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=28198&t=28198 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
