Is there anything improper about defining the default gateway as a host on a network that none of the interfaces belong to (provided that there's a route table entry for this network)?
- posted
15 years ago
Default GW on Different Network
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- posted
15 years ago
You mean your subnet is 172.16.1.0 /24 and you want to use a GW of
192.168.1.1 (routable from 172.16.1.0 router)?Couple of things:
1) Some operating systems will not let you do this. 2) You have to depend on proxy-arp on the router with 172.16.1.0 interface. 3) The PC somehow has to know that he has to arp for a gateway that's no on his subnet.If you're talking about router, recursive routing will take care of it.
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- posted
15 years ago
To make an analogy, a default gatewaty is like a door and an IP subnet is like a room. If the door that leads out of the room is actually in another room, then how do you get out? Having a default gateway IP address which is not in the host subnet is against best practices of networking and is generally a bad idea.
----- Scott Perry Indianapolis, IN
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- posted
15 years ago
Yes.
I did a quick test and see that both XP and a cheap Linksys wireless router (BEFW11s4) allow the GW to be on a different network than the interface. Do Cisco routers? What OS are you thinking of that prevents this?
Do you mean that the client will arp for the designated gateway's mac addr and the closest router will have to respond for the GW because it blocks the broadcast? Do they do this automatically? If not, how do you turn on proxy arp? In general, is it a good idea to enable this?
When the PC wants to send a packet off net, won't he broadcast an arp request (assuming no arp cache entry) regardless of whether the designated GW is on his network or not? If so, this issue depends on the proxy arp point you brought up above, right?
Sorry, I don't get what you mean. Please explain.
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- posted
15 years ago
Well, with an entry in the route table, you would be able to walk through the "other room" to get to the door. Right?
Actually, your comment that this is against best practices answers my question fine. But several interesting issues that I don't quite get yet have been raised so I think it's worth persuing them until I understand.
When I originally posted the question, I was thinking of the situation where a client is either connected to one of several wireless routers or connected via Cat5 and I was considering using the edge router (the one provided by the ISP) as the default gateway for all situations. I see that this is not a good idea because of arp issues.
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- posted
15 years ago
Recursive routing means a next hop address normally reachable directly needs a route to get there. So if I tell you to get to 1.1.1.0 you need to go via 2.2.2.0, and the route to 2.2.2.0 is via 5.5.5.254, and you are 3.3.3.1, you're next hop to 5.5.5.254 is 3.3.3.254. This is generally a bad thing . Its like saying to get from a to b you go via c, but c is accessible by d. Recursive routing can cause loops and timeouts.
Bob Sim> >
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- posted
15 years ago
That would work. Your wireless clients would ARP out the wireless router IP address as their LAN default gateway. The wireless router in turn ARPs out the ISP provided router as its default gateway. Although the wireless clients could detect this using a traceroute, they are not aware of any MAC addresses or ARP communication outside of their broadcast domain by its very definition.
----- Scott Perry Indianapolis, IN
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- posted
15 years ago
Mostly older operating systems and embedded systems. Routers will let you do it, however.
For example, one thing Unix admins used to do was to set it's own interface as the default gateway. This tells the OS that *THE ENTIRE* world of IP is recheable via the NIC and that the stack should arp for EVERY IP. So if a router receives an arp frame for some far away subnet/IP, the (local) router will answer the arp on behalf of the far away IP. Proxy arp is on by default on all cisco routers. It is *not* a good idea because you can deliver a packet on behalf of someone who doesn't know your network. IE someone just plugs in a PC and can start talking to anyone in your network due to proxy-arp.
The first first the the PC does is to figure out if the destination IP is on the local subnet or a remote subnet. If it's local, then the PC will send out an arp request. If the PC decides that the destination IP is *not* on the local subnet, he will arp for the gateway because the PC will know that he has to punt to the GW.
Other's explained recursive routing so I'll skip this one.