SEC-1.6 IOS Zone Based Firewall (ZBFW)

Greetings programs!

Today we’re talking about zone based firewall.


Zone based firewall is a stateful firewall available as a feature on cisco routers running ios and ios-xe. It’s capable of using nbar to identify traffic and and can perform deep packet inspection (DPI) on a few protocols (the most notable being http). Interestingly it can use Trustsec Security Group Tags (SGTs) as a matching condition.

It offers a cost effective solution for a couple of common cases. One would be small branch offices utilizing an internet connection for Guest internet and a backup dmvpn tunnel.

ZBFW provides a strong alternative to the care and feeding of a separate firewall in some situations, especially now that IOS-XE is capable of running some pretty cool containerized security apps like Snort and Stealthwatch learning network.

There are a couple of tradeoffs to using ZBFW that I noted when I was playing with it. The biggest one is classes in the service policy cannot be reordered or inserted inline.  you have to delete and re-create the policy, then add it back in to the zone pair.  That could lead to some change management headaches.

With that out of the way, let’s go ahead and take a look at this thing.


What does Stateful mean?

The term stateful in the context of a firewall means the firewall builds and maintains a connection table based on traffic it receives on it’s interfaces.  It uses this information to automatically allow the return traffic when it sees the match in the connection table.

A row in a generic connection would look something like this:


This would be a http connection from to  when the return packet from comes back the firewall sees the match in the connection table and automatically allows the traffic. This neat little trick is what makes a firewall a firewall in the sense that most people understand it.

Basic ZBFW operation

for starters have a quick look at this cheat-sheet that shows a simple use case involving internet access for some corporate computers and guest endpoints.

The way it works at a high level is classes are use to match traffic. A policy calls the classes and sets the action for the matching traffic .  Finally the policy is attached to a zone-pair.  A zone pair defines the source and destination interfaces that the policy will be applied to.

Definitions of terms


Zones define interfaces that share a common security policy. Traffic can move freely between interfaces in same zone.

It’s important to know, Interfaces that belong to security zones cannot communicate with interfaces that do not belong to security zones. This is something to keep in mind when designing the deployment and when troubleshooting.

Classes and polices:

There are two kinds of classes and policies. Layer 3/4 and layer 7.


Layer 3/4 Class maps

Layer 3/4 class maps are traffic selectors. Classes are used to identify what traffic we want to apply an action to. Class maps can match based on protocol (NBAR), access-lists, another class map for compound conditions, security group tags (trustsec), and user groups.

layer 7 class maps

A layer 7 class map is used for Deep packet inspection DPI. Most commonly used with HTTP to match attributes or content of http traffic. It’s outdated since most web traffic is encrypted now, but it’s there so I’m mentioning it.

Policy maps

Layer 3 Policy maps

Layer 3 Policy maps contain a list of classes, and the actions we want to perform on the classes.

There is a built-in class called class-default which matches anything not explicitly matched. It can be very useful for troubleshooting to call that class and add the log action to it to see what’s being blocked.

Layer 7 policy maps

Layer 7 policy maps apply actions to the traffic identified by the layer 7 classes.  These classes cannot be called directly from a service policy.  they are called under a layer 3/4 class in a layer 3 policy map that identifies the traffic flows we want to perform DPI on.

Here’s a example

Class-map type inspect http CM-L7-HTTP-1
match response header length gt 5000
policy-map type inspect http PM-L7-HTTP-1
class CM-L7-HTTP-1
Class-map type inspect CM1
match protocol http
policy-map type inspect PM1
class CM1
service-policy http PM-L7-HTTP-1


Important note about classes in policies

Note:  Classes need to be called in most to least specific order.  For example the there was a class  matching on tcp then a class matching on http, the http rule would never fire.  To re-order the classes, you must delete and re-create the policy, then re-add it back to the zone pair.


There two types of nesting. nested classes and nested policy maps

Nested classes are used for creating compound matching conditions. i.e. example inspect these protocols for those hosts.

Nested policy maps are used in deep packet inspection. layer 7 policy maps are called under a class in a layer 3 policy map

Nested class example.  this accomplishes the goal of allowing a specific list of protocols for a specific host.  It’s a basic compound condition.

access-list 100 permit ip any
class-map type inspect match any CM1-L4
match protocol http
match protocol dns
match protocol icmp
Class-map type inspect match-all CM1-L3
match access-group 101
match class CM1-L4

Parameter Maps

There are quite a few different kinds of parameter maps. I’m going to cover a couple of them briefly.

Layer 3/4 maps are used primarily for DDOS prevention. they are attached to the action in a policy map  i.e. “inspect PARAM-MAP”  where param-map is a parameter map.

The most common layer 7 parameter map is regex which is used to match strings in http traffic. It’s applied as an argument to a layer 7 class map.

There is a global inspect parameter map, and this where NBAR2 for protocol classification can be enabled, as well as general connection controls.

Zone pairs

A zone pair defines what traffic is allowed to pass from a source zone to a destination zone. Being stateful, the zone pair builds a connection table based on outgoing traffic. return traffic that matches an entry in the connection table is allowed.

The self zone

The self zone is a special built in zone that’s used to control traffic to and from the control plane of the router.

The self zone has some different behaviors and restrictions from normal zones, and it’s worth taking a closer look at it.

Zone pairs involving the self zone are not stateful. They behave a lot more like access control lists. There have been changes to how self works from version to version, so be mindful of that and test the your image before deploying self zone on a live network. For the version of IOS I tested 15.6(2)T, this is what I found:

By default traffic is allowed to and from the control plane to any zone unless the self zone is added to a zone pair, and a policy attached to it.

If a zone pair is created using self but no service policy is attached, traffic is still allowed.

If a service policy is attached, traffic in that direction is now restricted to what’s permitted in the policy, however, traffic in the other direction is not affected.

Because the self zone is not stateful, you must use the pass argument instead of inspect to allow the traffic, as the inspect directive has no meaning.

ZBF configuration workflow

1. diagram out your zones and policies
a. determine if you need nested classes
i. i.e. match x protocols for y hosts
2. define zones
3. define parameter maps if called for (advanced)
4. create traffic selectors
protocol matching
5. create policy maps
a. call class(es)
b. set action
6. create zone pairs and assign policies
7. assign interfaces to zones
8. test policies
9. review output to verify

Best practices

1. good naming convention: type, direction.
a. ex: for layer 3/4 class map something like: CM-IN2OUT
b. ex: layer 7 CM-L7-BLOCKED-SITES
2. call class-default with drop log action to catch errors
3. pro tip: you can flip a class between match-any and match-all by just re-inputing the command. don’t have to remove re-add

Verification commands

1. show zone security – shows zone interface assignment
2. show zone pair security – verify overall configuration
3. show policy-map type inspect zone-pair – shows statistics for zone pair
4. show policy-map type inspect zone-pair sessions | s Established – shows connection table info.
5. show run | s class|policy|parameter|zone – basic dump of the config elements

Challege lab

This lab topology takes ZBFW and places it in the context of a typical branch office network doing Direct Internet Access (DIA).  The zip contains the challenge and a solution.  enjoy!

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