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
_______________________________________________ For more information regarding industry leading CCIE Lab training, please visit www.ipexpert.com
