C9200 run config question

[Nicole Hudson]

The following line is in the running config of my C9200: 

ip dhcp relay override giaddr link-selection

I haven't been able to find a clear explanation for what this command means. I only found it has something to do with BGP which we are not using. 

Is this needed in a basic configuration? What exactly does this command mean?

Thanks!

[MHM Cisco World]

You use mpls 

And there are many sites use same vrf 

This sub-option is use to assign specific IP range for each site

MHM

[Giuseppe Larosa · Accepted solution]

Hello @Nicole Hudson ,

ip dhcp relay refers to the capability to convert DHCP broadcast packets to DHCP unicast packets sent to a remote DHCP server.

The giaddr is an internal field in the relayed  packet that is populated with the IP address of the SVI that received the original broadcast request with the intent to allow the DHCP server to pick up an address from  the right DHCP pool.

An explanation is reported in the link below:

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipaddr_dhcp/configuration/xe-3s/dhcp-xe-3s-book/dhcp-relay-svr-option-82-xe.html

>> The DHCP Relay Server ID Override and Link Selection Option 82 Suboptions feature enables the relay agent to be part of all DHCP message exchanges by supporting the use of two suboptions of option 82: server ID override and link selection. This design results in all DHCP messages flowing through the relay agent, allowing for greater control of DHCP communications. 

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In short is your Cat9200 working at Layer 3 ?

Are there any ip helper-address x.x.x.x configured under user facing SVI interfaces (VLAN interfaces) ?

if not that command does not do anything in your device because it is not acting as a DHCP relay

Hope to help

Giuseppe