I am studying about type2 Lsa in OSPF and many places have pseudonode mentioned. I am having hard time understanding about the pseudonode concept. Could anyone explain about this? Also, second question, how does type2 Lsa help build the spf tree in OSPF?
Type 2: network LSA -- For broadcast networks only; this LSA is flooded by the DR and lists OSPF-speaking routers on the network.
spf tree in OSPF?
When the Link State database is synchronized within an area, the Dijkstra algorithm is run against it in two passes to form the shortest-path (SPF) tree. The first pass against the SPF database forms the branches, or router adjacencies within the area. The second pass adds all leaves or stub networks to the tree. When OSPF builds the tree, it determines the shortest path to each destination based on the sum cost to the destination. The lower the cost is, the more preferred the route is. The cost of a route is the sum of all costs of outgoing interfaces to that destination. Oddly enough, RFC 2328 offers no specific values for cost. Nortel Networks, for example, implements OSPF under RFC 2328 and uses the same formula to generate cost as Cisco Systems. In multivendor environments, take the extra time to see how cost is calculated because it will help OSPF have a consistent view of the entire internetwork.
Cisco routers calculate OSPF cost as (108/BW) rounded down, where BW is the configured or default bandwidth of the interface.
Cost will be changed based on the interface and speed.
I am studying about type2 Lsa in OSPF and many places have pseudonode mentioned. I am having hard time understanding about the pseudonode concept. Could anyone explain about this?
pseudonode = think of it as a virtual router that the ospf physical routers interfaces in a broadcast/mulit-access network connect to.
second question, how does type2 Lsa help build the spf tree in OSPF?
LSA type 2 ( network) are created from designated routers (DR) on multiaccess-links and lists all its routers connected to it (pseudonode ). The LSA is only advertised (flooded) in the area it was created. This virtual router has no ospf cost associated with it and as such isn't included in any SPF calculation, Only the cost of the actual broadcast link of the routers interface is calculated meaning that two ospf neighbors on a broadcast/mulit-access link have their respective ospf cost calculated based on their ospf interface, so this virtual router (pseudonode) takes no part in any SPF process.
Please rate and mark as an accepted solution if you have found any of the information provided useful. This then could assist others on these forums to find a valuable answer and broadens the community’s global network.