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Concerning ACL with DHCP.

thonghawkyen
Level 1
Level 1

I have a router with 2 FE interfaces :

(1) interface FastEthernet0/0

=> ip address 137.55.70.1 255.255.255.0

=> duplex auto

=> speed auto

Note : This interface/subnet i hv a DHCP server connected as 137.55.70.2.

(2) interface FastEthernet0/1

=> ip address 137.55.71.1 255.255.255.0

=> ip helper-address 137.55.70.2

=> duplex auto

=> speed auto

Scenario (1) - OK

-------------------------

(1) I hv defined ACLs as followed :

=>access-list 101 permit ip 137.55.71.0 0.0.0.255 host 137.55.70.2

=>access-list 104 permit udp 137.55.71.0 0.0.0.255 host 137.55.70.2 eq bootpc

=>access-list 104 permit udp 137.55.71.0 0.0.0.255 host 137.55.70.2 eq bootps

(2) Applied to F0/0 :

=>ip access-group 104 out

Result : Clients connected to F0/1 subnet get DHCP IP addresses.

Scenario (2) - Not OK

--------------------------------

(1) Use the same ACL applied to F0/1 :

=> ip access-group 104 in

(2) And added the following line in the global configuration mode :

=>ip forward-protocol udp

(3) Remove 104 and applied 101 to F0/1 :

=> ip access-group 101 in

Result : Clients connected to F0/1 subnet CANNOT get DHCP IP addresses.

P/S : It is not as simple as i thought. Appreciate if anyone can help. Thank you very much.

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