Cisco IOS DHCP Server + classless static routes on DHCP clients

[Vitor Afonso Strabello]

Hi there, I was trying to find if it's possible to add the option for static routes for DHCP clients on Cisco IOS DHCP config mode. I'm looking to add a settings as defined on RFC 3442, like this one, set on ISC DHCPd server:

Global settings:

option rfc3442-classless-static-routes code 121 = array of integer 8;

option ms-classless-static-routes code 249 = array of integer 8;

And for the subnet declaration:

option rfc3442-classless-static-routes  24, 192, 168, 30, 192, 168, 10, 1;

option ms-classless-static-routes       24, 192, 168, 30, 92, 168, 10, 1;

Is it possible?

Thanks!

Vitor

[pascal · Accepted solution]

Yes, the fun part is converting it into a format IOS will accept.  You could try:

ip dhcp pool 0

   option 121 ip 24.192.168.30 192.168.10.1

   option 249 ip 24.192.168.30 92.168.10.1

If that doesn't work, change the "ip" to "hex" and convert each of your decimal bytes into hex.

[Vitor Afonso Strabello]

Hi Pascal, how are you?

Almost one year ago, I noticed this was my post HAHA

It worked for me, but I need to add more networks on options 121 and 249. Example, now I'm running with 2 different networks and I would like to use /16 to summarize both. I need to recall if this way worked on DHCPd server, it was a quite long time ago. I got rid from a VM with DHCPd serving these options from my home lab.

Here are the current config I got working today, on a IOS DHCP server:

!

ip dhcp pool DATA

   network 192.168.110.0 255.255.255.0

   dns-server 8.8.8.8

   option 121 ip 24.192.168.120 192.168.110.1 24.192.168.130 192.168.110.1

   option 249 ip 24.192.168.120 192.168.110.1 24.192.168.130 192.168.110.1

!

And this is the routing output from my computer now. I just pasted the routes from above. Ignore default route:

vitor@mantis:/$ ip route

192.168.130.0/24 via 192.168.110.1 dev wlan0  proto static

192.168.110.0/24 dev wlan0  proto kernel  scope link  src 192.168.110.3  metric 2

192.168.120.0/24 via 192.168.110.1 dev wlan0  proto static

169.254.0.0/16 dev wlan0  scope link  metric 1000

default dev ppp0  scope link

vitor@mantis:/$

Thanks!

[pascal · Accepted solution]

When entering a static route for a /24 you can use ip notation because you have 3 significant octets.  With a /16 you only have 2 significant octets so you have to convert to hex:

option 121 hex 10C0.A8C0.A86E.01

[Vitor Afonso Strabello]

Hi Pascal,

When needing a /16 mask, we need specify only in hex format?

I will try this when returning home.

Thanks!

[Vitor Afonso Strabello]

Hi Pascal, it worked!

Also I got working this way:

option 121 hex 10:C0:A8:C0:A8:6E:01

Thanks!

[Oleksii Martyshchenko]

Thank you for syntax! You solved my problem in a moment!