01(config)#do sh arp
Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet  192.168.1.111           1   0009.b752.d780  ARPA   Vlan10
Internet  192.168.1.100           -   000f.8ffe.0980  ARPA   Vlan10
Internet  192.168.1.120           1   0014.6ad0.da00  ARPA   Vlan10
Internet  192.168.1.112           1   0009.b73f.ce80  ARPA   Vlan10
Internet  192.168.1.50            1   000f.2394.6c40  ARPA   Vlan10
Internet  192.168.1.1             0   c03f.0eab.d1ec  ARPA   Vlan10
Internet  172.16.50.2             1   000d.ed22.eb44  ARPA   Vlan50
Internet  172.16.50.50            1   000f.2394.6c40  ARPA   Vlan50
Internet  172.16.50.100           -   000f.8ffe.0980  ARPA   Vlan50
Internet  192.168.1.130           1   0011.5c2d.b8c0  ARPA   Vlan10

and  the fib entry is also eliminated...

sw3550_01(config)#do sh ip cef
Prefix               Next Hop             Interface
0.0.0.0/0            192.168.1.1          Vlan10


omitted
192.168.1.130/32     attached             Vlan10
192.168.1.255/32     receive              Vlan10
224.0.0.0/4          drop
224.0.0.0/24         receive

or specifically...

sw3550_01(config)#do sh ip cef vlan 69

sw3550_01(config)#