Failover ISP setup - can't ping Internet from standby interface

[DazOG]

Hi

I have a working "ip sla" setup, following the instructions here: https://www.cisco.com/c/en/us/support/docs/ip/ip-routing/200785-ISP-Failover-with-default-routes-using-I.html and elsewhere.

The problem I've got is that in the example above the test IP is the next hop router, which is across the link being monitored.  In my case, the next hop is in the same server rack so it is pointless to monitor it to determine if the line is up or not, because it always will be unless the NTE is powered off.

If I try and ping e.g. 8.8.8.8 by doing:

ping 8.8.8.8 source-interface GigabitEthernet0/0/0 

or

ping 8.8.8.8 source-ip 123.0.0.1

(where 123.0.0.1 is the IP of GigabitEthernet0/0/0)

..I don't get any reply, unless i am already routing via the next-hop to Gi0/0/0.  If i am routing via the 2nd ISP, NAT'ing to a different block of public IPs, then I can't access the Internet via the 1st ISP.

Just to reiterate - the NTE is in the same rack as my equipment, i.e. 123.0.0.2 is in the same rack as 123.0.0.1, so it is pointless to use 123.0.0.2 as a test IP for reachability.

Any thoughts?  I feel like this is a NAT issue, but not sure how I can fix it?

[MHM Cisco World · Accepted solution]

Use 

Ip route 8.8.8.8 255.255.255.255 isp interface(you want to track) permanent 

This command force router to use specific isp and make track correct work.

MHM

[DazOG]

Thanks. I guess that would make that IP (Google’s DNS) inaccessible via whichever ISP is currently being used, wouldn’t it? 

[MHM Cisco World]

That correct but

We usually use two dns ip 8.8.8.8 and  8.8.4.4 

And also if interface down this static route is remove from rib.

So it have really some disadvantages.

MHM

[DazOG]

Thanks.  I've chosen another public DNS server that we aren't using for anything else anyway.