Virtual Interface IP address

Unanswered Question
Feb 1st, 2008

Hi,


In all Cisco documentation 1.1.1.1 is always specified as Virtual Interface IP address.


Here is my concern. When wireless client gets an IP address (through DHCP), 1.1.1.1 is entered as DHCP server which means that at 50% lease time client will attempt DHCP Request to 1.1.1.1


1.1.1.1 is not routable on the wired network, but controller makes sure that it is accessible on the wireless network.


The problem is if you have both wired and wireless connected, default gateway through wired connection wil have less cost, so client will attempt to send traffic to 1.1.1.1 using wired interface and will obviously fail.


At 83.5% of lease time client will attempt full DHCP process starting from Discovery and I guess this is when the IP on wireless NIC will be renewed, cause those Discovery messages are to 0.0.0.0....


Is my understanding correct?


Thanks!

David

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dancampb Sat, 02/02/2008 - 06:52

The virtual interface IP is not visible on the wired network. The wireless clients will show the DHCP server as 1.1.1.1 and send their renewal request to this address. When the request hits the controller it then sends a unicast DHCP renewal request to the server address specified on the interface mapped to the WLAN. When the request hits the wire the 1.1.1.1 address isn't seen at all. Its just used internal to the controller.


If you are seeing an issue (very few clients do) you can disable DHCP proxy on the controller. This will then show the actual DHCP server's address. Keep in mind though that as of 4.2 the dhcp proxy function changed. Prior to 4.2 all disabling this did was show the actual server's address. As of 4.2 the DHCP request is forwarded to the wire as a broadcast so you may need to add the IP helper command to the VLAN's default gateway so the request is forwarded to the DHCP server.

dknov Sat, 02/02/2008 - 11:02

Hi,


With both wired and wireless active at the same time (when laptop is docked for example)DHCP Request to 1.1.1.1 will fail cause it will be sent using wired interface and not wireless (default route metric in the host's routing table). This will cause this DHCP Request to fail at 50% lease time.

Next step is DHCP Discover at 83.5% of lease time. Destination IP will be 255.255.255.255 and it will be generated specifically on wireless interface, since this in the interface that needs to renew the IP.


This is exactly my question. Will this Discover to 255.255.255.255 go through wireless interface, cause this is the interface that needs IP addresses renewed. If so, I assume it will succeed and client's IP address will be renewed.


A bit worse than renewing at 50% lease time as a normal DHCP process, but better than loosing the IP at all when 100% lease time hits. 83.5% is somewhere in between...


Comments?


Can you also advise how do you disable DHCP Relay function on the controller? Do you need to remove DHCP IP addresses in WLAN configs?


Another question is about needing WLAN for Management interface. Is it needed for any reason? WLANs are always documented to be bound to Dynamic Interfaces and not Management, however Management interface does appear in the drop down menu under WLAN configuration. Thoughts?


Thanks!

David

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