David Ramsden – Network engineer, general geek, petrol + drum and bass head

Cisco Crypto ACLs – Do they really need to match?

When starting out with IPsec tunnels it seems to be a common misconception that the crypto ACL, sometimes referred to as the encryption domain or the interesting traffic, must match 100% or be mirrored at both peers or the tunnel won't come up. This isn't strictly true. Whilst the ISAKMP phase 1 and IPsec phase 2 proposals must match, the crypto ACL can be different.

Assume that at the local peer traffic to be encrypted originates from and is destined for The crypto ACL would be:

But what about the following?

IPsec phase 2 can still be established even though the crypto ACL isn't mirrored at the local and remove peer. The local peer specifies but the remote peer specifies In this scenario IPsec phase 2 can only be initiated from the peer that has the larger subnet. This is true for both Cisco ASA and IOS.

And in the example above, in the local peer's ACL there's a deny ACE but none on the remote peer's ACL. In this scenario any traffic originating on the local peer from destined to won't traverse the tunnel. The device (ASA or IOS router) will look at the next crypto map in the sequence and try to match traffic there. If no crypto maps are found it'll flow unencrypted out of the egress interface.

Obviously be careful with mismatching subnets and using deny ACEs in the crypto ACL because you may end up with traffic trying to enter the wrong tunnel and other strange things happening.


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Automating mass Cisco IOS upgrades

This morning I needed to upgrade the IOS on 29 Cisco 3560G switches. Rather than login to each one, clean up the flash storage, FTP on the IOS image and set the boot image, I wrote a simple shell script and used clogin from RANCID to automate this task. Of course, nearly every Network Configuration Management platform that's any good should be able to do this but I prefer the personal touch.

The commands required on the switch were as follows:

First I tell IOS to not prompt on file operations. This makes automation easier as there's no need to deal with questions. Then I clean up the flash storage on the switch by removing any old IOS images. The IOS image is copied from an FTP server to the flash storage. The file prompt is put back to defaults and the boot system variable is set to the new IOS image. Finally the configuration is committed to NVRAM because at some point the switch will need to be reloaded.

The shell script will read in a list of IP addresses to connect to and then using clogin it'll login to each switch and execute the commands above.

The script I wrote is as follows:

A file called ips.txt has the list of IP addresses for the switches (one IP address per line). The commands listed above go in to a file called commands.txt. And lastly there's a file called clogin.txt that contains the login details that clogin needs. This would look like:

This tells clogin that there's no need to enter enable and to first try SSH and followed by telnet.

When the script is run it will grab the first IP address in ips.txt, execute clogin to login to the switch and then execute each command in commands.txt. When clogin exits, the IP address in ips.txt will be removed and placed in to a file called processed.txt. The script then prompts if it should continue to the next IP address, allowing you to review what happened to make sure the IOS image copied on OK.

This allowed me to upgrade 29 switches, whilst watching some morning TV and sipping a coffee with my feet up. All that's required now is a reload of each switch.


Automating Cisco switch swap outs

So you can't automate the entire process unfortunately. You're still going to need to pull a late night and get your hands dirty...

Recently I was tasked with swapping out 4 old Cisco 10/100Mb switches with new 10/100/1000Mb switches. The old switches were a combination of Cisco 3560, 2950 and 3548 series. The old switches also had some old configurations that needed to be updated and the interface configurations weren't consistent. The interfaces also had different VLAN configurations and this was my main concern. What if I made a mistake? It's not very realistic to chase 192 ports and make sure every single one was working as expected.

Going back to a previous blog, should network engineers be programmers, writing a script to speed up the configuration process and eliminate any mistakes was the answer.

The process would be:

  1. Get the existing IOS configuration.
  2. Find the interfaces.
  3. Convert the old 10/100 ("FastEthernet") interfaces to 10/100/100 ("GigabitEthernet") interfaces.
  4. Extract the important parts of the interface configuration (description, speed/duplex if set, VLAN configuration, trunk configuration etc).
  5. Pump out the converted interfaces.
  6. Copy/paste in to a template after manually making sure it looks good.
  7. Upload the configuration to the startup-config of the new switch.
  8. Swap the switch out.

First of all, the existing IOS configurations are stored in SVN. They get here via RANCID. So I had the old configurations easily to hand. Also if anything did happen to go wrong on the night, this served as a good reference point.

My weapon of choice for this scripting task was Perl. The script I wrote took in an existing IOS configuration and extracted the physical interfaces and SVIs, converted them from FastEthernet to GigabitEthernet and grabbed all of the important stuff such as description, speed/duplex, VLAN configuration, trunk configuration, IP configuration it's an SVI etc.

Here it is:

I ran this against each of the 4 existing switch IOS configurations, checked the output quickly and then copied/pasted the interfaces in to my switch template that has everything else set such as Spanning Tree configuration, errdisable recovery, NTP, AAA, SNMP, syslog etc etc. VTP would take care of the VLAN database once the switch was connected to the core.

I connected each of the new switches to my laptop and configured the Vlan1 SVI with a temporary IP, then TFTP'd the switch template to the startup-config along with the IOS version required. Turn the switch off and on again to make sure it looks good and job done on the configuration front. Each switch took a very small amount of time to configure and I could be safe in the knowledge that all the interfaces were correct.

The 4 switches were swapped out in the dead of night. It took 4 hours from start to finish, including testing and monitoring. Out of roughly 192 ports there was only one device which didn't work the next morning and that was due to an auto-negotiation issue.In my book it was a very successful, painless and efficient change. One which I wasn't particularly looking forward to but thanks to a bit of scripting ended up being easy.


Editing Cisco IOS ACLs

If you've administered Cisco PIX or ASA security appliances, you'll know how easy editing ACLs is. If you want to insert a new rule in to an existing ACL you can easily insert it where you want. For example:


This will insert the rule at position 12 of the outside_access_in ACL, pushing the existing rule at position 12 down to position 13 and re-ordering everything.

In Cisco IOS there's no obvious way to do this when working with ACLs. A lot of the time people will copy the ACL out, edit it in a text editor, "no access-list" the ACL and then paste in the modified ACL. Which does work but can be risky when working remotely as it's easy to lock yourself out of the IOS device. This can happen if you don't remove the ACL from interfaces before deleting the ACL.

But you can edit ACLs on the IOS device itself when using an extended ACL. Lets create an ACL:


If you view this ACL you'll notice line numbers:


Lets say you need to add another rule before the deny (sequence number 20). Enter the extended ACL:


Now you can insert a new rule by specifying a sequence number less than the deny rule (which is sequence 20):


If you view the ACL you'll see the new rule:


What you can now do is resequence the ACL so that all the sequence numbers are sequential. For example if you wanted the sequence numbers to start at 10 and go up in increments of 10:


If you want to delete a specific rule:


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