Automatic VLAN Assignment (AVA) - Problem with option routers

Glenn Satchell Glenn.Satchell at uniq.com.au
Wed May 14 23:51:31 UTC 2008


Hmm, same switch port with two different subnets - that's a shared
network. Try putting a shared-network statement around the two subnets,
eg:

shared-network "somename" {
  # default subnet
  subnet 10.10.100.0 netmask 255.255.255.0 {
  ...
  }
# phone subnet
  subnet 10.10.200.0 netmask 255.255.255.0 {
    option routers 10.10.200.254;
    option broadcast-address ...;
    ...
  }
}

regards,
-glenn

>Date: Wed, 14 May 2008 17:33:25 +0200
>From: Tarik Gasmi <tarik.gasmi at rz.uni-freiburg.de>
>
>Hi there,
>
>we are trying to set up Automatic VLAN Assignment (AVA) on an Alcatal
>switch with an ISC DHCP server.
>
>IP phones should automatically be assigned to a dedicated phone VLAN
>(and Subnet) for Voip Traffic, while when a PC is plugged to the same
>switch port it stays in the default data VLAN (Subnet).
>
>Purpose is to ensure traffic separation between Data and Voip, e.g. for
>QoS and security issues.
>
>DHCP Configuration:
>***snip***
>
># default subnet
>subnet 10.10.100.0 netmask 255.255.255.0 {
>  option routers 10.10.100.254;
>  option broadcast-address ...;
>  ...
>  # if it is an IP Phone ...
>  if substring (option vendor-class-identifier, 0, 11) = "OptiIpPhone" {
>    option vendor-encapsulated-options
>     01:07:53:69:...:   # vendor option code
>     02:04:00:00:07:82:   # phone VLAN ID tag
>     03:1b:73:...;  # some information where to find the SIP Server
>  }
>  pool {
>    range 10.10.100.1 10.10.100.239;
>  }
>}
>
># phone subnet
>subnet 10.10.200.0 netmask 255.255.255.0 {
>  option routers 10.10.200.254;
>  option broadcast-address ...;
>  ...
>}
>
>host ip-phone {
>  hardware ethernet ...;
>  fixed-address 10.10.200.2;
>}
>***snap***
>
>AVA is realized in 2 DHCP Discover-ACK Cycles:
>
>   1. the phone sends out a DHCP Discover message
>   2. the DHCP server sends back a DHCP Offer with the Option 43 Vendor
>Specific Options in a special format, the phone will accept the Offer,
>send back the DHCP Request, and save the VLAN ID provided by the DHCP
>server in the Vendor Specific Options. and with an IP Adress from Pool
>10.10.100.1 - 239.
>   3. After the DHCP server has send the ACK message to the phone, the
>phone will release the leased IP address and start a new DHCP Discover
>cycle using the now known Voice VLAN ID tagging
>   4. the phone gets the fixed-address 10.10.200.2 in the Voip Subnet
>
>the phone gets the correct IP in the Voice VLAN.
>the problem is that the phone does not get sent the correct routers
>option in the DHCP Offer of the second cycle (10.10.200.254) but the one
>from the default subnet (10.10.100.254). due to this misconfiguration
>(IP and Router in different subnets), the phone cannot communicate.
>
>Strangely, everything is fine - the phone gets the correct router option
>- when this option is specified in the host-declaration also, an not
>only in the subnet-declarations. see the following ...
>
>***snip***
>host ip-phone {
>  hardware ethernet ...;
>  fixed-address 10.10.200.2;
>  option routers 10.10.200.254;
>}
>***snap***
>this works
>
>We want to avoid to have to add the specific router option to every
>ip-phone host declaration (we will have thousands of them).
>
>Any ideas or suggestions, what the problem could be?
>
>Thanks and best regards,
>
>Tarik Gasmi
>
>-- 
>________________________________________
>
> Tarik Gasmi
> Rechenzentrum Universität Freiburg
> Hermann-Herder-Str.10 / 79104 Freiburg
>
> Email: tarik.gasmi at rz.uni-freiburg.de
> Tel.:  +49 (0)761 203-4674
>________________________________________
>



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