On Tue, Oct 27, 2009 at 07:21:54PM +0100, Martin Kraus wrote:
Hello. I'm running ospf from a router against two other routers on the same ethernet segment. Those two routers a both capable of routing given prefixes, but I prefer one of them to do the actual routing and use the other one as a backup router in case of emergencies. How can I set such a preference? I thought of using ospf_metric2, but it is supposed to work only when ospf_metric1 isn't set, which is (checked using show route all). For some reason the configuration does what I need, but I can't figure out why. Could anyone shed some light on this?
I suppose that both routers propagate some external route and you want to set metric for that external route. For this purpose, you can use Type 1 metric (ospf_metric1) or Type 2 metric (ospf_metric2), both will work. Just set smaller value to more preferred router. If router receives both routes and both use Type 2 metric, then it compares just these metrics and uses the smaller one. If router receives both routes and both use Type 1 metric, then it compares these metrics icreased by IGP metric between both routers, which is the same value for both routes in your case. RFC 2328: Type 1 external metrics are expressed in the same units as OSPF interface cost (i.e., in terms of the link state metric). Type 2 external metrics are an order of magnitude larger; any Type 2 metric is considered greater than the cost of any path internal to the AS. -- Elen sila lumenn' omentielvo Ondrej 'SanTiago' Zajicek (email: santiago@crfreenet.org) OpenPGP encrypted e-mails preferred (KeyID 0x11DEADC3, wwwkeys.pgp.net) "To err is human -- to blame it on a computer is even more so."