Date: Fri, 28 May 1999 08:56:32 +0200 From: Alex Bligh <amb@gxn.net> To: andreas@klemm.gtn.com, andreas.klemm.ak@bayer-ag.de, freebsd-net@freebsd.org, zebra@zebra.org, Kunihiro Ishiguro <kunihiro@zebra.org> Subject: Re: [zebra 553] OSPF eequal-cost paths, which algorithm, how exactly load balancing ? Message-ID: <E10nHVI-0005YN-00@sapphire.noc.gxn.net> In-Reply-To: Your message of "Fri, 28 May 1999 08:40:02 %2B0200." <19990528084002.A41138@titan.klemm.gtn.com>
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Andreas, > I'm looking for the algorithm how OSPF does a routing decision and > what kind of load balancing is been done, between the two 4 MBit > leased lines, when OSPF has equal-cost paths like this. > Cisco IOS 11.2 or 11.3. Roughly: When Cisco has (by any meothod) two equal cost routes installed in the RIB (i.e. routing table), both get installed in the FIB (i.e. forwarding table). When a packet comes to be forwarded, generally some form of route cache is used, which is normally just a hash of the destination IP being routed to (+/- hardware accellerated routing on higher end boxes, CEF etc. etc.). However, occasionally (normally the first packet *to* a given host) this caching algorithm will miss, and a lookup will be made in the FIB. Where there is more than one entry, these algorithm round-robbins between each of them (well actually I think it may chose randomly between them, which is the same thing in practice). So if you enter "no ip route-cache" on the interface, you will find packets (whereever they are to) round robin between the equal cost routes, thus load sharing. But when you have route-caching switched on (normally), you will find all the traffic to a given destination goes the same way, but providing you have traffic to a large number of destinations, in practice you get good load sharing (don't try putting 2 newsfeeds down 2 2Mb lines this way though). The situation is more complex when you have protocols like EIGRP which will on later IOS versions do *non* equal load sharing. I believe what they do then is tag the relevant load sharing weight onto the RIB entry which propagates to the FIB and modifies the round robin algorithm. However I have not tested this in practice. -- Alex Bligh GX Networks (formerly Xara Networks) To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-net" in the body of the message
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