01-27-2023 07:05 AM
I am looking at making HSRP aware of a DHCP relay on a number of SVI's in a Catalyst 9400 series switch. The command that i have found to make an interface aware of DHCP relay is :
ip helper-address 10.10.10.10 redundancy HSRP_1
but the "redundancy" <name of group> section does not appear on my SVI's, so im wondering if it is possible to add awareness to an SVI and if so, can i use the same DHCP server address for multiple SVI's using this method ?
Best Regards,
Sabeel
01-27-2023 07:21 AM
Hi Sabeel!
I do not believe that modern IOS-XE software supports HSRP-aware DHCP Relay capabilities, as relevant configuration guides do not list HSRP-aware DHCP relay as a supported feature that can be configured.
From my research, this feature was added to de-duplicate relayed DHCP messages on redundant first-hop routers, as duplicate relayed DHCP messages caused some DHCP servers to malfunction. My suspicion is that modern DHCP server software applications do not have these same issues, so having multiple routers relay the same DHCP message to the same DHCP server is acceptable in modern day networking.
I will note that I personally have seen this type of configuration (wherein multiple first-hop routers running an FHRP relay DHCP messages to the same DHCP server such that the DHCP server is receiving "duplicate" DHCP messages) running in production environments without any issues. DHCP servers were able to receive, parse, and respond to the duplicate DHCP messages without issue.
I hope this helps - thank you!
-Christopher
01-27-2023 08:48 AM
Hi Christopher,
Thank you for your detailed explanation, so are you saying its supported by default on modern OS's ?
With the current config, there is no issue with DHCP, but currently our customer has 1 server and no redundancy. We are ripping the old solution out and adding new one which contains a primary and secondary DHCP server running windows DHCP. being as a secondary will be in the picture, I wanted to ensure that the DHCP process is smooth and traffic is not being sent to the wrong server, so I felt that this implementation may help with that which is why I explored it.
Best Regards,
Sabeel
01-27-2023 08:22 AM
I will try config EEM for this case if the HSRP aware is not work.
01-27-2023 08:41 AM
Hi MHM,
How can EEM benefit DHCP traffic being relayed on SVI's ? Do you have any documentation that can explain this ?
Best Regards,
Sabeel
01-27-2023 09:10 AM - edited 01-27-2023 09:11 AM
first, what is relate of add new Server with HSRP aware DHCP relay?
for your Q, I will try EEM that detect status of standby and when status change from standby->active the action cli will add ip helper address under the SVI.
read this about HSRP aware
""With the above configuration Bootstrap Request Message will be initiated only by HSRP Active Router R2_DHCP_Relay because it is the HSRP active router (due to higher HSRP priority configured). Now, the DHCP server receives only one DHCP Discover message from only one router and it sends the reply message to only one of the two routers from where it received it. ""
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