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DHCP on a 2621 through a trunk to a 2950

Greetings. In preparing to work on the policy-based routing for the question that was answered earlier today I decided I needed to work out some underlying routing problems first. I'm using my home lab for what should be a simple proof of concept. I have a 2621XM connect to a 2950 via a 1Q trunk. I have 2 sub-interfaces created on the 2621 for 2 different providers (Cox cable and SBC DSL). I have both in my lab. Both sub-ifs are set for DHCP. I haven't created any of the network behind the border router yet.

interface FastEthernet0/0 no ip address speed 100 full-duplex ! interface FastEthernet0/0.10 description TO SBC DSL encapsulation dot1Q 10 ip address dhcp no snmp trap link-status ! interface FastEthernet0/0.20 description TO Cox Cable encapsulation dot1Q 20 ip address dhcp no snmp trap link-status

The 2950 has 1 trunk interface with vlans 10 and 20 allowed and 2 other interfaces in vlan 10 and 20 respectively. Other than that it is basically blank a blank config. I did however create vlan 10 and 20 interfaces with no IPs and unshut them, though I'm not sure if I technically needed to.

interface FastEthernet0/1 description TO 2621 Fa0/0 switchport trunk allowed vlan 10,20,30 switchport mode trunk speed 100 duplex full spanning-tree portfast spanning-tree bpduguard enable ! interface FastEthernet0/2 description TO SBC DSL modem switchport access vlan 10 no cdp enable spanning-tree portfast spanning-tree bpduguard enable ! interface FastEthernet0/3 description TO Cox cable modem switchport access vlan 20 no cdp enable spanning-tree portfast spanning-tree bpduguard enable ! interface Vlan10 no ip address no ip route-cache ! interface Vlan20 no ip address no ip route-cache ! vlan 10 ! vlan 20

Fa0/1 TO 2621 Fa0/0 connected trunk full 100

10/100BaseTX Fa0/2 TO SBC DSL modem notconnect 10 auto auto 10/100BaseTX Fa0/3 TO Cox cable modem connected 20 a-full a-100 10/100BaseTX

Vlan1 unassigned YES NVRAM administratively down down Vlan10 unassigned YES unset administratively down down Vlan20 1.1.1.1 YES manual up up FastEthernet0/1 unassigned YES unset up up FastEthernet0/2 unassigned YES unset down down FastEthernet0/3 unassigned YES unset up up

The problem I'm running into is that the 2621 is not picking up an IP address via DHCP. I have confirmed that my 1Q tunnel is working by statically assigning Fa0/0.20 on the 2621 an IP as well as vlan 20 on the 2950 and pinging between them. CDP is also working. Why wouldn't the 2621 pick up an IP through the cable modem? I don't have the DSL service hooked up to the 2950 at the moment because I'm using it to research solutions (and write this message).

Any ideas or suggestions? I don't have enough Ethernet interfaces in the router to do 2 providers and an inside LAN connection so I'm using a switch for interconnection purposes. This should in theory work just fine.

Thanks J

Reply to
J
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Your setup is almost identical to mine with the exception of the model of switch (and cable provider). There were two things that would stop my router from getting DHCP... 1.) The incomming ACL on the interface - you apparently have not ACL's at all 2.) The switches BPDU's - you are filtering them already.

With that said, it should be working. However, here's some suggestions. You didn't have to create the VLAN interfaces on the switch.

You should probably remove the IP address from interface VLAN 20. Your config doesn't show the IP address, but your interface list does. - (They don't match.) Did you modify the listing of your configuration before posting?? You said you havn't created the network behind the router - Put the switch on that VLAN after you've created it.

J.Cottingim

Reply to
jcottingim

Thanks for the reply. I had a 3548 to play with as well as lots of Cabletron gear but I figured I'd stick with something simple and save the better hardware for something else. I may have to span the ports and see if my DHCP request is getting through the switch. I'm sure though that the trunk is ok. The IP information is old, from previous testing. I can't see any errors but that doesn't mean there aren't any. I'll see what a sniff turns up. Thanks for the info.

J
Reply to
J

The problem is now resolved. After sniffing and finding nothing out of the ordinary I reconnected the cable modem directly to the remaining unused port on the router and configured that port for dhcp. It pulled an IP within seconds. This got me thinking. We're doing MAC filtering at our own ISP on all cable modem customers. We allow only 1 MAC behind the cable modem unless the customer is a business-class customer or if they''ve purchased multiple IPs. I called Cox to inquire if they were filtering on the number of IPs as well. Both the 2950 and the

2621 MACs would both count towards the total. Cox was also doing this kind of filtering. The tech was very helpful and made a few changes necessary to address the problem. I reconnected the cable modem to the switch and voila! It worked. Now my only problem is trying to get the 2600 to act as a PPPoE client... Thanks for your assistance. J
Reply to
J

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