Forum Discussion

viks_96432's avatar
viks_96432
Icon for Nimbostratus rankNimbostratus
Sep 25, 2012

How does F5 LTM route traffic to a subnet it's not aware of

I have the below setup, A VIP is configured with IP 192.168.0.3 for telnet service with snat automap to loadbalance all connections to 192.168.1.2.

 

 

PC1 (10.0.0.2/24) <--> (10.0.0.1/24) RTR1 (192.168.0.1/24) <--> (192.168.0.2/24) F5 LTM(192.168.1.2/24) <---> (192.168.1.1/24)Server1

 

 

There is not default route configured on the LTM.

 

 

When i initiate a telnet to 192.168.0.3 from PC1 ( 10.0.0.2), it works and I just don't know how it does ? without proper routing configured on the F5 - Can someone help me to understand how F5 is returning traffic 10.0.0.2 ?

 

 

 

Thanks,

 

Viks

 

5 Replies

  • This is the Auto Last Hop feature at work. This is enabled by default; rather than consulting the routing table to find a path to route return traffic to, the system simply returns it to the MAC address it came from; in this case the MAC for 192.168.0.1. No routes required!
  • You can disable this feature but it's enabled by default as it can be quite useful and remember it only applies to traffic directed at a Virtual Server. Don't think it's a security risk because it's effectively routing when you don't want it to, it won't do this for other traffic.

     

     

    You can globally disable this feature via System> Configuration > Local Traffic > General but obviously, ensure you have relevant TMOS/LTM routes in place first.