All, I received some feedback from a couple of individuals. Thanks. I grabbed John T. Moy's OSPF book and found in Chapter 6 a very good/clear explanation of the use of Distance Vector for inter-area routing.
Back to reading, Chris > -----Original Message----- > From: Kane, Christopher A. > Sent: Wednesday, December 05, 2001 11:00 AM > To: '[EMAIL PROTECTED]' > Subject: OSPF inter-area routes calc (doyle and rfc 2328) > > All, > > Can someone help shed some light on OSPF's inter-area route calculation? > > I have been reading Doyle's TCP/IP Volume 1. It's a great book. But I've > stumbled across something that confuses me and I'm hoping someone can > help. This email is kind of long, but that's because I've included snips > from Doyle's book as well as RFC 2328. I thought I had a good grasp on > OSPF. I understand the 2 level concept of the Areas, router types, network > types, DR/BDR, etc.., but then Doyle threw me for a loop. He explains OSPF > in Chapter 9. Chapter 10 is about IS-IS. As Doyle begins explaining IS-IS > he makes the following statement: > "Recall from Chapter 9, that OSPF runs its SPF algorithm to compute routes > within an area, but that inter-area routes are computed using a distance > vector algorithm." I do not understand this statement. I've reviewed > Chapter 9 again and can't find where he explains that inter-area routes > are computed using a distance-vector algorithm. Below are the snips from > the RFC. > > RFC 2328 Section 3.2 "Inter-area routing". Here, John Moy and group > explain: > "When routing a packet between two non-backbone areas the backbone is > used. The path the packet will travel can be broken up into three > contiguous pieces: an intra-area path from source to an area border > router, a backbone path between the source and destination areas, and then > another intra-area path to the destination. The algorithm finds the set of > such paths that have the smallest cost. Looking at this another way, > inter-area routing can be pictured as forcing a star configuration on the > Autonomous System, with the backbone as hub and each of the non-backbone > areas as spokes." > > Further, Section 4.1 "Inter-area routing" explains: > "For inter-area routing, no other routing information is pertinent. In > order to be able to route to destinations outside of the area, the area > border routers inject additional routing information into the area. This > additional information is a distillation of the rest of the Autonomous > System's topology. This distillation is accomplished as follows: Each area > border router is by definition connected to the backbone. Each area border > router summarizes the topology of its attached non-backbone areas for > transmission on the backbone, and hence to all other area border routers. > An area border router then has complete topological information concerning > the backbone, and the area summaries from each of the other area border > routers. From this information, the router calculates paths to all > inter-area destinations. The router then advertises these paths into it's > attached areas. This enables the area's internal routers to pick the best > exit router when forwarding traffic to inter-area destinations". > > And finally, Section 16.2 "Calculating the inter-area routes" > The inter-area routes are calculated by examining summary-LSAs. If the > router has active attachments to multiple areas, only backbone > summary-LSAs are examined. Routers attached to a single area examine that > area's summary-LSAs. In either case, the summary-LSAs examined below are > all part of a single area's link state database (call it Area > A).Summary-LSAs are originated by the area border routers. Each > summary-LSA in Area A is considered in turn. Remember that the destination > described by a summary-LSA is either a network (Type 3 summary-LSAs) or an > AS boundary router (Type 4 summary-LSAs). For each summary-LSA: > (1) If the cost specified by the LSA is LSInfinity, or if the LSA's LS age > is equal to MaxAge, then examine the the next LSA. > (2) If the LSA was originated by the calculating router itself, examine > the next LSA. > (3) If it is a Type 3 summary-LSA, and the collection of destinations > described by the summary-LSA equals one of the router's configured area > address ranges (see Section 3.5), and the particular area address range is > active, then the summary-LSA should be ignored. "Active" means that there > are one or more reachable (by intra-area paths) networks contained in the > area range. > (4) Else, call the destination described by the LSA N (for Type 3 > summary-LSAs, N's address is obtained by masking the LSA's Link State ID > with the network/subnet mask contained in the body of the LSA), and the > area border originating the LSA BR. Look up the routing table entry for BR > having Area A as its associated area. If no such entry exists for router > BR (i.e., BR is unreachable in Area A), do nothing with this LSA and > consider the next in the list. Else, this LSA describes an inter-area path > to destination N, whose cost is the distance to BR plus the cost specified > in the LSA. Call the cost of this inter-area path IAC. > (5) Next, look up the routing table entry for the destination N. (If N is > an AS boundary router, look up the "router" routing table entry associated > with Area A). If no entry exists for N or if the entry's path type is > "type 1 external" or "type 2 external", then install the inter-area path > to N, with associated area Area A, cost IAC, next hop equal to the list of > next hops to router BR, and Advertising router equal to BR. > (6) Else, if the paths present in the table are intra-area paths, do > nothing with the LSA (intra-area paths are always preferred). > (7) Else, the paths present in the routing table are also inter-area > paths. Install the new path through BR if it is cheaper, overriding the > paths in the routing table. Otherwise, if the new path is the same cost, > add it to the list of paths that appear in the routing table entry. Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=28302&t=28302 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
