12-03-2008 11:41 AM - edited 03-06-2019 02:47 AM
On a 4506 L3 switch, a predecessor configured 'no ip route-cache cef' on the vlan SVIs. I would like to re-enable cef, but am not sure if it is actually disabled on these interfaces. 'Sh int switching' seems to confirm the configuration by showing all packets process switched for the vlans. On the other hand, 'sh ip cef' has entries for those vlans , and 'sh int vlan stats' shows most all packets being switched in hardware (not processed). So is cef enabled on these interfaces, or not?
Thank you for any help you can provide.
12-03-2008 11:51 AM
Hi,
it's quite simple. To enable cef switching you have to use the command 'ip cef' in global config. You can see 'ip cef' in your running-configuration. Normally if cef is disable you can't see any output with the command 'show ip cef' ...
router#show ip cef
%CEF not running
So, the ip cef in your case is enabled (default behavior).
The 'no ip route-cache cef' is used only to disable cef mechanism for a specific interface, and just for it.
For more infos:
HTH
Andrea
12-03-2008 01:08 PM
Thank you, Andrea. What you say would be fine, except that the interfaces on which cef is disabled are the same ones on which the 'show' commands say cef IS enabled. For example, on interface vlan 21 is configured 'no ip route-cache cef'. But 'sh ip cef' and 'sh int vlan 21 stats' both indicate that cef IS enabled for vlan 21.
12-04-2008 03:49 AM
what about 'show cef interfaces'?
IOS version?
Thank you
Andrea
12-04-2008 04:05 AM
better, check the specific 'show adj detail'. Will show how many packets/bytes are CEF switched out of each next-hop/adjacency
Regards
Andrea
12-05-2008 02:05 PM
"Ip cef" does not show in the config -- IOS version 12.2(31). Yet CEF is running.
And it definitely appears to be running on the interfaces configured with 'no ip route-cache cef'. "Show adj detail" has entries. At this point, I'm concluding that CEF is not allowed to be disabled on this switch. An article at http://searchnetworkingchannel.techtarget.com/generic/0,295582,sid100_gci1261924,00.html indicates this, although I would think that the attempt to disable it would have yielded an error message, rather than just allowing the "no ip route-cache cef" to be entered, and then just having cef continue to run.
Even so, I'm curious how the output from 'sh int switching' seems to contradict the output for 'sh int vlan stats'. As you can see from the following, all traffic is switched in hardware per the first output, and process-switched per the second output.
12fl4506#sh int vlan21 stats
Vlan21
Switching path Pkts In Chars In Pkts Out Chars Out
Processor 0 0 0 0
Route cache 0 0 0 0
Hardware 1011470547 1236839140642 488636118 182140106530
Total 1011470547 1236839140642 488636118 182140106530
12fl4506#sh int vlan21 switching
Vlan21 ICN Broadcast Domain
Throttle count 0
Drops RP 0 SP 0
SPD Flushes Fast 0 SSE 0
SPD Aggress Fast 0
SPD Priority Inputs 0 Drops 0
Protocol Path Pkts In Chars In Pkts Out Chars Out
IP Process 344776 39992701 14731 1031170
Cache misses 0
Fast 0 0 0 0
Auton/SSE 0 0 0 0
ARP Process 105336 7258882 127120 7627200
Cache misses 0
Fast 0 0 0 0
Auton/SSE 0 0 0 0
Thanks for your input.
12-05-2008 03:19 PM
Hello Rick, Andrea.
I think these last shows help to understand better what the device is doing with this configuration.
This C4506 is a multilayer switch that is based on CEF.
CEF is used to populate tables used for forwarding.
Multilayer switching is effective:
the first show provides
Switching path Pkts In Chars In Pkts Out Chars Out
Hardware 1011470547 1236839140642 488636118 182140106530
the second one says:
Protocol Path Pkts In Chars In Pkts Out Chars Out
IP Process 344776 39992701 14731 1031170
the process switched packets are very few in comparison to hardware based switched ones.
These should be routing protocol messages, CDP, other L2 protocols, some packets with options.
From the shows we can say that the command
no ip route-cache cef under SVI vlan 21 is not effective: very few packets are actually processed by the SVI interface vlan21 and most of them are already not manageable by CEF = require process switching.
This interpretation is more near what happened/happens to a C6500 in Hybrid Mode: when MLS is effective the SVI vlan interface counters were reflecting the process switched traffic: for example exporting Netflow data from the MSFC SVIs has been meaningless.
In Native Mode you can see correct counters on physical interfaces and also on SVI Vlans interfaces.
Sorry for the long post.
Hope to help
Giuseppe
12-05-2008 08:18 PM
Thank you, Giuseppe --
These commands have always seemed vague to me. This just confirms it. So not only is the 4506 not turning of CEF on the interfaces as it should be doing, but also the output of 'sh int vlan 21 switching' is not displaying correctly. I concur. The 'Fast' row should be showing the Fast and CEF-switched traffic -- that's how other articles have described it. Overall, another case of Cisco making us spin our wheels trying to figure out what they're doing with our data. Sorry for the frustrated tone --- and thanks for your input.
12-06-2008 02:37 AM
More info basically what Guiseppe said.
http://www.cisco.com/en/US/docs/switches/lan/catalyst4500/12.2/44sg/configuration/guide/cef.html
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