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MSM710 VSC DHCP relay agent "subnet selection"

 
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ndoudna
Frequent Advisor

MSM710 VSC DHCP relay agent "subnet selection"

Does anyone know what the point of the "Subnet selection" fields are (address and mask) in a VSC's DHCP relay agent configuration?

 

Somehow one of my VSCs on a lab MSM710 got configured with a subnet in the "subnet selection" Address field, so, it had 172.17.100.0 there. That turned into the controller having a strange subinterface in it called "br0:R2", with the IP address 172.17.100.0.

 

Worse, the controller sourced much of its IP traffic from this invalid address, instead of from its LAN port IP address.  That included unicast UDP discovery responses to an AP, so the AP never received them and were permanently stuck in "Waiting for acceptance" state, because it never got its UDP discovery responses.  The controller also sourced pings and ARPs from this bogus address, so it couldn't ping many hosts on its own subnet (some answered, some didn't).  It also responded to pings to that bogus address.

 

It took a long time to track down where this "br0:R2" interface came from, but I finally found a line in the text config, in a VSC config, that read "dhcp relay subnet 172.17.100.0".  When I deleted the "subnet selection" address from the VSC, the controller rebooted and came back without  its LAN port assignment!   But also without the "br0:R2" interface, and finally, the controller was sending out UDP discovery packets with its LAN port address as the source, and the AP could process them and complete its controller discovery.

 

I know the strange "br0:R2" interface's address is a bug, but now I'm wondering what on earth the VSC "subnet selection" in the DHCP relay agent is there for in the first place.  It's not in the controller's DHCP relay agent config either.

 

thanks,

noemi

2 REPLIES 2
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ISoliman
Super Advisor

Re: MSM710 VSC DHCP relay agent "subnet selection"

What are you tring to accomplish ?? this option is used to mark option 82 on the DHCP request when the controller relies that so the clients will get the correct IP, where is the DHCP server is ?? on the same as the LAN port ?? if yes and you are using version 5.3.x then this will not work.

 

 

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ndoudna
Frequent Advisor

Re: MSM710 VSC DHCP relay agent "subnet selection"

>>

       What are you tring to accomplish ??

>>

 

Avoiding the bug in which putting in a "subnet selection" creates an interface with a bogus IP address on the controller, and the controller uses that interface as a source IP address.

 

>>

       this option is used to mark option 82 on the DHCP request when the controller relies that so the clients will get the correct IP

>>

 

Looks like option 82 a container for relay agent information, the Circuit ID and Remote ID.  The VSC does have those fields under "Informatin option,"  but that's not the problem area.

 

Below "Use the following server" section, there's another section named "Subnet selection," with two boxes, one labelled "Address:" and one labelled "Mask:"   Do those have any purpose in life, aside from breaking APs' controller discovery?

 

The MSM710 is running 5.5.1.0-01-9677, but the "Subnet selection" field appeared in a prior version (I forget which).  I can't find anything about DHCP relay agents that says "subnet selection" is relevant.

 

 

>>

      where is the DHCP server is ?? on the same as the LAN port ?? if yes and you are using version 5.3.x then this will not work.

>>

 

Actually, it didn't matter where the DHCP server was, or if the controller was configured for DHCP server or DHCP relay.   As long as there was an address in the Address field of Subnet selection, the controller sourced IP packets from that address (and used it for ARP Sender IP Address), and AP controller discovery couldn't complete.

 

Anyone know what that "Subnet selection" is for in a VSC's DHCP relay agent section?

 

thanks,

noemi