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Interface Status UP, Protocol UP

Posted on 2014-07-21
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Last Modified: 2014-07-29
I wonder in which case we can see Interface Status UP, Protocol UP even when  nothing plugged in

Thank you
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Question by:jskfan
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9 Comments
 
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by:Don Johnston
Don Johnston earned 1336 total points
ID: 40209355
I've never seen that on a physical interface.  Unless you did something like loop the interface.
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Author Comment

by:jskfan
ID: 40209386
I have just tested it.

if you turn keep alive off on an interface (not loopback interface), you will not be able to ping after the keep alive period expires. however if you run show ip interface brief , the interface will still show the Status is UP and the Protocol is UP.
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by:Craig Beck
Craig Beck earned 664 total points
ID: 40209929
The keepalive tells the port whether to monitor the link-state or not.  If you issue the no keepalive command on a FastEthernet interface, for example, the status will go UP/UP.
if you turn keep alive off on an interface (not loopback interface), you will not be able to ping after the keep alive period expires.
Kind of...

Once you disable keepalives on an interface there is no keepalive timer, so the interface should stay up indefinitely.  If you assign an IP address to an Ethernet interface it will always be UP/UP so it will always be pingable if a packet is routed to it from another interface.  However, if you do this with an SVI it might behave differently depending on the hardware platform.

It doesn't work on all interfaces though in my experience... serial interfaces, for example, won't do this.
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Author Comment

by:jskfan
ID: 40211475
On R1  I issued no keep alive command, then I shut down the interface connecting R1 to R2, the Shut Down command was typed only on R1.
I issued ping command from R2 to R1, I got ping reply from R1 for few times , then could not ping it, I guess the keep alive period expired.

On R1 , I issued the command : show ip interface brief ,  on the interface I shut down , I still I can see the status UP and protocol UP.
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Don Johnston earned 1336 total points
ID: 40211507
If the interface is shut down, it should not show as UP/UP. It should show "Administratively Down".

Is this a real router or a simulator?
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Author Comment

by:jskfan
ID: 40224651
This is GNS 3
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Author Comment

by:jskfan
ID: 40224655
real IOS though
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LVL 47

Expert Comment

by:Craig Beck
ID: 40224892
GNS is a bit different at some things. I'd test that with real kit.
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Author Closing Comment

by:jskfan
ID: 40228158
Thank you
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