--- Jim Cunning
I disagree. An end host with a single interface, as in this situation, has no business trying to make load-balancing decisions that would be best made by the router owning the multiple interfaces with the information about the state of those interfaces as well as
knowledge about traffic on them from other end hosts. Moreover, router redundancy is handled by routers, not hosts, as it is very undesirable
hosts actually have to make decisions about what router to attempt to reach. VRRP, for example, makes it simple to hosts not to need to worry about what router to contact.
Agreed, but VRRP/HSRP was not subject here ...
are still some routers which don't support that, subject was whether it makes sense :) and to me it does if you don't have any other options. Not that I would ever do that but I've seen folks doing it
Tuesday, 3 June 2003 at 9:05pm, Martin wrote: [snip] the that there this
way for redundancy or just for heck of it they could.
This is getting intriguing.... Assuming it makes sense, what I'd like to understand then, on a system with a single ethernet interface and a route table with two default routes, how is the kernel going to know which of the two gateways to use? Suppose one of the gateway hosts has its far side interface down for some reason--how will the end host learn that gateway router isn't really usable?
I don't know about the kernel but I guess this should be up to routing daemon to make the inteligent decision which gateway/NIC to use. The only difference having 2 NICs and 2 default gateways pointing to each of them versus 1 NIC and 2 default gateways is NIC level redundancy but same routing daemon functionality must be there in both cases. This type of setup doesn't provide the best redundancy for the reason you've mentioned, you'd need vrrp/hsrp 'tracking' functionality or you have to run some routing protocol between the to gateways.
I suppose that the router will return an ICMP "Host not reachable" or "Network not reachable", but I don't see how that will help.
You also mentioned load-balancing earlier. I'm aware of load balancing a web site with multiple servers using DNS round robin or with NAT, and there are others, such as supersparrow, which uses BGP, IIRC. But how does one do load balancing from a single host with a single network interface and using only a static route table?
the genuine purpose of having multiple routes to the same destination prefix is redundancy and/or load balancing. Of course it's a matter of routing daemon implementation, I assume not all of them (Linux ones) can do per packet or per flow/session load balancing with static or dynamic routing. I believe, zebra should be able to do so. Otherwise, why would be this option available if it doesn't make sense at all?
I'm not trying to be stubborn about this--I truly would like to know, and if you've seen people do it, please describe it.
Imagine host with one NIC connected to switch where you also have two gateways with same RT and slow serial link. You choose it's up to your host routing daemon functionality/config to use one or both gateways simultaneously. This should work flawless with possible caveats but folks sometime do so because they want to have more control and didn't like the idea they have to relay only on network to do forwarding decisions. In this particular case the host traffic would always go through just one link if vrrp/hsrp or one gateway is used/set up. Optimal links utilization can be achieved with vrrp/hsrp as well but not from one host perspective. Martin
Thanks.
Jim
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