Cisco ping source loopback3/20/2024 ![]() "Finally, you will have to exempt the protected traffic from NAT on the loopback" But if you really remove this command, the ping won't work" In that case, Ethernet uses the loopback as a gateway to reach the subnet in question. So, we need to reach the remote protected subnet in order to virtually forward traffic through the Ethernet interface. "Again, the loopback is not a physical interface. Why I said that? Because if you issue debug crypto ipsec, you will notice that the other peer will try to negotiate the tunnel with the 42.x.x.x on ethernet0.156 and it will tells you invalid local address." Without it, the router will think that the endpoint address is the physical interface and the tunnel will never negotiate since the public IP is not defined in the physical interface. To accomplish this, the following command is important to instruct the router to treat the loopback address as the VPN endpoint. "Because the public IP is defined in the loopback interface, it must be our VPN endpoint. ![]() Why I said that? Because if I remove the map off one of them, the tunnel won't negotiate" Applying the map on both of them is crucial. It's the job of the physical interface, which is the ethernet in my case because it's the actual WAN interface. Since the loopback is a virtual interface, it cannot negotiate the tunnel. "Apply crypto map on both the loopback interface and the Ethernet sub-interface. Please feel free to correct me if I am wrong. I am not going to mention other config such as IKE/IPSec proposals, IPSec transform sets, interesting traffic ACL.,etc as you already familiar with. Here is the most important config for a loopback to function as VPN tunnel endpoint along with my humble technical explanation according to my understanding so far. I understand parts of the configuration but other parts I could not understand. However, I still don't understand how it completely works. After lots of experiments, I could finally get it to work. ![]() I just wanted to mention that TCL is actually a full scripting language and can do a lot more than this.I've spent two days figuring out how can I use the loopback interface as the tunnel endpoint. These are all simple TCL scripts that you can easily memorize and utilize during your studies, lab exam or even in production. As we know it’s not enough to just have pings, we also want to ensure our traffic engineering is working correctly: To demonstrate this point further, lets run a trace route on those addresses too. For just pings you can change the source, change packet size, change the repeat count, etc. All it requires is a simple change:Īs you can see, you aren’t limited to what command you can run in the loop. For example, let’s say I want to run that same script but source the traffic from my loopback. I also want to show you that you can run more than just a simple ping. If I can write a script with 10-20 addresses in it, I can copy that script to all the devices in my network and check connectivity. This may not seem like much with 4 addresses, but think scale. So you can see once I hit enter after the final bracket IOS runs a ping test for each IP. So let’s run this first to see what the output looks like: I’m just keeping it to 4 to show you the output when I run it. We also could have 100 addresses in there. We could call it “X” or “loopbacks” or whatever. Basically, for each item in the variable named “addr” run the ping command for that item. It helps to understand basic programming logic here, but it’s not too complicated to understand. Since the script is super simple I figured I’d share it here.įirst here’s a version of the script, and I’ll explain the syntax: tclshįirst we need to enter the TCL shell on IOS. I’m not an expert at TCL by any stretch of the imagination, but I’ve been using a similar script in my lab work and even starting using it in production when making any network-wide changes. That’s where TCL scripting can come in handy. Manually running 20 ping tests on 20 different devices is going to waste valuable minutes. When taking the CCIE, one of the things you need to do is check connectivity across your network quickly.
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