Tammy,

Adjacency table comes from the concept of CEF.

Where Adj. table will keep the L2 Address of all the next hops.

and another table i.e. FIB table will keep the copy of routing table (means use this next-hop-address to reach at this destination).

So, when router has packet for particular destination, router will look in its FIB table, and come to know about the next hop address to reach at this destination.

and, at the same time router will look at the adj table, to know the mac-address of this next-hop.

in this way, router doesn't have to look at the routing table to destined the packet to particular location & thus packet processing is done with L2 speed.

1.  CAM table records the incoming packet's MAC address, Port & VLAN.to enable Switching at Layer 2.

2. The Adjacency table records IP address and Layer 2 header for the IP and then references the TCAM table and at this point the switch will have enough information to rewrite the packets headers and send them out the egress port..... The idea is that all of the information is loaded into the Adjacency table & TCAM table as these are run on the ASIC's which makes it hardware/wire speed Layer 3/MLS switching.

The Adjacency table is a seperate entitity to the FIB table. the adjacency table GETS the information from the FIB table as shown in my original post.

The conecpts are quite difficult to get you head around once you first see it but i went off and read some extra stuff to try and understand it more and cam across the link below if i remember rightly!

Rather than me rewrite waht has already been written......(as i might confuse you! ) This is a snippet from Cisco's "Understanding CEF" (link below)

http://www.cisco.com/en/US/products/hw/routers/ps167/products_tech_note09186a00801e1dc2.shtml

Cisco Express Forwarding (CEF) switching is a proprietary form of scalable switching intended to tackle the problems associated with demand caching. With CEF switching, the information which is conventionally stored in a route cache is split up over several data structures. The CEF code is able to maintain these data structures in the Gigabit Route Processor (GRP), and also in slave processors such as the line cards in the 12000 routers. The data structures that provide optimized lookup for efficient packet forwarding include:

• The Forwarding Information Base (FIB) table - CEF uses a FIB to make IP destination prefix-based switching decisions. The FIB is conceptually similar to a routing table or information base. It maintains a mirror image of the forwarding information contained in the IP routing table. When routing or topology changes occur in the network, the IP routing table is updated, and these changes are reflected in the FIB. The FIB maintains next-hop address information based on the information in the IP routing table.

  Because there is a one-to-one correlation between FIB entries and routing table entries, the FIB contains all known routes and eliminates the need for route cache maintenance that is associated with switching paths such as fast switching and optimum switching.

• Adjacency table - Nodes in the network are said to be adjacent if they can reach each other with a single hop across a link layer. In addition to the FIB, CEF uses adjacency tables to prepend Layer 2 addressing information. The adjacency table maintains Layer 2 next-hop addresses for all FIB entries.

HTH

regards

Inayath