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Cisco IOS and Radius

Introduction

Cisco NAS equipment has become quite popular of late, but being Cisco equipment running IOS, the configuration can be a bit non-obvious to the unfamiliar. This document aims to describe the most common configuration options to make your Ciscos interoperate with radius as you would expect a well-behaved NAS to do.

IOS 12.x

For Cisco 12.x (12.0 and 12.1), the following AAA configuration directives are suggested:

aaa new-model
aaa authentication login default group radius local
aaa authentication login localauth local
aaa authentication ppp default if-needed group radius local
aaa authorization exec default group radius local
aaa authorization network default group radius local
aaa accounting delay-start
aaa accounting exec default start-stop group radius
aaa accounting network default start-stop group radius
aaa processes 6

this such configuration works very well with most radius servers. One of the more important configurations is:

aaa accounting delay-start

This directive will delay the sending of the Accounting Start packet until after an IP address has been assigned during the PPP negotiation process. This will supersede the need to enable the sending of Accounting-Request packets with Acct-Status-Type = Interim-Update, as described below for IOS versions 11.x

The above it will use the radius server to authenticate your inbound telnet connections. You will need to create an entry in your users file similar to the following to allow access: 
!root Password.Cleartext := "somepass" Service-Type = NAS-Prompt-User

This will let a user in for the first level of access to your Cisco. You will still need to enable (using the locally configured enable secret) to perform any configuration changes or anything requiring a higher level of access. The username !root was used as an example here, you can make this any username you want, of course.

Unique Acct-Session-Id’s

Just a note to all cisco ISPs out there who want RFC 2866 compliance need to enable the hidden command: radius-server unique-ident <n>

Minimum IOS: 12.1(4.1)T.

Acct-Session-Id should be unique and wrap after every 256 reboots.

You must reboot after entering this command to take effect. If not, you will observe after 10 minutes of entering this command, the following message. 
%RADIUS-3-IDENTFAIL: Save of unique accounting ident aborted.

IOS 11.x

For Cisco 11.1, you normally use:

aaa new-model
aaa authentication ppp radppp if-needed radius
aaa authorization network radius none
aaa accounting network wait-start radius

to get the Cisco to talk to a radius server.

With IOS 11.3

aaa accounting update newinfo

If you want the IP address of the user to show up in the radutmp file (and thus, the output of radwho).

This is because with IOS 11.3, the Cisco first sends a Start accounting packet without the IP address included. By setting update newinfo it will send an accounting Interim-Update packet which updates the information.

Also you might see a lot of duplicates in the logfile. That can be fixed by:

aaa accounting network wait radius
radius-server timeout 3

To disable the Ascend style attributes (which is a VERY good idea!):

radius-server host X.Y.Z.A auth-port 1645 acct-port 1646

To enable the Ascend style attributes (which we do NOT recommend!):

radius-server host X.Y.Z.A auth-port 1645 acct-port 1646 non-standard

To see Vendor-Specific.Cisco.AVPair attributes in the Cisco debugging log:

radius-server vsa accounting

Cisco 36xx & 26xx, keeping the NAS IP static

The Cisco 36/26 by default selects (it seems at random) any IP address assigned to it (serial, ethernet etc.) as it’s RADIUS client source address, thus the access request may be dropped by the RADIUS server, because it can not verify the client. To make the cisco box always use one fixed address, add the following to your configuration:

ip radius source-interface Loopback0

and configure the loopback interface on your router as follows:

interface Loopback0
ip address 192.0.2.250 255.255.255.255

Use a real world IP address and check the Cisco documentation for why it is a good idea to have working loopback interface configured on your router.

If you don’t want to use the loopback interface of course you can set the source-interface to any interface on your Cisco box which has an IP address.

Credits

More Information

For more information, the following page on Cisco’s web site may help:

http://www.cisco.com/univercd/cc/td/doc/product/access/acs_serv/vapp_dev/vsaig3.htm