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cef, briefly...

VERSION 1 
Visibility: Open to anyone
Created on: Apr 19, 2012 6:11 AM by arteq - Last Modified:  Apr 19, 2012 6:12 AM by arteq

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)#

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