nanog mailing list archives
Re: two interfaces one subnet
From: Curtis Maurand <cmaurand () xyonet com>
Date: Tue, 12 May 2009 13:37:03 -0400
Try this: http://www.linuxfoundation.org/en/Net:Bridge --Curtis Patrick W. Gilmore wrote:
On May 11, 2009, at 5:40 PM, Ben Scott wrote:On Mon, May 11, 2009 at 5:28 PM, Hector Herrera <hectorherrera () gmail com> wrote:On Mon, May 11, 2009 at 2:22 PM, David Devereaux-Weber <ddevereauxweber () gmail com> wrote:... both> interfaces are on the same subnet, the OS sees the same router (gateway) address on both interfaces, and the results are sub-optimal ... around 50%packet loss.packet loss is probably due to the network switch having to re-learn the location of the MAC address constantly as it sees packets on two or more ports with the same MAC address (think STP loops).My understand of the scenario is: Two physical interfaces, each with a unique IP address, in the same Ethernet broadcast domain, on the same IP (sub)network. If that's the case, the MAC address won't change. The cards stay put. So a layer two switch will be none the wiser. The reason this doesn't work (for most implementations) is that most IP routers look only at the destination IP address, and keep no state. (Here, I'm using "router" to include the routing engine built-in to any full IP implementation, not just dedicated equipment from Cisco, et. al.) So we have a host with IP addresses A and B on the same subnet. A packet comes in from some other host X. The application software does whatever it does, and sends a response. The router looks at the destination IP address X, and sees that it has two routes, A and B. Depending on implementation, the router may send everything out the first interface it finds in the routing table (e.g., use A and ignore B), or round-robin between the two, or who-knows-what. Either way, if the packet *from* X was addressed *to* B but the response comes back from *A*, then host X is going to drop the packet as invalid/irrelevant/etc.You are assuming facts not in evidence. It doesn't matter which physical interface transmits the packet. For instance, if I ping a router's loopback interface, there is nothing stopping the router from making the loopback the source IP address of the return packet even though the (virtual) loopback interface _obviously_ did not physically transmit the packet.Another example: Imagine a web server with two uplinks in _different_ subnets running Quagga. Now assume the web server gets an HTTP request and the route back to the requesting host changes before all the packets are returned. Does the download break? Sure, if you use an implementation too broken for words. If not, things work just fine.Could everyone please stop coming up with "if people are stupid and break things, things don't work" examples. We all agree on that.Back in reality land, things that broken tend not to be used. (And please no jokes about cisco or microsoft or whatever.)
Current thread:
- Re: two interfaces one subnet, (continued)
- Re: two interfaces one subnet Patrick W. Gilmore (May 11)
- Re: two interfaces one subnet Ben Scott (May 11)
- Re: two interfaces one subnet James Hess (May 11)
- Re: two interfaces one subnet David Coulson (May 11)
- Re: two interfaces one subnet Chris Adams (May 11)
- Re: two interfaces one subnet Patrick W. Gilmore (May 11)
- Re: two interfaces one subnet Ben Scott (May 11)
- Re: two interfaces one subnet Patrick W. Gilmore (May 11)
- Re: two interfaces one subnet Martin Hannigan (May 12)
- Two interfaces one subnet (summary) Ivan Pepelnjak (May 14)
- Re: two interfaces one subnet Curtis Maurand (May 12)
- Re: two interfaces one subnet Chris Meidinger (May 12)
- Re: two interfaces one subnet David Devereaux-Weber (May 11)
- Re: two interfaces one subnet Chris Meidinger (May 11)
- Re: two interfaces one subnet Ben Scott (May 11)
- Re: two interfaces one subnet Chris Meidinger (May 11)
- Re: two interfaces one subnet Patrick W. Gilmore (May 11)
- Re: two interfaces one subnet Scott Howard (May 12)
- Re: two interfaces one subnet Patrick McManus (May 13)