interface vlan

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May 5th, 2008
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I've seen several implementations of "interface vlan <id>" on many Catalyst switches for interVLAN routing but on a single instance of a core switch only.

Recently, I've seen 2 core switches connected with each other via HSRP and with interface vlan <id> configured on each switches -- same vlan id created for both switches. The interface vlan <id> was configured to have a virtual ip and a priority id for HSRP.

My question is, in these kind of scenario, what would be the reason, chances or cause that a virtual interface (layer 3 - switch virtual interface) would go into a down state ? I am just looking for a clear answer/picture since it's the physical interface that often encounter these type of problems most of the time.



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srue Mon, 05/05/2008 - 15:16
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they're probably running HSRP between the two switches in case one of the switches goes down altogether, not in case one vlan interface goes down.

This way, when a switch reboots (or crashes!) the other switches vlan interfaces take over via HSRP and routing is still enabled.

They are also probably trunking between the switches (right?) so in theory the vlan interfaces shouldn't go into a down state at all, with the exception of a reboot.

ankbhasi Tue, 05/06/2008 - 02:16
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Hi Joseph,

Very well explained by Steven and very less likely you will see any vlan interface in particular going down. If the core switch 1 vlan interface does not find any physical port configured for itself it will go down.

Generally on core switches there are no end users and generally we find trunk connections so lets say the trunk connection or that particular physical port goes down then the vlan interface will also go down. Also sometimes bymistake if you delete some vlan from vlan database then also its layer 3 interface goes down.

As we all know anything can happen in networks :)



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