Date: Fri, 28 May 1999 13:15:05 +0200 From: ANDREAS.KLEMM.AK@bayer-ag.de To: " - *amb@gxn.net" <amb@gxn.net> Cc: " - *andreas@klemm.gtn.com" <andreas@klemm.gtn.com>, " - *freebsd-net@freebsd.org" <freebsd-net@freebsd.org>, " - *zebra@zebra.org" <zebra@zebra.org>, " - *kunihiro@zebra.org" <kunihiro@zebra.org> Subject: Antwort: Re: [zebra 553] OSPF eequal-cost paths, which algor Message-ID: <0006800011799081000002L012*@MHS>
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Hi Alex, first of all: sorry, if you get some dirty characters, Lotus Notes cannot be configured, to be pretty 7Bit ASCII compliant and readable :-/ Thanks for your nice answer. Could you perhaps point out, what actually happens. Do I have 2 x 2 x 2 Mbit = 8 Mbit throughput for IP traffic ? Or does the router decide to choose one route over one router pair ? If the latter is the case, what criterium the router chooses, which path to use ? The roundrobin functionaliy you mention I know from having _one_ router and two interfaces then turning on the fast cache, so doing processor switching. But here the question is, _how_ do the packets or sessions flow when having to router _pairs_ ?! Does OSPF really do something like roundrobin packet for packet (1st packet uses route over R1-R2, 2nd packet uses the other equal-cost path). Another question is, when routing IPX, Appletalk and also turning on bridging, it might happen, that one path is more loaded then the other (spanning tree). Then the paths have the same OSPF cost, but are differently loaded, what happens then ? Is one path preferred and after what algorithm ? Thanks Andreas /// amb@gxn.net on 28.05.99 11:00:07 An: kunihiro@zebra.org @ INTERNET, zebra@zebra.org @ INTERNET, freebsd-net@freebsd.org @ INTERNET, Andreas Klemm@BAYERNOTES, andreas@klemm.gtn.com @ INTERNET Kopie: Thema: Re: [zebra 553] OSPF eequal-cost paths, which algorithm, how 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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