The best solution is to use /32 loopback addresses.When a CCIE Lab task prohibits you from doing that, a 'ip ospf network point-to-point' solves that as you receive a label for the entire /24 which works fine, but as long as you have the choice, use /32 addresses on the loopback itself.
-- Regards, Rick Mur CCIE2 #21946 (R&S / Service Provider) Sr. Support Engineer – IPexpert, Inc. URL: http://www.IPexpert.com On Fri, Sep 25, 2009 at 12:10 PM, Dande Rajasekhar (drajasek) < [email protected]> wrote: > Well said Rick.Thanks a lot J > > > > <Also with the default behavior of OSPF, when you use /24 networks to > configure a loopback, you run into problems because OSPF advertises them as > /32 and then LDP on the PE router <advertises a label for the /24 prefix, > but the upstream router expects a label for a /32 prefix, so it get's > discarded and the LSP is broken. > > > > Then What is the best solution for this ? RFC 5328 ? or ip ospf p-p under > loopback ? > > > > > > > > > > *From:* Rick Mur [mailto:[email protected]] > *Sent:* Friday, September 25, 2009 3:34 PM > *To:* Dande Rajasekhar (drajasek) > *Cc:* [email protected] > *Subject:* Re: [OSL | CCIE_RS] Inter-area LSP > > > > Hi Dande, > > > > Indeed, as long as you don't use any summarization you still receive the > /32 loopback address through OSPF areas, however the reason to separate an > OSPF domain is to get control over your routes and being able to summarize > in the perfect spots. Therefore you barely see GOOD Service Provider > networks having a separation in areas. You almost always have one big OSPF > area 0 or IS-IS level-2 network, which is also very good for implementing > MPLS Traffic Engineering. > > But to answer your question, indeed you still receive /32 networks when you > don't implement any kind of summarization. > > > > About your second question, it's not 100% required to have /32 loopback > networks advertised to have an LSP, still it's a very strongly recommended > best practice. When you are using MPLS VPNs, the MPLS forwarding of VPN > routes is done by looking up the label for the BGP next-hop, this is done on > the IP address, so you will have the best control and are certain that the > traffic is perfectly label switched when this label is based on a /32 prefix > instead of an entire /24. When you don't use peerings on loopback addresses > you run into problems with Penultimate Hop Popping as the label stack is > removed 1 hop too early. You really need to receive labeled packets on a PE > router to make a decision in which routing table the traffic should be > looked up and forwarded. > > > > Also with the default behavior of OSPF, when you use /24 networks to > configure a loopback, you run into problems because OSPF advertises them as > /32 and then LDP on the PE router advertises a label for the /24 prefix, but > the upstream router expects a label for a /32 prefix, so it get's discarded > and the LSP is broken. > > > > I hope I answered your question with this :-) > > > > > -- > > Regards, > > Rick Mur > CCIE2 #21946 (R&S / Service Provider) > Sr. Support Engineer – IPexpert, Inc. > URL: http://www.IPexpert.com > > On Fri, Sep 25, 2009 at 10:35 AM, Dande Rajasekhar (drajasek) < > [email protected]> wrote: > > Guys, > > > > What does that mean by redistributing PE loopbacks into all > areas.Doesnt loopback/32 flow across ospf areas if we don’t use area range > at ABR.Also why do we need /32 loopback address for setting up LSP? > > > > > > From RFC 5328: > > > > IGP route leaking consists of redistributing all specific PE loopback > > addresses across area boundaries. As a result, LDP finds in the RIB > > an exact match for its FEC and sets up the LSP. As a consequence, > > the potential benefits that a multi-area domain may yield are > > significantly diminished since a lot of addresses have to be > > redistributed by ABRs, and the number of IP entries in the IGP Link > > State Database (LSDB), RIB, and Forwarding Information Base (FIB) > > maintained by every LSR of the domain (whatever the area/level it > > belongs to) cannot be minimized. > > > > > > Thanks & Regards, > > * Rajasekhar Dande* > * Software Engineer* > * ERBU 7600 India** > * > [email protected] > Phone: +91 80 4426 1143 > > Cisco home page <http://www.cisco.com/> > > *Error! Filename not specified.* > > * Cisco Systems, Inc.* > > > > > > > _______________________________________________ > For more information regarding industry leading CCIE Lab training, please > visit www.ipexpert.com > > >
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