cisco 851 and 2 LANs

Unanswered Question

There are 2 LANs connected to FastEth 4, which has primary and secondary addresses:

int f 4

ip add

ip add second

I can ping hosts from both net only when I provide the source address:


ping sour

ping sour

Without source the second net doesn't respond

ping - DOESN'T work!

I've never come to something like this before. I also have a HP router and it works. What can be done with this?

ps: I want cisco to be a router for these 2 LANs.

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Richard Burts Sun, 09/16/2007 - 10:39


The symptoms sound like there might be an issue with the configuration of the default gateway on the PCs. It sounds like they can access addresses within their own subnet but have difficulty in accessing addresses outside of their own subnet. Typically this would be an issue about the default gateway of the PC. Can you confirm what is the configured default gateway for the PC in the 2.2.2 network?



May be I dont understand something but I don't have any idea what default gateway those hosts have. They are not under my control. I only have ethernet outlet and the information that it is connected to dummy L2 switch which serves for 2 LANs. As I said I can ping hosts on those LANs whis the only limitation that I need to provide the source address for the "secondary" net. In my opinion it means that packets that go out of FastEth4 have default src address. When I explicitly change it with the source directive the result is positive, but it's not enough. I want packets destined for to go out F4 with return address (not which they don't know where to send).

I guess we can safely rule out notion that F4 doesn't answer arp requests on behalf. In this case "ping sour" wouldn't come back.

I'm stuck. I have pretty much the same configuration in my office LAN with HP router having secondary address and packets for both LANs go without troubles.

Any ideas?

Richard Burts Sun, 09/16/2007 - 12:58


I am slightly confused and hope that you can clarify. In the original post you mentioned a HP router that works with this. I am not clear whether the HP is at your office (not where you currently are) and works there or whether the HP is where you currently are and works in the situation when the Cisco does not. Perhaps you can clarify this?



I see. I guess I should not have mentioned HP at all. It's a distraction, please, don't pay attention to it.

In my original post I simplyfied things a bit. Now I feel I nedd to explain it all.

There are 2 places A & B.

A is where I work and where I am located. It has 2 nets /24 & /28 and HP edge router with outside ip /29.

B is a remote place, location unknown. There is cisco 851 there which has primary ip /28 and to which I have a route. It also (obviously) connected to lan which I don't care about.

What I am interested in is another LAN ( is on the same switch with Cisco see it, can ping it, provided I specify a source address. But I can not give source address to routing rules.I need to reach from A as I do primary B.

Hope it clarifies things, tough now I feel something is not right.

Richard Burts Mon, 09/17/2007 - 07:18


The symptoms that you describe make a bit more sense now. And they sound like a problem with end stations not with the router. If the router can ping the end stations when you specify a source address in 2.2.2 then it demonstrates that there is a good physical path and demonstrates basic IP connectivity. If the router can not ping the end stations if you use the default source address then it is likely that the end stations can not communicate with anything that is not on the local LAN. That is an end station problem and not a router problem.



But this is exactly what I want - the router to appear "local" for both LANs. Now when I thought it over again I feel I need NAT for packets source address mangling.

May be this picture explanes things better than the verbal content.>R(HP)-<

R - is router

+ - physical connection

Let me repeat an objective once again: reach hosts on subnet from

Richard Burts Mon, 09/17/2007 - 08:29


Are you saying that you want 1.1.1.x to see the router as local, for 2.2.2.x to the router as local and for 1.1.1.x and 2.2.2.x to not see each other as local? If so then you need to configure it as VLANs instead of as secondary addresses. If 1.1.1.x and 2.2.2.x are in the same VLAn then they are not separated.



Richard Burts Mon, 09/17/2007 - 09:27


I have re-read the posts in this thread. I continue to believe that the symptoms you describe suggest that the issue is the default gateway on the end stations. I know that you have explained that you do not control these end stations. But for to access 2.2.2.x the end stations must have a default gateway of the address on the router (

In one of your posts you mention the possibility of trying to use NAT to solve this issue. I do not believe that NAT will solve this problem. If you do not control those end stations then I believe that you need to communicate with whoever does control them and verify what they are currently configured with as default gateway.




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