ethernet to ethernet routing on 2621

[elsmore2700]

I'm rusty on my cisco skills so I wonder if I'm missing something simple here. Router is configured with LAN on E0/0 and WAN T1 on S0/0. I want to add a backup for egress (web surfing, etc) traffic through E 0/1 to cable modem/SMC 8013 router. Very basic configuration here.

With current config, no route statement is necessary. With new config, I added new route statments:
route 0.0.0.0 0.0.0.0 serial0/0
route 0.0.0.0 0.0.0.0 fastethernet 0/1 <next hop> 100 (distance metric).

As long as S0/0 remains connected, traffic flows normally. It is possible to ping the other side of the cable connection (upstream) from the router. If I shut down the serial interface though, devices on the LAN can't reach the internet as intended.

Thoughts?

 

[rudeboy]

What about your NAT statements? Can you post the config?

 

[elsmore2700]

No NAT to deal with. I actually think I stumbled upon the answer--at least I hope so. I've pasted the information below. Does this seem plausible as the cause of my problem?


"MAKE SURE that you turn off route caching/fast switching,


CODE
interface INTERNET1
no ip route-cache

interface INTERNET2
no ip route-cache

because if you do not the traffic WILL NOT be load balanced correctly. Once a destination is learned out of one interface all subsequent traffic will keep going out that interface without a route table lookup. Once you turn that off, traffic will be truly load balanced. "

 

[rudeboy]

Once your Serial0/0 is shut down, the router now knows that this is not available for routing packets. The router should then send all packets out the other interface. I think that the ip route-cache is important when load balancing as far as performance goes, however when one interface goes down, you are no longer talking about load balancing. If you are not doing NAT, I am guessing that you have a PUBLIC IP on your LAN. Are you running BGP?

 

[elsmore2700]

Thanks for the response. No, there's no routing enabled except for the static routes I added. There is a public IP on the WAN/serial interface and the WAN/FastE is NAT'd by the SMC cable modem/router.

ISP cloud Cable modem - Comcast cloud
| |
|public IP | 10.x
|S 0/0 | E 0/1
|----------------|
| cisco 2621 |
| IOS-12.1 (17) |
|----------------|
| LAN
| public IP (/26 block)
| E 0/0

 

[rudeboy]

I know that it seems that I am stuck on NAT but per your diagram, I am assuming the following;
1)s0/0 has a public IP (T1)
2)e0/1 has a private IP (=>Cable Modem)
3)e0/0 has a pulic IP (LAN connection)

If this is the case, then the CISCO is doing no NAT at all. Your cable modem/router is probably set up to NAT traffic from the 10.x.x.x network, but your LAN is coming from a public subnet. Can you configure your cable modem/router to NAT your public LAN block? If not, you should set up a NAT rule in your cisco when going out E0/1.

 

[elsmore2700]

I see. I wish I had the device in front of me to test this out. You're suggesting that I've got to configure reverse NAT from the LAN (public IP) to a static 10.x.x.x address for the traffic to go out E0/1. The E0/1 interface won't handle egress packets with a public IP that is outside of the address pool of the cable modem.

Of course if I just NAT'd the entire LAN side with the same numbering scheme as the cable modem, this would all be easy.
Well I'll try it out later this week.