Bootstrapping virtual Cisco XR routers

Cisco IOS XR virtual routers support something called CVAC which allows passing the initial configuration of the router via a virtual drive. It’s rather similar to Cloud-init, if you are familiar with that, or with what Juniper calls config-drive.

It’s fairly simple, you create one or two config files, bake them into an ISO file and mount that ISO as a CD-ROM drive when you start the VM. XR will check if there’s a CD-ROM attached and if so it’ll do the equivalent of copy CONFIG_FILE running-config.

The two potential config files are

  • iosxr_config.txt - normal XR config file
  • iosxr_config_admin.txt - admin mode config file

Create them like you would any other config file. In my case I just want to bootstrap the image with a mgmt-IP, NETCONF server running and some initial user credentials.


hostname router-{0:0>2}
interface MgmtEth0/0/CPU0/0
 ipv4 address 10.0.0.{0}/24
 ipv6 address 2001:db8:0::{0}/64
 no shutdown
ssh server v2
ssh server netconf port 830
netconf agent ssh


username my-user
 group root-system
 secret 5 $1$p00f$r3aLlyS3crEt.Yo

The above config is for XR For I need to modify the SSH & NETCONF configuration for it to work.

ssh server v2
ssh server netconf vrf default
netconf-yang agent ssh

YMMV depending on what version you are on. I have a Python script doing all of this, which explains the use of {0} for formatting/substitution.

Anyway, once you have the config files, bake them into an ISO with mkisofs:

mkisofs -output vms/{0}-config.iso -l -input-charset iso8859-1 -V config-1 --relaxed-filenames --iso-level 2 iosxr_config.txt iosxr_config_admin.txt

You don’t have to use both config files if you don’t want to.

Now start the VM with the ISO attached as a CD. I boot the whole thing using good ol’ qemu - no fancy libvirt (for now) - so for me it’s:

qemu-system-x86_64 -enable-kvm \
	-display none -daemonize -m 4096 \
	-serial telnet:,server,nowait \
	-hda vms/40.qcow2 -cdrom vms/40-config.iso
	... (followed by LOTS of NICs - snipped out for brevity) ...

There’s an alternative syntax for qemu using -drive and if you are using libvirt you need to edit the XML config. See this Cisco page for some examples.

While this takes care of mgmt IP, starting the NETCONF server and creating an initial user, it doesn’t generate SSH keys and since we can only pass a config file via CVAC it’s a bit tricky to ask the box to generate keys. It is actually possible to put “do crypto key generate” in the config file and it will run this in operational mode. Unfortunately XR will interactively prompt for the key length so this method doesn’t work out. On IOS it is possible to specify the key length on the command line so if you are on IOS (or XE) you are in luck.

Since I’m doing all this for a CI environment, it’s not very important to have unique keys, the virtual routers aren’t reachable from anything but the CI runner VM anyway, so I decided to pre-seed the base image with a key.

I don’t want to modify the original image I got from Cisco, so I moved it aside and used it as base for my new image.

mv iosxrv-k9-demo- original-iosxrv-k9-demo-
qemu-img create -f qcow2 -b original-iosxrv-k9-demo- iosxrv-k9-demo-
... start VM with iosxrv-k9-demo- image ...

XR will prompt you to configure the root-system username the first time i starts, which I did, I then run crypto key generate rsa after which I wipe the configuration clean:

ios con0/0/CPU0 is now available

Press RETURN to get started.

!!!!!!!!!!!!!!!!!!!! NO root-system username is configured. Need to configure root-system username. !!!!!!!!!!!!!!!!!!!!

         --- Administrative User Dialog ---

  Enter root-system username: my-user
  Enter secret:

... snip ...

User Access Verification

Username: my-user

RP/0/0/CPU0:ios#crypto key generate rsa
Thu May 19 10:22:47.611 UTC
The name for the keys will be: the_default
  Choose the size of the key modulus in the range of 512 to 4096 for your General Purpose Keypair. Choosing a key modulus greater than 512 may take a few minutes.

How many bits in the modulus [1024]: 4096
Generating RSA keys ...
Done w/ crypto generate keypair
Thu May 19 10:24:28.884 UTC
RP/0/0/CPU0:ios(config)#commit replace best-effort
Thu May 19 10:24:33.474 UTC

This commit will replace or remove the entire running configuration. This
operation can be service affecting.
Do you wish to proceed? [no]: yes
Thu May 19 10:24:37.633 UTC
Thu May 19 10:24:40.823 UTC
RP/0/0/CPU0:ios(admin-config)#commit replace best-effort
Thu May 19 10:24:45.583 UTC

This commit will replace or remove the entire admin running configuration. This
operation can be service affecting.
Do you wish to proceed? [no]: yes

and you’ll be back to where it prompts you to go through the initial config of root-system username - i.e. “factory reset” but with a SSH key loaded.

Do you know of way to generate a key through CVAC or similar? Please reach out to me (contact details in the page footer)!

Written on May 19, 2016