Saturday, July 17, 2010

CDP Holdtime and LabUpdate

You may have shut down a link or suspected a link failure but seen "sh cdp neigh" still displaying an active connection, well that is all due to the CDP Holdtime.  It has to run down to 0 before the neighbor is removed from the table.  The default is 180 seconds, which can certainly lead to some confusion if you are troubleshooting a recently failed link.  The default can be changed with the "cdp holdtime" global parameter.  Once you're used to looking for it, you can see that the holdtime is unusually low for the failed link, which should lead you to figure it really is down w/out waiting for the entire holdtime to expire:

Rack1SW2#scn
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
S - Switch, H - Host, I - IGMP, r - Repeater

DeviceID   Local Intrfce Holdtme Capability      Platform      Port ID
BB2            Fas 1/0           125          R S I            3725           Fas 0/0
Rack1SW1 Fas 1/9            15            R S I            3725           Fas 1/9

Notice that the Holdtime will keep decrementing and be unusually low (15 here) which indicates you may have a problem before you'll actually see the neighbor drop off completely.

As for my R&S lab, b/c I've just taken on a new large project I'm going to have to move my lab back til next Spring.  I'm just able to work through about 10 pages / day of INE Workbook 1, so it'll be a while before I finish!  Once I've finished WB 1, I'll then do full labs for a couple of months before attempting the real thing.

2 comments:

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