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tanascius
Asked: 2010-12-23 08:26:13 +0800 CST

Problem with connecting two different networks

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I have two networks: 192.168.13.0/24 (blue) and 192.168.15.0/24 (green).
Computer A is connected to the 13-net, only. Computer B has two interfaces, one in each network.
There is third computer that acts like a router and connects the 13-net to the 15-net (only in this direction).

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Now, I'd like to ping 192.168.15.100 from computer A to B. Unfortunately there is never a reply. But when I use a hub instead of a switch it works.

In my opinion the ping packet travels through the switch to the router (which is the default route/gateway for A). The router sends the packet back to the switch to B. Probably B receives it on its 15-net interface but answers with it's 15th interface? Is this possible?
The problem is, that B may have only a gateway 192.168.13.50 - but I am not really sure of it (B is a embedded system with limited configuration possibilities).

Can anyone explain what happens here? Thank you!

routing networking

3 Answers

  1. In both cases the ping starts by an arp requests to get the MAC address of the router. Then the ping message is wrapped in a layer 2 packet with the router for destination.

    What happens When you use a hub:

    • the ping is broadcasted to all clients (B x 2 and the router),
    • the B.13.100 interface drops the packet,
    • but its 15.100 interface gets it, being probably in promiscuous mode (ignoring the MAC target),
    • thus B replies directly to A on its 13.100 interface.

    Using a switch,

    • B does not get the packet directly. The switch knows that the A packet is directed to the router (thanks to its destination MAC address) and does not involve the other ports.
    • The router should normally get the packet on its 13.50 interface,
    • and should forward it via its 15.50 interface, directed to B.
    • B should get it on its 15.100 interface and reply from its 13.100 interface
    • A should get it directly, it doesn't.

    There is probably something wrong at the router.
    Try a traceroute command from A, using the switch.

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  2. What you are probably seeing is this:

    1. Packet Goes out interface on A (13.10)
    2. Gets Routed to B via Router (15.100 via router)
    3. Gets Recieved by B (15.100)
    4. B goes to reply and says "Oh Hey I'm on the same network!"
    5. Packet goes out interface on B(13.100)
    6. Gets Recieved by A (13.10)
    7. A say's I'm not expecting this from 13.100 drops packet

    You can confirm this by running a packet trace on A & B while sending the ping, look at the headers and check the source & destination addresses.

    The only real way to solve this is to add a /32 routing entry on host B to force the packet out 15.100 but that doesn't scale well.

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  3. a) Do a tracert from computer A to computer B, that should reveal what hops its taking
    b) What are the Default Gateways Set on computer B ?
    c) What type of switch it is ? Layer-2, Layer-3 ?
    d) can you try disabling the 13.100 NIC on computer B completely and then reboot that machine and ping it to see if that works ?

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