OSPF Version 2 Open Shortest Path First RFC

OSPF Version 2 (Open Shortest Path First) (RFC 2328) Speaker: Sandra Date: 2012/03/20 1

Outline �Definition �Advantages �Disadvantages �Shortest-Path Tree �Routing patterns �Link State Advertisements (LSAs) �OSPF Connection �Hello packet message �Reference 2

Definition �OSPF (Open Shortest Path First ). It is designed to be run internal to a single Autonomous System. Each OSPF router maintains an identical database describing the Autonomous System's topology. From this database, a routing table is calculated by constructing a shortest- path tree. 3

Advantages � 1. Fast convergence. � 2. OSPF is a Link State Algorithm. � 3. OSPF supports Variable Length Subnet Masks (VLSM). � 4. OSPF uses multicasting within areas. � 5. After initialization, OSPF only sends updates on routing table sections which have changed, it does not send the entire routing table. 4

Disadvantages � 1. OSPF is very processor intensive due to implementation of SPF algorithm. OSPF maintains multiple copies of routing information, increasing the amount of memory needed. � 2. OSPF is a more complex protocol to implement compared to RIP. 5

Shortest-Path Tree R 1 R 2 R 3 R 4 R 5 R 6 R 2(1) R 1(2) R 2(5) R 2(4) R 4(2) R 3(2) R 4(5) R 5(2) R 5(3) R 3(2) R 5(3) R 6(2) R 4(3) R 5(4) R 6(3) R 1 2 1 R 2 5 R 4 2 R 6 R 3 4 R 5 2 R 5 3 R 6 6

Routing patterns Backbone Areas area 0 ABR(Area Border Router) ASBR (Autonomous System Boundary Router) area 1 area 2 AS(Autonomous System) 7

Link State Advertisements (LSAs)(1/2) �Router LSA: Router link advertisements generated by each router for each area it belongs to. �Network LSA：Network link advertisements generated by designated routers (DRs) giving the set of routers attached to a particular network. �Network Summary LSA ： Area Border Router (ABR) takes information it has learned on one of its attached areas and it can summarize it (but not by default) before sending it out on other areas it is connected to. 8

Link State Advertisements (LSAs)(2/2) � ASBR Summary LSA: This is solved by an Area Border Router flooding the information for the router (i. e. the Autonomous System Boundary Router) where the type 5 originated. �AS External LSA: Generated by the ASBR and provides links external to the Autonomous System (AS). �Group Membership LSA : Group membership link entry generated by multicast OSPF routers. �NSSA(Not-So-Stubby Area) External LSA: not receive external LSAs from Area Border Routers, but are allowed to send external routing information for redistribution. 9

OSPF Connection E D Hello B A C 10

Hello packet message �Router ID D �Hello/Dead lntervals �Neighbors �Area ID �Router priority �DR(Designated Router) ip address B �BDR (Backup Designated Router) ip address E Hello C A 11

Summary(1/2) �OSPF is an interior gateway protocol based on the shortest path first algorithm. �Each router in an OSPF network stores information about router interfaces and their relationship to neighboring routers in a link state database. �The SPF algorithm uses a measurement of the overhead required to send a packet over a particular interface to calculate the cost of a particular route. �The largest entity in an OSPF network is the autonomous system, which is further divided into areas; all OSPF routers within a single area have identical databases. 12

Summary(2/2) �OSPF routers can have all interfaces in one area, interfaces in multiple areas, or interfaces that connect to other routing protocols. �All traffic between areas travels through Area 0, the backbone. �Routers that share a common segment are neighbors; the Designated Router and Backup Designated Router form adjacencies with their neighbors for the purpose of synchronizing databases. 13

Reference �OSPF_Tutorial �RFC 2328 14