Ah, excellent stuff Tyson! Indeed Cristian, like Tyson was explaining remember when you redistribute from an IGP into BGP the weight attribute gets set to 32768 whereas the routes you learn from the other PE over BGP will have a weight of 0 and highest weight wins. So, in the case where you have an OSPF backdoor link (which is why we need the sham-link in the first place) if you advertised the sham-link end point into OSPF on PE1, it would go down to the CE, over the backdoor link to the other CE and finally to PE2 where it would be redistributed into BGP with a weight of 32768. Meanwhile, PE1 would also be sending the sham-link endpoint over the iBGP session between PE1 and PE2 and when PE2 received that prefix it would have a weight of 0. The higher weight would take precedence, and you would be back to the same problem : )
On Fri, Apr 30, 2010 at 12:49 AM, Tyson Scott <[email protected]> wrote: > If the SHAM link is directly advertised into OSPF it is always going to be > preferred over the BGP route as it would show up in the BGP table with the > preferred weight of 32768 for the IGP generated route and break your > sham-link. > > It is a safety precaution to prevent you from screwing up what you are > trying to fix ;). > > Here is an RFC that can go into greater detail if you are interested in more > specifics. > > http://tools.ietf.org/html/draft-rosen-vpns-ospf-bgp-mpls-06 > > Regards, > > Tyson Scott - CCIE #13513 R&S, Security, and SP > Technical Instructor - IPexpert, Inc. > Mailto: [email protected] > Telephone: +1.810.326.1444, ext. 208 > Live Assistance, Please visit: www.ipexpert.com/chat > eFax: +1.810.454.0130 > > IPexpert is a premier provider of Self-Study Workbooks, Video on Demand, > Audio Tools, Online Hardware Rental and Classroom Training for the Cisco > CCIE (R&S, Voice, Security & Service Provider) certification(s) with > training locations throughout the United States, Europe, South Asia and > Australia. Be sure to visit our online communities at > www.ipexpert.com/communities and our public website at www.ipexpert.com > > -----Original Message----- > From: [email protected] > [mailto:[email protected]] On Behalf Of Joe Astorino > Sent: Thursday, April 29, 2010 8:03 PM > To: Cristian Nedelcu > Cc: ccie_rs; ccie_sp > Subject: Re: [OSL | CCIE_RS] sham-link > > Because that is what the cisco documentation says : ) I know it is > not the greatest answer, but that is all we are told in the > documentation. It does not really specify WHY you are not supposed to > advertise the sham-link endpoint into OSPF, only that it MUST be > advertised into BGP and that it should not be advertised into OSPF. > > With that being said, if you lab this up you will find that when you > advertise the loopback into BGP inside the VRF it will get > redistributed into OSPF anyways...which according to the documentation > you don't want but it will work fine. You could also filter to make > sure that it does not go into OSPF during redistribution. > > > > On Thu, Apr 29, 2010 at 5:57 PM, Cristian Nedelcu > <[email protected]> wrote: >> why does OSPF end-points of sham-link must be advertised as BGP routes and >> not OSPF ? >> >> _______________________________________________ >> For more information regarding industry leading CCIE Lab training, please >> visit www.ipexpert.com >> >> > > > > -- > Regards, > > > > Joe Astorino - CCIE #24347 > Sr. Technical Instructor - IPexpert > Mailto: [email protected] > Telephone: +1.810.326.1444 > Live Assistance, Please visit: www.ipexpert.com/chat > eFax: +1.810.454.0130 > > IPexpert is a premier provider of Self-Study Workbooks, Video on > Demand, Audio Tools, Online Hardware Rental and Classroom Training for > the Cisco CCIE (R&S, Voice, Security & Service Provider) > certification(s) with training locations throughout the United States, > Europe, South Asia and Australia. Be sure to visit our online > communities at www.ipexpert.com/communities and our public website at > www.ipexpert.com > _______________________________________________ > For more information regarding industry leading CCIE Lab training, please > visit www.ipexpert.com > > -- Regards, Joe Astorino - CCIE #24347 Sr. Technical Instructor - IPexpert Mailto: [email protected] Telephone: +1.810.326.1444 Live Assistance, Please visit: www.ipexpert.com/chat eFax: +1.810.454.0130 IPexpert is a premier provider of Self-Study Workbooks, Video on Demand, Audio Tools, Online Hardware Rental and Classroom Training for the Cisco CCIE (R&S, Voice, Security & Service Provider) certification(s) with training locations throughout the United States, Europe, South Asia and Australia. Be sure to visit our online communities at www.ipexpert.com/communities and our public website at www.ipexpert.com _______________________________________________ For more information regarding industry leading CCIE Lab training, please visit www.ipexpert.com
