[nsp] Cisco CEF FIB and Adjacency table

From: Yu Ning (yuning@ns.chinanet.cn.net)
Date: Tue Sep 12 2000 - 05:25:17 EDT


Hi,

Cisco CEF/dCEF is quite a useful feature in the GSR platform. FIB(Forward Information
Base) and Adjacency Database(AD) is component of CEF. In CCO document, it's said that FIB
is derived from RIB, can any one tell me why the FIB+AD architecture is faster than
traditional RIB+ARP ?

After a search, I only found some vague description on Chesapeake, that:
"
The FIB is somewhat of a copy of the Routing Table. Recursive routes are resolved, so that FIB entries all reference a specific next-hop neighbor. A 256-way M-Tire structure is used with a patented algorithm to find matches with at most 4 lookups. The Adjacency Database (AD) contains the Layer 2 rewrite string (new header), and an interface descriptor. It gets populated in the background by ARP and by mapping statements. I've been thinking of the AD as containing outbound interface and next hop information, with FIB entries referencing this information.

"

To be more specific, can any one tell me why FIB is more efficient (in lookup speed)
than RIB ?


thanks!

Yu Ning
-------------------------------------------
(Mr.) Yu(2) Ning(2)
ChinaNet Backbone Operation
Networking Dep.,Datacom Bureau
China Telecom.,Beijing,P.R.C
+86-10-66418105/66418121/66418122
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