Zone Based Firewall Transparent Mode

Cisco’s zone based firewall is normally used with layer 3 interfaces but you can also use it as a transparent firewall. If you have no idea what zone based firewalls are then I suggest you first take a look at my basis ZBF configuration example. If you haven’t configured layer 2 bridging before then you should start with the transparent IOS firewall example. Having said that, let’s configure a Zone based firewall in transparent mode. This is the topology that I will be using:

zbf transparent mode

Above we have 3 routers. R1 and R3 are in the same layer 2 segment because we’ll configure R2 to bridge the FastEthernet 0/0 and 0/1 interfaces. Once this is done we’ll configure the Zone Based Firewall. I will use a very simple example, by default all inter-zone traffic is denied. I want to configure R2 so that it will permit only ICMP traffic from R1 to R3 (and the return traffic). Let’s get started!

Configuration

First we’ll configure bridging:

R2(config)#bridge crb 
R2(config)#bridge 1 protocol ieee 

R2(config)#interface fastEthernet 0/0
R2(config-if)#bridge-group 1

R2(config)#interface fastEthernet 0/1
R2(config-if)#bridge-group 1

I don’t need a layer 3 interface on R2 so we’ll go for concurrent routing and bridging with IEEE spanning-tree. The two FastEhernet interfaces have been added to the bridge group.

If you decide to use bridge irb, the layer 3 bridge interface will automatically belong to the ZBF self zone.

I will create a LAN and WAN zone. R1 will be in the LAN zone and R3 in the WAN zone. We’ll also add the interfaces to the correct zone and create a zone pair for traffic from our LAN to the WAN.

R2(config)#zone security LAN
R2(config)#zone security WAN

R2(config)#interface fastEthernet 0/0
R2(config-if)#zone-member security LAN

R2(config)#interface fastEthernet 0/1
R2(config-if)#zone-member security WAN  

R2(config)#zone-pair security LAN-TO-WAN source LAN destination WAN

With the zones in place, we can create a security policy. We’ll use NBAR to match on ICMP traffic and create a policy-map that uses the inspect rule:

R2(config)#class-map type inspect ICMP
R2(config-cmap)#match protocol icmp

R2(config)#policy-map type inspect LAN-TO-WAN 
R2(config-pmap)#class ICMP
R2(config-pmap-c)#inspect

Last but not least we have to attach that policy-map to the zone pair:

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Forum Replies

  1. hello mr.rene
    thank you for this nice article
    mr.rene i want to say something and wish you understand me because my english not strong too much
    i love your way and your books it was so clear and helpful
    i was wondering if you working on new books?
    if not,so as one of people who loves your work i (kindly) suggest you make either
    new “how to master” ccna/ccnp security books or “how to master” ccie r&s and recommend first one
    i am sure there is people agree with me and i sure they trust in your work
    so sorry that i talk too much
    best wishes

  2. Helpful to us.You carry us to key points.Thanks Mr.Rene Molenaar.

    Best Regards,
    Herbert.

  3. Hi Rene,

    Please help, I lab it up and it is not working. As shown in the show command below there is no icmp packet seen.

    R2#show policy-map type inspect zone-pair
    
    policy exists on zp LAN-TO-WAN
     Zone-pair: LAN-TO-WAN
    
      Service-policy inspect : LAN-TO-WAN
    
        Class-map: ICMP (match-all)
          Match: protocol icmp
    
       Inspect
            Session creations since subsystem startup or last reset 0
            Current session counts (estab/half-open/terminating) [0:0:0]
            Maxever session counts (estab/half-open/terminating) [0:0:0]
            Last session created neve
    ... Continue reading in our forum

  4. Hi Kenneth,

    The config looks ok. Without ZBF, can you ping between R1 and R3?

    What did you use to test this? On Cisco VIRL, it doesn’t work for me. On real hardware it does.

    Rene

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