With the new modular policy framework (MPF) introduced in ASA versions 7.x and 8.x, the firewall administrator is now able to apply policing and rate limiting to traffic passing through the ASA appliance. I got a few questions from people how this functionality works and decided to throw in a quick example below which you can easily modify accordingly to match your needs.

Scenario:

We want to rate limit a local internal host when accessing a specific external public server. The local host is 192.168.1.10 and the external public server is 100.100.100.1. We need to limit the traffic to 100kbps and burst size 8000.

Configuration Snippet:

ASA(config)#access-list rate-limit-acl extended permit ip host 192.168.1.10 host 100.100.100.1

ASA(config)#class-map rate-limit
ASA(config-cmap)#match access-list rate-limit-acl

ASA(config)#policy-map limit-policy
ASA(config-pmap)#class rate-limit
ASA(config-pmap-c)#police output 100000 8000

ASA(config)#service-policy limit-policy interface outside

In order to be able to monitor and troubleshoot your Cisco ASA firewall, you need to understand the difference between connections and translations.

Refer to the diagram above for an explanation about Connections and Translations.

A Connection works at the Transport Layer and includes the Source IP/Source Port and the Destination IP/Destination Port. Connections are subsets of Translations. You can have many connections open that are all using the same Translation. For example, a connection shown above is originated from Internal source host 192.168.1.1 with source port 1030 towards a Destination host (public Web Server) 100.100.100.1 on Destination Port 80.

A Translation works at the IP Layer and includes the Real IP Address and the Mapped (Translated) IP Address. Using NAT or PAT, a Real IP address is translated to a Mapped IP address and vice-versa. From the diagram above, the Real IP address 192.168.1.1 is translated to a Mapped IP address 20.20.20.1.

Connection Related Commands

ASA# show conn
ASA# show conn details
ASA# show local-host

The above commands will display the current active connections and information details about each connection. An example output is the following:

TCP outside:100.100.100.1/80 inside:192.168.1.1/1030 idle 0:00:05 bytes 1965 flags UIO

Translation Related Commands

ASA# show xlate
ASA# show xlate detail
ASA# clear xlate

The commands above enable you to display or clear the contents of the translation table. An example output is the following:

NAT from inside:192.168.1.1 to outside:20.20.20.1 flags i

A single Cisco ASA appliance can be partitioned into multiple virtual firewalls known also as “Security Contexts”. Each security context acts as a separate firewall with its own security policy, interfaces and configuration. However, some features are not available for virtual firewalls, such as IPSEC and SSL VPN, Dynamic Routing Protocols, Multicast and Threat Detection.

All firewall models (except ASA 5505) support multiple security contexts. By default, all models support 2 security contexts without a license upgrade (except the ASA 5510 which requires the security plus license).

Each security context that you create on the appliance includes its own configuration file (filename.cfg) stored on local flash memory. This configuration file contains the security policy, the included interfaces and the virtual firewall configuration of the specific security context. By default, an admin context is always created having a configuration file “admin.cfg“. This is just like any other security context except that when a user logs in the admin context then he has full administrator access to all other security contexts.

When you convert the appliance from single context mode to multiple context mode (using the command “mode multiple“) the firewall converts the current running configuration into two files: a new startup configuration that comprises the system configuration, and “admin.cfg” that comprises the admin context (stored in the root directory of the internal Flash memory). The original running configuration is saved as “old_running.cfg” (in the root directory of the internal Flash memory).

Configuring Security Contexts

! Enable multiple context mode
ASA(config)# mode multiple

! Then reboot the appliance.
 

! Configure the administrator context
ASA(config)# admin-context administrator
ASA(config)# context administrator
ASA(config-ctx)# allocate-interface gigabitethernet0/1.10
ASA(config-ctx)# allocate-interface gigabitethernet0/1.11
ASA(config-ctx)# config-url flash:/admin.cfg
 

! Configure other contexts as required
ASA(config)# context customerA
ASA(config-ctx)# allocate-interface gigabitethernet0/2.100
ASA(config-ctx)# allocate-interface gigabitethernet0/2.200
ASA(config-ctx)# config-url flash:/customerA.cfg
 

! Configure other contexts as required
ASA(config)# context customerB
ASA(config-ctx)# allocate-interface gigabitethernet0/2.111
ASA(config-ctx)# allocate-interface gigabitethernet0/2.222
ASA(config-ctx)# config-url flash:/customerB.cfg
 

Changing between contexts and the system execution space:

When you connect with a console cable on the appliance, you will log in the system configuration (or the system execution space). The “system execution space” is the global appliance space from where you can then enter into specific security contexts. If you are logged in the “system execution space” and issue a “show run” command, this will ONLY show you the global system configuration and NOT the various security contexts configurations. You will need to log into a specific security context in order to change or see its configuration.

To change between the system execution space and a context, or between contexts, see the following commands:
 
! To change to a context named CustomerA, enter the following command:
ASA# changeto context CustomerA

! The prompt changes to the following:
ASA/CustomerA#

! To change back to the system execution space, enter the following command:
ASA/CustomerA# changeto system

! The prompt changes to the following:
ASA#