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Meshing and Backup-Links over WDS additionally to cable

Discussion in 'Networking Issues' started by Knicko, Sep 14, 2005.

  1. Knicko

    Knicko Network Guru Member

    Hi Guys,
    I have connected 5 houses either directly via cable or via the WDS-Option of my accesspoints (all WRT54G vith DD-WRT v22).

    As the cable-links as well as the WDS-connections (aps hang up, WDS/WPA connection gets lost) break down from time to time I would like to add additional backup links to keep the network up running in every case.

    I guess that simply connecting each WRT54G via WDS to each other in range aditionally will not work because of packet collisions (will it?).

    I've read that OSPF might be a solution for me; does anyone have a clue how routing-tables for LAN and WLAN should look like (the problem I have is that three of these APs are directly connected via the switched network without routers inbetween), which subnets to give to which house/AP and how windows-networking will continue working?

    A lot of questions, I know.
    I hope you can help me, I'm really going mad otherwise....
  2. DevilStick

    DevilStick Network Guru Member

    You do not have a routing problem. Since using WDS and wired links, you are creating loops in your network.

    Let's say you initially set up your network and a pc in house #3 wants to send a packet to a pc in house #2. Since your WRT54G in house #3 does not know which pc belongs to your destination IP, it broadcasts an arp request on all ports to get the mac address of the destination pc. (ARP who has ip xxx.xxx.xxx.xxx) All network devices in the same broadcast domain get this arp request and the device with the ip xxx.xxx.xxx.xxx creates a unicast arp reply (ARP xxx.xxx.xxx.xxx is MAC yy:yy:yy:yy:yy:yy). The pc which is connected to the router on house #2 creates this arp reply and now your router has a big problem to transmit this packet:

    Switches create virtual point-to-point connection. This is done by learning which mac addresses are connected to which ports and thus packets for know mac addresses are only sent to the learned port.
    But the router on house #2 got the arp request and thus the source mac-address of the pc in house #3 on 2 ports (the Wireless WDS port and the wired port). So it cannot decide where to send the packet and broadcats it again on all ports. The other routers on network do the same and thus the packet travelles als loop around the network until its Time-to-live expires.

    Which loops you can paralyze a complete network in seconds. But since big networks need redundance there is a solution for this problem. It's called Spanning Tree Protocol (STP).

    Spanning tree keeps care that there is exactly one way for a packet to travel between two devices. This is done by blocking the loop. All STP devices send Spanning Tree Packets to prove that a link is up. When now a link goes down - cable break - the spanning tree message from the directly connected device is not received in a given interval and the blocked port gets switch to forward mode and the backup line is set up.

    You have either to remove the wired links between house #3 and house #2 and the one between house #1 and house #2 or you have to use STP which is not supported on the official Linksys firmware. But since you use WDS, I assume you are using an alternate firmware which you should check for STP.

    I think DD-WRT is able to talk Spanning-Tree. There is at leat an option in the wan port configuration, but I think it also hast affect to the LAN ports.

  3. Knicko

    Knicko Network Guru Member

    Oh cool, I didn't know that this option in dd-wrt v22 also enables spanning-tree on the LAN site.

    I tried it out and spanning-tree information was relly exchanged. Unfortunately the connection now gets totally lost for about 30 seconds at some of the routers, now linked by LAN and WDS.

    Might it be that my network-configuration is responsible for? Do I have to replace the Switches by the WRT54Gs, meaning to switch the LAN-segments together directly into the WRT54Gs and not into the switches?

    Hope you can help ;)

    Here's a more detailled setup of the current LAN (cable) connections:
  4. DevilStick

    DevilStick Network Guru Member

    When I understand correctly, you have attached switches to your WRT54G and the cable creating the loop is not directly connected to the WRT54G, but to the switch.

    Maybe your switch does not forward spanning-tree packets and filter it out. Do you know ethereal or other sniffers?

    I think so, since you told me about STP packets on LAN ports which you saw.

    So connect your client to the switch and start a sniffer. When you do not see STP packets within a few seconds, then your switch is the problem and you should connect the cable which creates the loop directly to your WRT54G.

    Here is my suggestion for cabeling and where you have to run STP.



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