Forum Discussion

Terry_Rodecker_'s avatar
Icon for Nimbostratus rankNimbostratus
Nov 30, 2010

Cable Modem connection on F5 Link Controllers

We recently installed 2 F5 Link Controllers to allow for load balancing our ISPs. They are currently in the data flow and are handling our traffic but we are not load balancing between the 2 existing ISPs. We are attempting to connect some "cheap" bandwidth for Internet by adding a bank of cable modems and DSL modems to one of the extra interfaces on the F5s. I have the interface on the F5s added to the VLAN as a tagged interface (since we're going to have multiple cable/dsl modems on this connection and each will have their own subnet). The connectivity is as follows;



cable modem---switch----F5s



The switch has the VLAN created on it. The port where the cable modem is connected is an access port in that VLAN. The ports where the F5s are connected are set for 802.1q (aka Cisco Trunking, not to be confused with F5 trunking which would be Cisco's etherchannel). For testing purposes I configured an IP address for the VLAN in question on the switch.



The switch can ping the Cox default gateway.


The switch can ping the F5s


The F5s can ping the switch


The F5s cannot ping the Cox default gateway



We've gone over the configuration numerous times and are certain that the subnet masks and IP addresses are correct. We've had the ISP on the phone and they've checked the cable modem. They can see an ARP entry for the IP address that's on the switch but they do not see an ARP entry for the IP addresses on the F5s.



Has anyone tried connecting cable modems to Link Controllers before? If so, did you have this particular issue or anything similar to this?





3 Replies

  • Does the F5 have a route through the modem to the gateway? If you do a traceroute, where does it die?
  • Thanks. We got it fixed though I don't truly understand why it didn't want to work before.



    The F5s and the ISP were on the same subnet so no route was necessary. As it turns out, the ISP's cable modems don't like to be connected to Cisco equipment. It was directly connected to a Cisco 2960, which kept the F5s from working. I then placed a Cisco ASA (aka Pix Firewall) between the 2960 and the cable modem and the same thing, the ASA could access the Internet but nothing behind it could. I replaced the ASA with a $19 Linksys router and everything is working fine now.