Should I block icmp on my edge router or my firewall?

Originally, we were blocking icmp on our border router traffic (2811), but recently we changed this block on the firewall (ASA) instead. I have been informed that blocking on the router would cause too much overhead on the router, since it is now seen to inspect all traffic, and the firewall was better equipped for this.

What is the standard of the industry? Cisco recommend that?

Something like that, although I recommend you this announcement on the forum of firewall for confirmation.

! refuse the Fragments non-initial ICMP

access-list 101 deny icmp any any fragment

! permit messages "dest unreachable."

access-list 101 permit icmp any 3

! allow the message "time exceeded".

access-list 101 permit icmp any any 11

! allow the message "source quench"

access-list 101 permit icmp any 4

! license problem message "parameter.

access-list 101 permit icmp any any 12

! allow "echo reply" messages

access-list 101 permit icmp any any 0

! refuse all other icmp

access-list 101 deny icmp a whole

You could consider strengthening the unreachable destination too. They should look like this for each type and code that you want to allow:

! allow messages 'dest unreach - unreach port.

Acccess-list 101 permit icmp any any 3 of 3

See here:

http://www.IANA.org/assignments/ICMP-parameters

Tags: Cisco Security

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