计算机网络第8章：_路由协议：RIP,OSPF.ppt

* * * * * * * * * * * * * * * * * * * OSPF has to have at least one Area (Area 0 ). This area is the bacbone were all the other areas must be connected to. ABR: Area border Routers are routers that have interfaces in more than one area, generally between Area 0 and other areas. * * * * * DR : Designated Routers Reduce OSPF traffic on Multiaccess Links Store and Distribute neighbors LSDBs BDR exist as a backup for redundancy. DR election: OSPF Priority is used in the selection : 1 – 255, default = 1, 0 for noncandidate If priorities are the same, higher Loopback IP address. In case of no existing Loopbacks highest IP address of interfaces are used. OSPF Order of events: Establish router adjacencies Elect a Designated Router Discover Routes Select appropriate routes Maintain routing info Relies on IP packets for delivery of routing information (uses protocol number 89) * * * * * * * * * * * * * * * * * * * * OSPF Cost = Metric Cost is applied on all router link paths 16 bits number (1 – 65,535) Lower metric = more desirable Route decision is made on total cost of the path. Metric is derived from Bandwidth 100,000,000 Bandwidth Formula COST = OSPF Cost for T1 = 100,000,000 / 1,544,000 = 64 OSPF Path Cost All interfaces connected to the same link must agree on the cost of that link. Otherwise, the link will be considered down A Cisco router’s default cost for a serial link is 1784 ( 56Kbps bandwidth ) If the line is a slower speed, you must specify the real link speed Router (config-if) # ip ospf cost number Router (config-if) # bandwidth number 1 ~ 65535 Kbps Modifying OSPF Cost Metric Example A common situation requiring a cost change is in a multi-vendor routing environment. A cost change would ensure that one vendor’s cost value would match another vendor’s cost value. Another situation is when Gigabit Ethernet is being used. The default cost assigns the lowest cost value of 1 to a 100 Mbps link. Configuring OSPF Timers All routers in an OSPF area