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"Router Mode" in RV0xx, WRT54G, etc.

Discussion in 'Cisco Small Business Routers and VPN Solutions' started by fred3, Nov 7, 2011.

  1. fred3

    fred3 Network Guru Member

    I'm trying to get good, clear definition of what "Router Mode" DOES. Not "How to use it in this, that or another situation?" just "What does it DO?"

    I have some beliefs but they only go part of the way:
    - The opposite setting "Gateway" mode means NAT.
    - "Router" mode means NO NAT.
    I'd rather expect that, other than that, there would be no difference.

    Some people suggest that turning on "Router" mode turns the device into a "Bridge" but I don't get what they mean. Surely there is a routing table involved in "Router" mode, eh?

    Is there any good, clear definition of what "Router" mode does? The documentation just says something like "use Gateway mode if this is connecting to the internet and use Router mode if there are other routers on the network". Wow. "Other routers on the network" could mean just about anything!!

    I have one setup chaining RV042s from LAN to LAN to LAN to a gateway and to the internet.
    There are actually 3 sites with a private link that go like this:
    Site 1 to Internet
    Site 1 to Interim LAN to Site 2
    Site 1 to Interim LAN to Site 3
    Site 2 to Interim LAN to Site 1
    Site 2 to Interim LAN to Site 3
    Site 3 to Interim LAN to Site 1
    Site 3 to Interim LAN to Site 2
    There is an RV042 at each end connecting the interim LAN to the local LANs at each end.
    The two RV042s are in Router mode and entail added routes.

    Yet, it seems that these routers are not fully "bi-directional".
    The internet access for Sites 2 and 3 go through Site 1. They both have their WAN side on the Interim LAN.
    BUT if the same orientation is used for the RV042 at Site1, with the WAN on the Interim LAN, the system doesn't work.
    The WAN on the RV042 at Site 1 has to be on the Site 1 LAN and the LAN on the RV042 at Site 1 has to be on the Interim LAN in order for this to work. So, this implies it's not a bi-directional device even in "Router" mode.

    I think I'm missing something pretty basic here and maybe the one thing is a "bug" but I'd really like some help understanding what the "Router" mode is supposed to do in terms of function(s).
     
  2. heuristik

    heuristik Networkin' Nut Member

    i, too, was perplexed about router mode for my rvs4000. i tried to deploy it as a makeshift "layer 3 core switch" in the middle of my home network for a bit, but quickly ran into the same issues you're seeing. The core of each of these linksys routers is essentially, a bridge, two eth# ports, and a VLAN separation that is maintained by iptables. Therefore, to attempt to implement multiple independent logical routing definitions beyond the traditional Inside / Outside role isn't very practical on these things.

    When you flip to "router" mode, the only thing the linksycs actually does is enable RIP, near as I can gather. The iptables are still in place, and you have to manually forward traffic between the different segments. You can't really overcome the hardware logic that wants to segregate "WAN" side from "LAN" side, but you can trunk and tag on the LAN ports to get a close approximation of your stated intent. After futzing with my collection of wifi dd-wrt hackups and the rvs4000, i ended up finally resorting to just putting an old cisco 2960 in the middle as my core switch. Kind of weird to see a 48 port switch completely empty except for its two gbic slots, but it works. Even funnier is the 2960 can do everything BUT NAT, and the linksys stuff can't do much else :)
     
  3. Sfor

    Sfor Network Guru Member

    Router is by all means a connection between networks. In the simplest form it connects two networks routing traffic between them in both directions. The "router mode" is just such a basic and simple device. In order for it to work, both networks have to have different IP adress spaces.

    The "gateway mode" adds a NAT to the "simple router" making the gateway mode. This addition hides the internal structure and IP adress space of the LAN side. So, the LAN side is not accessible from WAN. In other words the gateway mode is a sort of onesided router.

    The bridge mode is yet another system, sending all what comes from one side to the other. It does not care about address spaces and such. The routers we are speaking of are not able to do it. The core of the RV0xx devices is in fact a VLAN capable ethernet switch with the network processor addition. Yet the software will not let to go below the router level when it comes to the WAN and LAN ports.

    VLAN separation is not done through IPtables. It is done on the ethernet switch level, so other than IP protocols are separated, as well. The RV0xx are equipped with ethernet switch chips with built in hardware VLAN capabilities (avalable in RV082 an RV016, hidden in RV042). So, even if the router routes back packets to the LAN, the switch chip will not allow them to go to the other VLAN.

    When working with IP based networks one does not should consider the devices to be uni or bidirectional. All the IP capable devices just do a simple work. If the target adress is inside LAN adress space the packets are sent directly to it. If the target is outside, the traffic is sent to the gateway. The router is a set of two simple IP network devices. One of them belongs to one network space, the other one to some other network space. It can not work properly, when the two IP adress spaces are the same. When a packet arrives router checks to what adress space it belongs and sents it through proper device (LAN or WAN). If the packet is not sent to either adress space the packet is sent to the gateway (which could be placed on WAN or LAN side if the router software makes it possible).
     
  4. fred3

    fred3 Network Guru Member

    Good answer, thanks.
     

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