What is 127.0.20.20

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Apr 16th, 2007
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Hi,


When I execute "show arp" in my 3750 switch, one of the line in the output shows;

Internet 127.0.20.20 - 0000.0000.0000 ARPA Virtual2


What is that? What it's use for? Anyone knows?


My IOS is c3750-ipservices-mz.122-35.SE2.bin


Thanks in advance

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Richard Burts Tue, 04/17/2007 - 04:06
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Danilo


The 127 network is for internal loopback addressing so it appears to be for some internal processing that is going on. The interface indicated is Virtual2 so I am guessing that you have something with virtual templates or similar. Perhaps you can clarify a bit what is in the configuration?


HTH


Rick

Richard Burts Wed, 04/18/2007 - 04:08
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Danilo


It is very interesting. I have looked at the config and do not see an obvious explanation. I wonder if it could be related to this:

switch 1 provision ws-c3750-48ts


The ARP entry:

Internet 127.0.20.20 - 0000.0000.0000 ARPA Virtual2

shows the address as associated with an interface on the local router of Virtual2. But if it is not something about stacking then I am not clear why the switch has created the virtual interface.


HTH


Rick

Danilo Dy Wed, 04/18/2007 - 04:59
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Hi Rick,


I'm playing with these new devices to check on their new features. Last week I stack this lab switch with another same model member switch, I properly removed that member switch from the stack and put in production. This lab switch currently is trunked to another same model lab switch (similar config) and hsrp in two 3845 routers. The line "switch 1 provision ws-c3750-48ts" is already there since I first boot up the switch fresh from the box.


I just discover the following in this switch;

- This ARP entry

- The null interface doesn't show in the config.



I also discover something in 3845 router Gi0/0&0/1 interface;

- When I unplug the cable, the interface still shows up/up. Need to put "keepalive #" on it. I haven't checked the FE if it has the same symptoms.


If I found out the source of that ARP entry I'll post it here :)

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