For simplicity, I have three routers Router A, Router B, and Router C. I have a DHCP server called Server D, and a client called Client E.
Router A is my core router which is a Cisco 3750 layer-3 switch
Router B is an older Cisco 3550 layer-3 fiber switch
Switch C is an older Cisco 3524 switch
Server D is a MS DHCP server
Client E is an XP laptop
Client E is connected to Switch C
Switch C has the following:
switchport access vlan 30
Switch C has port 24 configured as:
switchport mode trunk
switchport encapsulation dot1q
Switch C is connected to port 12 on 3750
port 12 on 3750 is configured as
switchport mode trunk
switchport encapsulation dot1q
Router B has port 1 configured as
switchport mode dynamic desirable
Router B connects to Router A by port 12 as
switchport trunk encapsulation dot1q
switchport mode trunk
Server D is connected to 3750 via port 29 by
switchport access vlan 10
With this setup ALL hosts can ping ALL other hosts
However, my problem lies with DHCP. Router A does NOT forward DHCP requests but router B does. Why is this?
Router A has the following:
interface Vlan30
ip address 10.1.2.200 255.255.255.0
ip helper-address 10.1.1.50
end
Router B has the following:
interface Vlan30
ip address 10.1.2.201 255.255.255.0
ip helper-address 10.1.1.50
end
When Client E performs a DHCP request, it is received by Router B and sent to Server D (which is OKAY).
However, the problem is when I disconnect Router B from the network - DHCP requests are NOT picked up by Router A.
I know this because - I did an Ethereal packet capture and I see the DHCP packets transmitted to Router B but not Router A and I did a debug ip dhcp server packet on both routers. Router B showed the packets but Router A never did, not even with Router B off.
I want to completely disconnect Router B from the network. Believe it or not, the only reason I have Router B still connected, is because of this DHCP problem. When Router B is disconnected, DHCP is the ONLY problem. The routing path actually exists through Router A because I can do a tracert and look at the routing tables. But why is DHCP not being forwarded by Router A? I have run out of things to try. Does anyone know? Thanks for help in advance!
