From: ccie2004@excite.com
Date: Tue Jul 19 2005 - 08:57:35 GMT-3
Hi All, Thx for your input on this. Godswill, I think as Brian has pointed out the synch issue is basically to prevent blackholing of traffic. The underlying concept is that if the IGP is not fully converged than do not advertise the route to your EBGP peer. Please feel free to correct me if I am incorrect on this. All, I was trying to get a real world answer on how in Transit AS's today this issue is dealt with (even with sync disabled we still have to figure out a way to prevent blackholing. I don't believe there is a way around that. Is There?). We see that with MPLS VPN's etc it eliminates the need for your core routers to run IBGP. But in traditional networks is this limitation still there. Does anyone(ISP's) actually redistribute BGP into their IGP. I just can't seem to grasp how is the situation dealt with because every solution I can think of has some problems or the other. Can someone provide an example of this being dealt with in real world networks.--- On Tue
07/19, Brian McGahan < bmcgahan@internetworkexpert.com > wrote:From: Brian McGahan [mailto: bmcgahan@internetworkexpert.com]To: oletu@inbox.lv, ccie2004@excite.com, ccielab@groupstudy.comDate: Tue, 19 Jul 2005 02:17:25 -0400Subject: RE: BGP synchronizationGodswill,Yes, the problem synchronization is designed to prevent is thedropping of traffic by non-BGP speaking routers in the transit path. Inorder to prevent this the logic of synchronization is that if the routeis installed in the IGP table all routers in the transit path (whichshould logically be running IGP) will have a route to the finaldestination. The solution is as you said, to suppress bestpathselection (and hence advertisement) if an IGP match is not found for theiBGP learned prefix.HTH,Brian McGahan, CCIE #8593bmcgahan@internetworkexpert.com Internetwork Expert, Inc.http://www.InternetworkExpert.comToll Free: 877-224-8987 x 705Outside US: 775-826-4344 x 70524/7 Support:
http://forum.internetworkexpert.comLive Chat: http://www.internetworkexpert.com/chat/>; -----Original Message-----> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On BehalfOf> Godswill Oletu> Sent: Monday, July 18, 2005 2:13 PM> To: ccie2004@excite.com; ccielab@groupstudy.com> Subject: Re: BGP synchronization> > I am trying to follow your logic, but I am finding it difficult to> reconcile> this line...> > ".....Synch is supposed to address the issue of a router in the middleof> your network which is not running IBGP and hence does not know how toget> to> a particular network that your IBGP routers are aware of....."> > Is that the issue that Synchronization really addressed?> > I think with BGP Synchronization, when an IBGP peer gets an update fora> route from another IBGP peer, it looks into its local IGP routingtable,> if> that route is present, he is free to advertise it to other BGP Peers,if>
otherwise, he will refrain from advertising it. In this scenerio, all> internal routers do not have to necessary run IBGP.> > To fulfill the requirements of BGP Synchronization, your IGP should be> aware> of all the routes in your network, so that when these routes become> available to BGP, it can accept them and send them out to external BGP> neighbors.> > eg> > interface fastethernet0/0> ip address 120.1.1.1 255.255.255.0> !> interface fastethernet0/1> ip address 10.1.1.1 255.255.255.0> !> ip route 50.0.0.0 255.0.0.0 120.1.1.1> !> router bgp 120> network 120.1.1.0> network 10.1.1.0> network 50.0.0.0> network 60.0.0.0> network 70.0.0.0> !> end> > You see that, BGP will not advertise networks 60.0.0.0 & 70.0.0.0,because> according to the limited configuration above, there is no reference to> them> in the router's routing table, but if you disable syncronization, BGPwill> accep!
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these networks and advertise them, despite the fact that noneof> your> routers know how to reach those networks.> > You can see that, if your BGP router advertise networks 60.0.0.0 &> 70.0.0.0,> there will be problem if traffic is sent to you destined for those> networks,> I believe this is the very problem BGP synchronization is setup to> prevent.> > Others can contribute, so that we can all learn, if this is not whatyour> question is about, you can throw in more light.> > my 0.2> > Note:> There are many other methods of making routes available to your BGP> router,> the network statement used above is just one of them.> > ----> Godswill Oletu> > > ----- Original Message -----> From: > To: > Sent: Monday, July 18, 2005 2:01 PM> Subject: BGP synchronization> > > > Hi All, I just came up with a question on BGP which I have beenasking> > myself but just can't seem to get
a handle on. It has to do with BGP> > Synchronization. I know this has been beaten to death and newer IOS> > versions have it disabled however my question is a combination ofthe> > underlying issue that Synch is supposed to address and BestPractises.> > Synch is supposed to address the issue of a router in the middle ofyour> > network which is not running IBGP and hence does not know how to getto> a> > particular network that your IBGP routers are aware of. BGP Best> practises> > say that never redistribute your EBGP learnt routes into yourinterior> > routing protocol. Thinking along those lines and if I am right how> exactly> > would you get reachability across your network. Would you usedefault> > routes on your non-BGP speaking routers or are there any otherdesign> type> > fixes that I am missing. thx> >> > _______________________________________________> > Join Excite! -
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