Can anybody advise of when CEF flushes entries in its forwarding table? Are there any commands to view that table and can timers to flush entries be influenced? I mean can I change the timer to flush the entry if the source/destination stream has been idle for a specific time.
Hi Istvan, oh dear, they've written a whole book on it. Looks like I had the wrong impression, I thought it had individual datastreams in it's cache (like 22.214.171.124 - 126.96.36.199), but it looks more like a routing table (dest 188.8.131.52 via x). I just wanted to randomise the load balancing more.
In my test net I want to load balance between 10.137.1.100 and 10.137.2.0/24. In the ip cef table I have prefix entries for 10.137.2.0/24 via (equal) next hop 10.137.1.11 and 10.137.1.10. I just want to know how does CEF do its load balancing decision between the two. (source/dest host, source/dest network, source only). And how often is the next hop decision renewed? Let's say there were no packets between 10.137.1.100 and 10.137.2.100 for x minutes, when would CEF choose a new next hop?
PS: I'll have a look at the book. There is not much written on the web about the ins and outs of CEF.
I've tested again in my test network. per-destination seems the default. It's good, it does do source/dest pair load balancing. And even on one single outgoing interface.
Fair enough the CEF next hop table doesn't time out. But there must be a more specific switching table (source/dest pair) that times out after a while. Is it the fast switching table? I'll have a look at the book, it seems to go quite in detail about the different switching methods.
I think I will have to try it out in the real network to see what the split is going to be.
I believe CEF is fully deterministic in how it "hashes" flows across multiple paths. The only impact with timers would perhaps be it tracking a flow's stats.
On some platforms/(later)IOSs, there's a configuration option to change how CEF hashes its flows. This seems similar in concept to switches supporting Etherchannel hash options.
The major difference between CEF and other "cache" methods, CEF treats every packet alike when making a packet's forwarding decision. Other cache methods do much forwarding decision work on a flow's 1st packet, "save" (cache) that decision, and apply it to subsequent flow packets.
CEF negates the need/benefit for other cache methods, except perhaps for flow cache. The reason being, although CEF improves forwarding decision performance especially for 1st packet, there's often other decision peformance issues such as ACLs. (NB: Incidently, at least for ACLs, on the higher router platforms, there's compiled ACLs to improve ACL processing without caching too.)
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