To prevent IP spoofing in a VLAN I configured the following access list

 ip access-list extended MITARBEITER
 permit tcp any
 permit udp any
 permit icmp any

 interface GigabitEthernet0/1.240
 encapsulation dot1Q 240
 ip address
 ip helper-address
 ip access-group MITARBEITER in
 ip nat inside
 standby 240 ip
 standby 240 priority 150
 standby 240 preempt

I tested it and when I remove ip access-group MITARBEITER in the DHCP request are not blocked by the router. (Using Packet Tracer). What is the problem?

  • 2
    You are only permitting traffic from that one /24 network into the router through that interface. You can simplify it by using ip on one line instead of the three different lines of tcp, udp, and icmp.
    – Ron Maupin
    May 2, 2023 at 21:43
  • But by using IP I would block protocols below L3 in the OSI model like. ARP, STP, CDP?
    – Rubus
    May 2, 2023 at 21:54
  • 1
    STP, CDP, etc. are link-only protocols that do not leave the interface on which they are received. ARP does not cross routers. You are no more blocking them with ip than you are by choosing only higher-layer protocols like TCP and UDP.
    – Ron Maupin
    May 2, 2023 at 22:00
  • 2
    It's an IP access-list, so it will only apply to IPv4 layer-3 traffic. ARP, STP, and CDP aren't IP.
    – Ricky
    May 3, 2023 at 0:30
  • Moreover, an IP(v4) ACL doesn't block IPv6 or other L3/L2 traffic...
    – Zac67
    May 3, 2023 at 7:40

1 Answer 1


DHCP Discover broadcast has to be whitelisted explicitly

    ip access-list extended MITARBEITER
 permit udp host eq bootpc host eq bootps
 permit TCP any
 permit UDP any
 permit icmp any

The list can be simplified to

ip access-list extended MITARBEITER
 permit udp host eq bootpc host eq bootps
 permit IP any

Now it works!

  • 1
    You are still using three lines where one would work.
    – Ron Maupin
    May 3, 2023 at 1:47

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