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I am in the middle of migrating to a new IP scheme. From 192.168.10.x/23 to 10.6.x.x/19. Our ISP handles layer 3 and we statically route from our L2/L3 Cisco 3750G switch stack to their L3 device.

Currently the static route is setup on our L2/L3 switch as such. This points to the ISP's handoff interface:

Cisco3750Stack#show run | begin ip route 0.
ip route 0.0.0.0 0.0.0.0 192.168.10.1

We have injected 10.6.x.x DHCP pools into the switch configuration and I have set the configuration on one test interface (gi2/0/43) to pull the new DHCP configuration along with new VLAN configuration. It was successful in pulling the new scheme to that particular interface.

My Question: Can I add a static route Cisco3750Stack#ip route 0.0.0.0 0.0.0.0 10.6.0.1 10 to the existing static route and be able to test connectivity with the new IP scheme assuming that the ISP has configured their side properly to receive the new 10.6.x.x/19 block?

IMPORTANT: I added an administrative distance of 10 to the end of this particular static route (shown above) so to not affect the current production route.

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    The AD will prevent the default route from being used until the default route with the lower AD is gone. – Ron Maupin Sep 15 at 19:24
  • Will the new route replace the old one? – Ron Trunk Sep 15 at 19:28
  • Eventually the new route will replace the old one. I'm just attempting to test at the moment without removing the existing route. – Craken Sep 15 at 19:42
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Can I add a static route Cisco3750Stack#ip route 0.0.0.0 0.0.0.0 10.6.0.1 10 to the existing static route and be able to test connectivity with the new IP scheme...?

No, you can't. As others have pointed out, since you've set the administrative distance to 10, this route will not be added to the routing table unless the other default route (with lower administrative distance) is removed.

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  • Could I add the new route without the administrative distance metric? – Craken Sep 15 at 20:14
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If all you're looking for is a way to make sure the new connection can pass traffic, you can add a route for something specific that won't interfere with your normal traffic, such as: ip route 8.8.8.8 255.255.255.255 10.6.0.1

When you're finished, you can simply remove that route and wait until you're ready to replace the default route with the new gateway.

To remove the temporary route, use: no ip route 8.8.8.8 255.255.255.255 10.6.0.1

Keep in mind, you may need a NAT statement on this router or up-stream, depending on where you do your NAT-ing.

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    I never thought to test with ip route 8.8.8.8 255.255.255.255 10.6.0.1. Great idea @Jesse P. – Craken Sep 21 at 15:18
  • @Craken You can use whatever you want (as long as it doesn't break your production traffic). Essentially, all you need to do is a ping or traceroute to verify the path is good but, since ICMP is often blocked, you may want to use something like one of the IP addresses that an FQDN for a website uses (like www.google.com) and then trying to access it on TCP/80 or TCP/443 to see if you get a response back. – Jesse P. Sep 21 at 15:57
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Can I add a static route Cisco3750Stack#ip route 0.0.0.0 0.0.0.0 10.6.0.1 10 to the existing static route and be able to test connectivity with the new IP scheme

No, you can't test in the way you described because your testing static route won't be active until and unless the existing route is disabled or deleted. Basically, a layer 3 device gives high priority to lower AD value route entries, and second priority is given to the next lowest AD value route entry. In your case, if the existing route has the lowest AD value and the newly configured route has an AD value of 10, all of the traffic will choose the existing route entry with lowest AD value.

To test your requirement, two options can be considered:

  1. Take the downtime and delete existing route entry, so that the newly configured route entry will be active and traffic will be forwarded based on this route.

  2. Configure static floating route entries with similar Administrative Distance values (AD). Traffic will be equally load balanced among the two routes, and you can verify that the second route is correctly working.

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