Forum Discussion

SolarJeans's avatar
Feb 16, 2023

F5 DNS Links

Hello Experts,

My F5 DNS now has 1 link to internet, now I am planning to add 1 more internet link to F5 DNS so it can be resilience.

Under DNS -> GSLB: Links -> Link List, can I add the additional internet link here so it can act something like LC function?

The DNS lookup will return 2 public IP based on which link it comes from. And if 1 link down the lookup will always go to health link.

4 Replies

  • SolarJeans For device communication you could add an additional link and use ECMP but that still leaves you with the DNS side issue. On the DNS side they will only know what name maps to which IP so if this is your only GTM you would need to have something similar to the following to get it to work properly and the GTM would respond to DNS queries on both ISP connections. 172,800 IN SOA <----- Existing ISP IP 172,800 IN SOA <---- New ISP IP

    • SolarJeans's avatar
      Icon for Cirrus rankCirrus

      Thank you.

      I am a little bit confused, so this cannot be done in dns, and need provision lc function?

      As 2 ISP links are located in same DC, and if both links are up, it will be round robin.

      What I concern is, if 1 ISP link down, will DNS still look for down link and reply the down link public IP? As my health mon will monitor the internal server and will show up even ISP link down. Or can I assign 2 health mon profiles in same virtual servers? One is monitoring ISP public IP and one is monitoring internal server IP, if 2 profiles assigned, this will monitor both profiles or either one?

      • Paulius's avatar
        Icon for MVP rankMVP

        SolarJeans Is this new ISP connection going to your GTM or a different device? If this is going to your GTM and one of the links goes down on the GTM and you have ECMP configured it will just use the other connection. If you have monitors also that will allow you to mark a link down because that ISP is having issues that can also bring the link down and only use the one. From the client side of DNS queries they will have a DNS response that shows both IPs but the client will query 1 and if the query fails it will query the other records because of how DNS queries work. So you cannot deal with your redundant links by just GTM DNS but you can achieve this using ECMP, some SLA monitors on the GTM, and then DNS records that show what the authoritative NSs are for the zone in question.

  • Yes, you can add the additional internet link in the F5 DNS GSLB Link List for load balancing and resilience. The DNS lookup will return IPs based on the available links, and if one goes down, it will route to the healthy link. raiderlink