ISP Link Load Blancing Use Case

Hello everyone,

I have a requirement to implement source-based outbound routing across two ISP links on a BIG-IP LTM and I am looking for guidance on the correct way to achieve this

Environment:

- BIG-IP LTM (version: fill in your version)

- Two ISP links: ISP-1 (Airtel) and ISP-2 (TCL)

- Behind the BIG-IP there is a firewall

- Behind the firewall there are two internal subnets:

- 10.20.0.0/24 — this subnet should use ISP-1 (Airtel) for internet access

- 10.50.0.0/24 — this subnet should use ISP-2 (TCL) for internet access

• The firewall's default gateway points to the BIG-IP internal interface

Requirement:

1. When both ISP links are UP:

- Traffic from 10.20.0.0/24 must go out via ISP-1 (Airtel)

- Traffic from 10.50.0.0/24 must go out via ISP-2 (TCL)

2. When ISP-1 (Airtel) goes DOWN:

- Traffic from 10.20.0.0/24 should automatically failover and go out via ISP-2 (TCL)

3. When ISP-2 (TCL) goes DOWN:

- Traffic from 10.50.0.0/24 should automatically failover and go out via ISP-1 (Airtel)

4. When the failed link recovers, traffic should automatically return to its preferred ISP.

In short — each subnet has a preferred ISP, but if that ISP is down, it should fall back to the other ISP automatically. Both failover directions must work.

What I want to know:

1. What is the correct and recommended way to achieve this on BIG-IP LTM?

2. What objects need to be configured — Virtual Servers, Pools, SNAT, iRules, routes?

3. How does the BIG-IP detect that an ISP link is down and trigger the failover automatically?

4. Are there any gotchas or common mistakes to avoid in this type of setup?

Any help, working configuration examples, or pointers to relevant documentation would be greatly appreciated.

Thank you!