ip route 0.0.0.0 0.0.0.0 ethernet 1 problem

[kcbell]

Below is my home network:

SW ---|Cisco Router|-----|Cable Modem|--
E0 E1
Cisco 3100

I have this Cisco 3100 router running my home network for the pass five years with the routing statement below.

ip route 0.0.0.0 0.0.0.0 ethernet 1

After I shut it off and blow all the dust off inside the router and powered it back up, I could no longer access the Internet.

Then I changed the topology with the following:

SW ---|Cisco Router|----|linux router|----|Cable Modem|--
E0 E1 old PC
Cisco 3100 192.168.0.1

I used the same routing statement as above. Nothing worked.
Then I changed the routing statement to

ip route 0.0.0.0 0.0.0.0 192.168.0.1

Then everything works. I understand why 192.168.0.1 would work because I am linking two router together.

But why “ip route 0.0.0.0 0.0.0.0 ethernet 1” that used to work but it does not work now. With high speed cable Internet, I don’t know what is the next router’s IP.

Any idea?

KC

 

[KiscoKid]

Technically speaking when you specify an interface as a next hop like that (aka connected route), you're relying on proxy arp to find a default gateway to a remote subnet.

Was there definetly not another device (in addition to the cable modem) attached to the Ethernet segment on interface E1?

 

[kcbell]

Kiscokid:

There should be another device connected to the cable modem. E1 got a DHCP address from the device. However, I cannot ping and connect do anything. I could ping my second IP but nothing else.

KC

 

[KiscoKid]

Whenever you have multiple devices on a multiple access (aka Ethernet) segment, there is always a chance a device you don't want to be your default gateway will respond to your proxy arp and become your default gateway - even though it may be uncapable of routing your IP packets.

It's generally not recommended to point default routes via an Ethernet interface like that to avoid the problem described above.

If the modem was the only device physically attached to E1 and absolutely nothing else, it should work ok however.

 

[kcbell]

KiscoKid:

I guess the question is why it does not work. Like I said the configuration was same as I shut down the router and now it doesn't work. Any idea? Thanks.

KC

 

[ccrossley]

Hello Gurus,

I need your help!

Site A (Headquarters) contains
(2) T1 Circuits
(1) Internet T1 circuit
(1) Point to Point to Site B T1 Circuit using a 1721 router

This site is also using an additional SBC router for internet and NAT.


Site B (remote site) contains
(1) Cisco 1721 router that is using point to point tunneling protocal to (1) T1 Circuit


Current Status:
I currently have all of site A working properly, distributing IP addresses, internet access, etc.

Problem:
Site B is not able to recieve internet or IP addresses. Should I configure the Site B Cisco Router to use
VPN or continue to try and use Point to Point tunneling?

Any suggestions/comments are greatly appreciated in advance.

Thx,

 

[jpm121]

If you have a point-to-point T1, you don't need any tunneling at all. You'd put site A in one subnet of private IPs, site B in another subnet, and use the routers to route between them. That's the whole reason for a point-to-point T1! Normally your carrier should be handling the movement of traffic between site A and B, and it is transparent to you.

 

[tech1994]

A little read about default gateway's and statics, might help you out.

http://www.ciscohq.com/FAQs/Cisco-FAQs/Cisco-IP-Default---Gateway.html