Cisco CCNA Study Guide - Router Alley

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CCNA Study Guide v2.62 – Aaron Balchunas 220 OSPF Designated Routers In multi-access networks such as Ethernet, there is the possibility of many neighbor relationships on the same physical segment. In the above example, four routers are connected into the same multi-access segment. Using the following formula (where “n” is the number of routers): n(n-1)/2 …..it is apparent that 6 separate adjacencies are needed for a fully meshed network. Increase the number of routers to five, and 10 separate adjacencies would be required. This leads to a considerable amount of unnecessary Link State Advertisement (LSA) traffic. If a link off of Router A were to fail, it would flood this information to all neighbors. Each neighbor, in turn, would then flood that same information to all other neighbors. This is a waste of bandwidth and processor load. To prevent this, OSPF will elect a Designated Router (DR) for each multiaccess networks, accessed via multicast address 224.0.0.6. For redundancy purposes, a Backup Designated Router (BDR) is also elected. OSPF routers will form adjacencies with the DR and BDR. If a change occurs to a link, the update is forwarded only to the DR, which then forwards it to all other routers. This greatly reduces the flooding of LSAs. DR and BDR elections are determined by a router’s OSPF priority, which is configured on a per-interface basis (a router can have interfaces in multiple multi-access networks). The router with the highest priority becomes the DR; second highest becomes the BDR. If there is a tie in priority, whichever router has the highest Router ID will become the DR. To change the priority on an interface: Router(config-if)# ip ospf priority 125 Default priority on Cisco routers is 1. A priority of 0 will prevent the router from being elected DR or BDR. Note: The DR election process is not preemptive. Thus, if a router with a higher priority is added to the network, it will not automatically supplant an existing DR. Thus, a router that should never become the DR should always have its priority set to 0. * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
CCNA Study Guide v2.62 – Aaron Balchunas 221 OSPF Neighbor States Neighbor adjacencies will progress through several states, including: Down – indicates that no Hellos have been heard from the neighboring router. Init – indicates a Hello packet has been heard from the neighbor, but twoway communication has not yet been initialized. 2-Way – indicates that bidirectional communication has been established. Recall that Hello packets contain a neighbor field. Thus, communication is considered 2-Way once a router sees its own Router ID in its neighbor’s Hello Packet. Designated and Backup Designated Routers are elected at this stage. ExStart – indicates that the routers are preparing to share link state information. Master/slave relationships are formed between routers to determine who will begin the exchange. Exchange – indicates that the routers are exchanging Database Descriptors (DBDs). DBDs contain a description of the router’s Topology Database. A router will examine a neighbor’s DBD to determine if it has information to share. Loading – indicates the routers are finally exchanging Link State Advertisements, containing information about all links connected to each router. Essentially, routers are sharing their topology tables with each other. Full – indicates that the routers are fully synchronized. The topology table of all routers in the area should now be identical. Depending on the “role” of the neighbor, the state may appear as: • Full/DR – indicating that the neighbor is a Designated Router (DR) • Full/BDR – indicating that the neighbor is a Backup Designated Router (BDR) • Full/DROther – indicating that the neighbor is neither the DR or BDR On a multi-access network, OSPF routers will only form Full adjacencies with DRs and BDRs. Non-DRs and non-BDRs will still form adjacencies, but will remain in a 2-Way State. This is normal OSPF behavior. * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
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