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Understanding how MSS DHCP_Server works on the WLC controller

  [KB34048] Show Article Properties


Summary:

This article explains the workflow of MSS DHCP_Server on the WLC controller.

For testing, demonstrating, or troubleshooting purposes, you may want to enable DHCP-Server on the controller.

Solution:

The WLC controller could act as a DHCP server and allocate IP addresses to the following items and is enabled by default:

  • Directly connected WLAs

  • Host connected to a new (un-configured) WLC2, WLC8, or WLC200, to configure the WLC using the Web Quick Start.  

Optionally, you can also configure the DHCP server on the WLC controller to provide IP addresses to the Distributed WLAs and to the clients.

The MSS (Mobility System Software installed on WLC controllers) DHCP server is implemented according to “RFC 2131: Dynamic Host Configuration Protocol” and “RFC 2132: DHCP Options and BOOTP Vendor Extensions”, with the following exceptions:

  • If the WLC is powered down or restarted, MSS does not retain address allocations or lease times.

  • The MSS DHCP server does not operate properly when another DHCP server is present on the same subnet.

  • The MSS DHCP server is configurable on an individual VLAN basis only and operates only on the subnets that you configure it. 

Note: Use of the MSS DHCP server to allocate client addresses is intended for temporary, demonstration deployments and not for production networks. Juniper Networks recommends that you do not enable MSS DHCP_Server on the WLC controller to allocate client addresses in a production network.

When MSS receives a DHCP Discover packet, the DHCP server allocates an IP address from the configured range or scope according to RFC 2131 and sends an ARP message to the IP address to be sure that it is available on the network. If the IP address is in use, then the DHCP server allocates the next address in the range or scope and resends ARP message again. The process continues until the MSS finds an unused address. MSS then offers the IP address to the Distributed WLA or client that sent the DHCP Discover. If there are no unused IP addresses left in the range or scope, then the MSS ignores the DHCP Discover packet and generates a log message.

If the client does not respond to the DHCP Offer packet which has been received from the MSS DHCP_Server within 2 minutes, then the offer becomes invalid and MSS returns the address to the DHCP pool.

The siaddr (Source Address) value in the DHCP exchanges is the IP address of the VLAN. The yiaddr (Yield Address) value is an unused address within the range or scope the server is allowed to use. In addition to an IP address, the Offer message from the MSS DHCP server also contains the following:


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