ZBF question on Cisco 1841

Hello Experts,

Can anyone explain why Zone Based Firewalls would inspect layer 7 protocols for common protocols such as http, smtp, pop3, imap, etc...

I'm reading about ZBFs and do not understand why a layer 7 deep inspection would be necessary when the firewall is already inpecting on 3/4 layer. Can anyone help me understand why?
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katredrumAsked:
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Nayyar HH (CCIE RS)Network ArchitectCommented:
As network attacks can occur at any Layer of the OSI model a Firewall would need to be able to inspect traffic upto layer-7 if it is going to prevent them. Some protocols as you've mentioned are well known to ZBF, with these protocols ZBF is able to open up secondary channels as neccessary as long as the primary channel is already explicitly permitted. An example would be FTP.

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