Help with 2620xm home network with cable modem SB4100

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May 7th, 2010
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Hi everyone,

I'm trying to set up a home network with Cisco 2620xm (1fa) with cable modem SB4100 through a 2950 switch, but can't get it to work.

I set up 2 Vlans..  Vlan 10 fa port 2 for cable modem, Vlan 20 as access ports for the rest. Port 1 is set as trunk which connects to router.

Router has two sub interfaces fa0/0.10 as IP NAT outside network for Vlan 10 and fa0/0.20 as IP NAT INSIDE network for Vlan 20.

IP route
ACL was set as standard to permitting the networks and Nat is linked with ACL and set to overload.

There is a catch in which my ISP uses DHCP assigned ip, however, it hasn't changed for a very long time.

There are definite faults in my setup but I don't know exactly what it is or if it's even workable because I read on other forums that a 'NAT on a stick' is needed with a loopback address.

Also there is the problem with the cable modem with one max mac, so I switched CDP, Keepalives, STP off on port 2.

Can someone please point me to the right direction?

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Giuseppe Larosa Sat, 05/08/2010 - 02:55
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Hello Matthew,

it is not clear if you have already done this.

However, you can make the router to get an ip address by DHCP service on cable modem like a PC by using

int f0/0.20

enc dot1q 20

ip address dhcp

no shut

on C2950 the only ports in vlan20 have to be port to cable modem and trunk port to router fas0/0

you can also use the DHCP option in a static route like

ip route dhcp

in order to be able to deal with possible changes over time

on switch port to cable modem:

no cdp enable

spanning-tree bpdufilter enable

switchport nonegotiate

! to switch off DTP

you don't need a loopback in your setup.

Hope to help


Paktol1088 Sat, 05/08/2010 - 13:04
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Hi Giuslar,

Thanks for your reply, I've tried a number of diferrent config based on your reply but it's still not working.

Fristly, what do I do with the other vlans? move all access ports to vlan 1?

second, 'DHCP' isn't an option with my router.Is this normal?

Port fa0/1 is set as trunk and fa0/2 (cable modem) is on it's own vlan

Giuseppe Larosa Mon, 05/10/2010 - 02:54
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Hello Matthew,

sorry I did an error the syntax to use the DHCP option in a static route is:

! checked on a C2811 with 12.4(15)T

ip route f0/0 dhcp ?

so in your case becomes

ip route f0/1.10 dhcp

>> Fristly, what do I do with the other vlans?

if you have other vlans / IP subnets and you want them to access the internet you need to add ip nat inside under the Layer3 subinterface

all LAN switch ports should be in a vlan different then vlan 10 or some user can access directly the cable modem.

in vlan 10 you should have only the trunk port to router and the port to cable modem

Hope to help



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