Catalyst 6500 Series Switch and Cisco 7600 Series Router Firewall Services Module Configuration Guide, 3.2
Introduction to the Firewall Services Module
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Introduction to the Firewall Services Module

Table Of Contents

Introduction to the Firewall Services Module

New Features

Security Policy Overview

Permitting or Denying Traffic with Access Lists

Applying NAT

Using AAA for Through Traffic

Applying Internet Filtering

Applying Application Inspection

Applying Connection Limits

How the Firewall Services Module Works with the Switch

Using the MSFC

Firewall Mode Overview

Stateful Inspection Overview

Security Context Overview

Introduction to the Firewall Services Module

The FWSM is a high-performance, space-saving, stateful firewall module that installs in the Catalyst 6500 series switches and the Cisco 7600 series routers.

Firewalls protect inside networks from unauthorized access by users on an outside network. The firewall can also protect inside networks from each other, for example, by keeping a human resources network separate from a user network. If you have network resources that need to be available to an outside user, such as a web or FTP server, you can place these resources on a separate network behind the firewall, called a demilitarized zone (DMZ). The firewall allows limited access to the DMZ, but because the DMZ includes only the public servers, an attack there affects only the servers and does not affect the other inside networks. You can also control when inside users access outside networks (for example, access to the Internet), by allowing only certain addresses out, by requiring authentication or authorization, or by coordinating with an external URL filtering server.

The FWSM includes many advanced features, such as multiple security contexts (similar to virtualized firewalls), transparent (Layer 2) firewall or routed (Layer 3) firewall operation, hundreds of interfaces, and many more features.

When discussing networks connected to a firewall, the outside network is in front of the firewall, the inside network is protected and behind the firewall, and a DMZ, while behind the firewall, allows limited access to outside users. Because the FWSM lets you configure many interfaces with varied security policies, including many inside interfaces, many DMZs, and even many outside interfaces if desired, these terms are used in a general sense only.

This chapter includes the following sections:

New Features

Security Policy Overview

How the Firewall Services Module Works with the Switch

Firewall Mode Overview

Stateful Inspection Overview

Security Context Overview

New Features

Table 1 lists the new features for Release 3.2.

Table 1 New Features for FWSM Release 3.2


BGP Stub Routing

The FWSM supports BGP stub routing. The BGP stub routing process advertises static and directly- connected routes but does not accept routes advertised by the BGP peer.

See the "Configuring BGP Stub Routing" section.

High Availability

Failover Preemption for Active/Standby Failover

You can configure failover preemption for units in an Active/Standby failover configuration. When the primary unit in an Active/Standby failover configuration fails, or if the secondary unit boots before the primary unit, the secondary, standby unit becomes active. Configuring failover preemption causes the primary unit to automatically become the active unit after a specified amount of time.

See the "Configuring Failover Preemption" section

AAA State Replication

FWSM synchronizes the user authentication table when Stateful Failover is enabled so the user does not have to authenticate once again after failover happens.

See the "Stateful Failover" section.

Application Inspection

SIP Enhancement

SIP enhancements allow FWSM to clear media connections on receipt of a 200 OK for BYE message, on receipt of a 200 OK for CANCEL message, or on receipt of 200 OK for 4xx/5xx/6xx Error messages. Previously media connections were cleared only due to an idle timeout. This enhancement also makes embryonic connections timeout no longer based on the configurable timeout sip-invite command or on the expiry field in the SIP invite message.

See the "SIP Inspection Enhancement" section.


This release introduces PAT support in RTSP. For the RTSP PAT feature, if the translated port is different from the original port in an RTSP control channel message exchange, the translated port number is included in the RTSP packet before it is sent to the server. This ensures that the server responds on the correct port for the client.

See the "RTSP Inspection" section.

H.323 GUP Support

The H.323 GUP support feature includes a separate inspection module that receives packets via dynamic inspection logic. This new inspection opens up pin-holes for establishing connection among Cisco gatekeepers working as clusters to provide gatekeeper redundancy to H323 gateways.

See the "H.323 GUP Support" section.

GGSN Load Balancing

The GGSN load balancing feature allows any GNS belonging to a GNS pool to respond to an SGSN request in order to achieve load balancing on the GGSNs. The inspection engine allows a set of GNS to respond to a request even if a GSN is not specified as the responder to the request in the GTP request message.

See the "GGSN Load Balancing" section.

Transparent Firewall

Transparent Firewall NAT Support

You can now configure NAT for a transparent firewall. This feature extends the NAT/PAT functionality to transparent mode thereby reducing the need for adding a new NAT/PAT device in the network. This feature is also very useful in cases where multiple VRFs with overlapping addresses are used. NAT per VRF is not supported on the Catalyst 6500 series switches and the Cisco 7600 series routers.

Introducing NAT support for transparent firewalls addresses the NAT per VRF requirement while offering the capability to run routing protocols through the firewall with a simple configuration.

See the "NAT in Transparent Mode" section.


NAT Bypass No Longer Creates NAT Sessions

In previous releases, even if you used NAT exemption or identity NAT, the FWSM created NAT sessions (xlates) for all flows. In Release 3.2, you can configure the FWSM to create xlates only when NAT is configured. By default, the FWSM creates NAT sessions for all connections even if you do not use NAT. For example, a session is created for each untranslated connection even if you do not enable NAT control, you use NAT exemption or identity NAT, or you use same security interfaces and do not configure NAT. Because there is a maximum number of NAT sessions, these kinds of NAT sessions might cause you to run into the limit. To avoid running into the limit, you can disable NAT sessions for untranslated traffic using the xlate-bypass command.

See the "NAT Session (Xlate) Creation" section.


Authentication Support When Sessioning To The System Execution Space

When you log in to the system execution space from the switch in multiple context mode, a new feature in FWSM Release 3.2 lets you use authentication using a AAA server or local database. Previously, the only method of authentication available was to use the login password defined in the system configuration. The new authentication method is enabled by the aaa authentication telnet console command in the admin context. If you upgrade to Release 3.2, and have this command already in the admin context configuration, then authentication for the system execution space is enabled using the specified server or local database, even if you did not intend to enable it. To use the login password instead, you must remove the aaa authentication telnet console command in the admin context.

See the "Authenticating Sessions from the Switch to the FWSM" section.

Direct Login Or Logout Using Virtual HTTP and SSH For User Authentication

In addition to direct login with virtual Telnet, you can now log in or out directly using HTTP and SSH.

See the "Authenticating Directly with the FWSM" section.

Virtual HTTP Hostname Support

You can now assign a hostname to the virtual HTTP server on the FWSM. When a user is forwarded to the virtual HTTP server to enter their AAA username and password, you see the hostname in the authentication dialog box message.This information helps differentiate the AAA prompt from the destination HTTP server prompt.

See the virtual http command in the Catalyst 6500 Series Switch and Cisco 7600 Series Router Firewall Services Module Command Reference.

Interactive Password Prompts With RADIUS For Authentication

With RADIUS servers, a user can now be prompted for a new password when authenticating.

See the "Authentication Methods" section.


TCP State Bypass

If you have asymmetric routing configured on upstream routers, and traffic alternates between two FWSMs, then you can configure TCP state bypass for specific traffic.

See the "Configuring TCP State Bypass" section.

Connection Timeouts For Non-TCP Traffic On A Per-flow Basis

You can now configure connection timeouts for non-TCP traffic using Modular Policy Framework. Formerly, you could only set global timeouts.

See the "Configuring Connection Limits and Timeouts" section.

Switch Integration

IOS Support For Autostate Messaging For Rapid Link Failure Detection

Using Catalyst operating system software Release 8.4(1) and higher or Cisco IOS software Release 12.2(18)SXF5 and higher, the supervisor engine can send autostate messages to the FWSM about the status of physical interfaces associated with FWSM VLANs.

See the "Enabling Autostate Messaging for Rapid Link Failure Detection" section.

Security Policy Overview

A security policy determines which traffic is allowed to pass through the firewall to access another network. The FWSM does not allow any traffic to pass through unless explicitly allowed by an access list. You can apply actions to traffic to customize the security policy. This section includes the following topics:

Permitting or Denying Traffic with Access Lists

Applying NAT

Using AAA for Through Traffic

Applying Internet Filtering

Applying Application Inspection

Applying Connection Limits

Permitting or Denying Traffic with Access Lists

You can apply an access list to allow traffic through an interface. For transparent firewall mode, you can also apply an EtherType access list to allow non-IP traffic.

Applying NAT

Some of the benefits of NAT include the following:

You can use private addresses on your inside networks. Private addresses are not routable on the Internet.

NAT hides the local addresses from other networks, so attackers cannot learn the real address of a host.

NAT can resolve IP routing problems by supporting overlapping IP addresses.

Using AAA for Through Traffic

You can require authentication and/or authorization for certain types of traffic, for example, for HTTP. The FWSM also sends accounting information to a RADIUS or TACACS+ server.

Applying Internet Filtering

Although you can use access lists to prevent outbound access to specific websites or FTP servers, configuring and managing web usage this way is not practical because of the size and dynamic nature of the Internet. We recommend that you use the FWSM in conjunction with a separate server running one of the following Internet filtering products:

Websense Enterprise

Sentian by N2H2

Applying Application Inspection

Inspection engines are required for services that embed IP addressing information in the user data packet or that open secondary channels on dynamically assigned ports. These protocols require the FWSM to do a deep packet inspection.

Applying Connection Limits

You can limit TCP and UDP connections and embryonic connections. Limiting the number of connections and embryonic connections protects you from a DoS attack. The FWSM uses the embryonic limit to trigger TCP Intercept, which protects inside systems from a DoS attack perpetrated by flooding an interface with TCP SYN packets. An embryonic connection is a connection request that has not finished the necessary handshake between source and destination.

How the Firewall Services Module Works with the Switch

You can install the FWSM in the Catalyst 6500 series switches and the Cisco 7600 series routers. The configuration of both series is identical, except for the following variations:

The Catalyst 6500 series switches supports two software modes:

Cisco IOS software on both the switch supervisor and the integrated MSFC (known as "supervisor IOS").

Catalyst Operating System (OS) on the supervisor, and Cisco IOS software on the MSFC.

For commands and configuration that are performed on the switch itself, both modes are described.

The Cisco 7600 series routers support only Cisco IOS software.

Both series are referred to generically in this guide as the "switch."

The FWSM runs its own operating system.

Using the MSFC

The switch includes a switching processor (the supervisor) and a router (the MSFC). Although you need the MSFC as part of your system, you do not have to use it. If you choose to do so, you can assign one or more VLAN interfaces to the MSFC (if your switch software version supports multiple SVIs. In single context mode, you can place the MSFC in front of the firewall or behind the firewall (see Figure 1-1).

The location of the MSFC depends entirely on the VLANs that you assign to it. For example, the MSFC is behind the firewall in the example shown on the left side of Figure 1-1 because you assigned VLAN 201 to the inside interface of the FWSM. The MSFC is in front of the firewall in the example shown on the right side of Figure 1-1 because you assigned VLAN 200 to the outside interface of the FWSM.

In the left-hand example, the MSFC routes between VLANs 201, 301, 302, and 303, and no inside traffic goes through the FWSM unless it is destined for the Internet. In the right-hand example, the FWSM processes and protects all traffic between the inside VLANs 201, 202, and 203.

Figure 1-1 MSFC Placement

For multiple context mode, if you place the MSFC behind the FWSM, you should only connect it to a single context. If you connect the MSFC to multiple contexts, the MSFC will route between the contexts, which might not be your intention. The typical scenario for multiple contexts is to use the MSFC in front of all the contexts to route between the Internet and the switched networks (see Figure 1-2).

Figure 1-2 MSFC Placement with Multiple Contexts

Firewall Mode Overview

The FWSM runs in two different firewall modes:



In routed mode, the FWSM is considered to be a router hop in the network.

In transparent mode, the FWSM acts like a "bump in the wire," or a "stealth firewall," and is not considered a router hop. The FWSM connects to the same network on its inside and outside interfaces. You can configure up to eight pairs of interfaces (called bridge groups) to connect to eight different networks, per context.

You might use a transparent firewall to simplify your network configuration. Transparent mode is also useful if you want the firewall to be invisible to attackers. You can also use a transparent firewall for traffic that would otherwise be blocked in routed mode. For example, a transparent firewall can allow unsupported routing protocols.

In multiple context mode, you can choose the mode for each context independently, so some contexts can run in transparent mode while others can run in routed mode.

Stateful Inspection Overview

All traffic that goes through the firewall is inspected using the Adaptive Security Algorithm and is either allowed through or dropped. A simple packet filter can check for the correct source address, destination address, and ports, but it does not check that the packet sequence or flags are correct. A filter also checks every packet against the filter, which can be a slow process.

A stateful firewall like the FWSM, however, takes into consideration the state of a packet:

Is this a new connection?

If it is a new connection, the firewall has to check the packet against access lists and perform other tasks to determine if the packet is allowed or denied. To perform this check, the first packet of the session goes through the "session management path," and depending on the type of traffic, it might also pass through the "control plane path."

Note The first packet for a session cannot be comprised of fragments for a packet that is larger than 8500 Bytes. The session will be established, but only the first 8500 Bytes will be sent out. Subsequent packets for this session are not affected by this limitation.

The session management path is responsible for the following tasks:

Performing the access list checks

Performing route lookups

Allocating NAT translations (xlates)

Establishing sessions in the "accelerated path"

Some packets that require Layer 7 inspection (the packet payload must be inspected or altered) are passed on to the control plane path. Layer 7 inspection engines are required for protocols that have two or more channels: a data channel, which uses well-known port numbers, and a control channel, which uses different port numbers for each session. These protocols include FTP, H.323, and SNMP.

Note The FWSM performs session management path and accelerated path processing on three specialized networking processors. The control plane path processing is performed in a general-purpose processor that also handles traffic directed to the FWSM and configuration and management tasks.

Is this an established connection?

If the connection is already established, the firewall does not need to recheck packets; most matching packets can go through the accelerated path in both directions. The accelerated path is responsible for the following tasks:

IP checksum verification

Session lookup

TCP sequence number check

NAT translations based on existing sessions

Layer 3 and Layer 4 header adjustments

For UDP or other connectionless protocols, the FWSM creates connection state information so that it can also use the accelerated path.

Data packets for protocols that require Layer 7 inspection can also go through the accelerated path.

Some established session packets must continue to go through the session management path or the control plane path. Packets that go through the session management path include HTTP packets that require inspection or content filtering. Packets that go through the control plane path include the control packets for protocols that require Layer 7 inspection.

Note For QoS compatibility, the FWSM preserves the DSCP bits for all traffic that passes through the FWSM.

Security Context Overview

You can partition a single FWSM into multiple virtual devices, known as security contexts. Each context has its own security policy, interfaces, and administrators. Multiple contexts are similar to having multiple standalone devices. Many features are supported in multiple context mode, including routing tables, firewall features, and management. Some features are not supported, including dynamic routing protocols.

In multiple context mode, the FWSM includes a configuration for each context that identifies the security policy, interfaces, and almost all the options you can configure on a standalone device. The system administrator adds and manages contexts by configuring them in the system configuration, which, like a single mode configuration, is the startup configuration. The system configuration identifies basic settings for the FWSM. The system configuration does not include any network interfaces or network settings for itself; rather, when the system needs to access network resources (such as downloading the contexts from the server), it uses one of the contexts that is designated as the admin context.

The admin context is just like any other context, except that when a user logs in to the admin context, then that user has system administrator rights and can access the system and all other contexts.

Note Multiple context mode supports static routing only.