DHCP Smart Relay

A DHCP (Dynamic Host Configuration Protocol) Relay agent relays DHCP requests from the client to the DHCP server, and relays the DHCP replies from the server to the client. It acts as a proxy, and can reduce the number of DHCP servers required in the network. The DHCP relay agent inserts its own IP address in the giaddr field (gateway address) of the DHCP request. The DHCP server looks into this IP address, identifys the DHCP client‘s subnet, and assigns an IP address accordingly.

The ExtremeXOS BOOTPRelay module is Extreme Networks' DHCP Relay agent. It is now enhanced to optionally insert the secondary addresses of the interface.

DHCP clients can now be dynamically assigned both public and private addresses in an effort to reduce administrative overhead. ExtremeXOS can now configure the BOOTPRelay agent to insert both primary and secondary address(es) of the client-facing interface as a gateway address in the DHCP packet. At any given point in time, the DHCP client can be assigned one IP address.

You can insert different addresses in the giaddr field (gateway address) in two different ways:

Parallel mode: the switch simultaneously relays multiple DHCP Discover packets, each containing a different IP address as the gateway address. The relay agent receives a DHCP Discover request from the DHCP client. The relay agent makes multiple copies of this DHCP DISCOVER request, inserts each IP address of the client-facing interface in the giaddr field (gateway address) in each one of these copies, and relays all these DHCP DISCOVER packets simultaneously to the DHCP server.
The DHCP server has the responsibility to assign the correct IP address in the correct subnet by choosing the DHCP DISCOVER packet to respond to. Similarly, the DHCP client is responsible for accepting the appropriate DHCP OFFER from the DHCP server.

Note
Only the DHCP DISCOVER request is sent in multiple copies, with different IP addresses as the gateway address in each. All other DHCP packets are relayed normally.
Sequential mode: the switch relays a DHCP DISCOVER packet for each IP address sequentially.
The relay agent receives a DHCP DISCOVER request and inserts the primary address of the client-facing interface as the gateway address, and relays this packet to the server. The switch counts the number of times a DHCP client sends out a DHCP request without receiving a DHCP OFFER message. After three retries, the relay agents sets the secondary address as the gateway address in the next DHCP Discover request that gets relayed. If the DHCP server still does not respond after three retries, the next secondary address is used as the gateway address, and so on cyclically.

Configuring DHCP Smart Relay

Issue the following commands to configure te DHCP Smart Relay feature:

To configure DHCP smart relay mode to include the secondary IP address as giaddr at VR level: configure bootprelay {ipv4 | ipv6} include-secondary {sequential | parallel | off} {vr vr_name}
To configure DHCP smart relay mode to include the secondary IP address as giaddr at VLAN (Virtual LAN) level: configure bootprelay {ipv4 | ipv6} {vlan} vlan_name include-secondary {sequential | parallel | off}
To unconfigure any smart relay configuration that was specified at the VLAN level: unconfigure bootprelay {ipv4 | ipv6} {vlan} vlan_name include-secondary
To display various DHCP BOOTP Relay statistics:
- show bootprelay
- show bootprelay ipv6
- show bootprelay configuration ipv4
- show bootprelay configuration ipv6

Supported Platforms

This feature is supported on all platforms.

Published March 2020prev | next

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