Cisco® CCNP Routing Exam Notes : Types Of Ospf Routers, Ospf Lsa Types, And Ospf Commands

(Exam: 642-801)

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21. OSPF:

1. An OSPF area is a collection of networks and routers that has the same area identification.

2. The following are the types of OSPF routers:

  • Internal router: An internal router has all the interfaces in the same area. All internal routers maintain same link state databases.
  • Backbone router: Backbone routers reside on the perimeter of Area 0, with at least one interface connected to backbone (Area 0)
  • Area Border Router (ABR): ABRs are routers that have interfaces attached to multiple areas. It may be noted that these routers maintain separate link-state databases for each area that they are connected. They are capable of routing traffic destined for or arriving from other areas.
  • Autonomous System Boundary Router (ASBR): This router has at least one interface to the external network (another autonomous system). This autonomous network can be non-OSPF. ASBRs are capable of route redistribution. Redistribution is the ability of a router to import routing information from non-OSPF networks, and distribute the same in OSPF network for which it is responsible and visa versa.

3. LSA Types:

  • LSA Type 1: Router link entry, generated by all routers for each area to which it belongs. These are flooded within a particular area.
  • LSA Type 2: Network link entry, generated by designated router (DRs). Type 2 LSAs are advertised only to routers that are in the area containing the specific network.
  • LSA Type 3 and Type 4: Summary link entry, these LSAs are generated by area border routers (ABRs). These are sent to all routers within an area. These entries describe the links between the ABR and the internal routers of an area. These entries are flooded throughout the backbone area and to the other ABRs.
  • LSA Type 5: Autonomous System External Link Entry, these are originated by ASBR. These entries describe routes to destinations external to the autonomous system. These LSAs are flooded throughout the OSPF autonomous system except for stubby and totally stubby areas.

4. The sequence of steps followed in OSPF operation are as below:

  • Establish router adjacencies
  • Elect DR and BDR
  • Discover Routes
  • Choose appropriate routes for use
  • Maintain routing information.

5.The command "show ip ospf database" displays the contents of the topological database maintained by the router. This command also displays router id and the ospf process id. ip ospf interface can be used to check whether the interfaces have been configured properly. The command also gives the timer intervals, including hello intervals, and neighbor adjacencies.

7.OSPF keeps up to six equal-cost route entries in the routing table for load balancing.

8.OSPF uses Dijkstra algorithm to calculate lowest cost route. The algorithm adds up the total costs between the local router and the each destination network. The lowest cost route is the preferred route when there are multiple paths to a given destination.

9. OSPF has the following advantages over Distance Vector protocols such as RIP:

  • Faster convergence: OSPF network converges faster because routing changes are flooded immediately and computed in parallel.
  • Support for VLSM: OSPF supports VLSM. However, please note that RIP version2 also supports VLSM.
  • Network Reachability: RIP networks are limited to 15 hops. On the other hand, OSPF has practically no reachability limitation.
  • Metric: RIP uses only hop count for making routing decisions. This may lead to poor efficiency in some cases. For example, that a route is nearer but is very slow compared to another route with plenty of bandwidth available but few more hops away. OSPF uses "cost" metric to choose best path. Cisco uses "bandwidth" as metric to choose best route.
  • Efficiency: RIP uses routing updates every 30 seconds. OSPF multicasts link-state updates and sends the updates only when there is a change in the network status

10. The path cost in OSPF network is calculated using bandwidth. The formula used is [10 <8> divided by Bandwidth]. For example, the cost of a 56kbps serial link is 1785. The default cost of a 10mbps Ethernet is 10.

22. When a serial line is configured on a Cisco router, the default bandwidth is 1.544Mbps. If the line is slower speed, "bandwidth" command can be used to specify the real link speed. The cost of the link will then automatically correspond to the changed value.

23. You must manually configure a static route to configure DDR (Dial on Demand Routing). DDR is widely used as a backup route, in case of failure of primary link.

24. Route Summarization:

Route summarization is calculated as below:

Step 1:

1. Take the first IP: 172.24. 0 0 1 1 0 1 1 0.0
2. Take the second IP: 172.24. 0 0 1 1 0 1 0 1.0
Note that we are not really concerned about the octets that have equal decimal values. This is because they don�t come into play while calculating summarization route, in this case.

Step 2:

Count the number of bits in the third octet that are aligned (or lined up) with same values. In this case 6 bits are lined up in the third octet. The summarization route is calculated by adding this number (6) to the octets preceding the third (first and second octets).
Therefore, the number of bits in the summarized route is 8+8+6 = 22

Step 3:

Calculate the decimal equivalent for third octet with 6 bits as given in the matching binary. That is 0 0 1 1 0 1 x x. Note x is because it corresponds to non matching binary number. It is equal to 128*0 + 64*0 + 32*1 + 16*1 + 8*0 + 4*1 or 32+16+4 or 52.
Therefore, the summarized route is:

25. While evolving a network addressing scheme for an organization, you need to assign a different network number for each subnet. Also, you need to set aside one network number for each WAN connection.

26. Representing a subnet mask with / notation:

Consider an IP subnet mask of The same be represented as /25. This is arrived at, by taking the binary equivalent of (= 11111111.11111111.11111111.10000000). Count the number of ones', there are 25 of them. Therefore, the same can be written as /25.

27. The following are link state routing protocols:


28. OSPF - LSA, LSR, and LSUs:

1. LSA (Link State Advertisement): LSAs are included in the database description packets (DDPs or DBDs). LSA entries include link-state type, the address of the advertising router, the cost of the link, and the sequence number.

2. LSR ( Link State Request): When a slave router receives a DDP (Database Description Packet), it sends an LSAck packet. Then it compares the received information with its own information. If the DDP has more recent information, the slave router sends a link-state request (LSR) to the master router.

3. LSU ( Link State Update): LSU packet is sent in response to LSR (Link-State Request) packet that is sent from a slave router to a master router. LSU contains complete information about the requested entry.

4. In an OSPF environment,

  • A DDP (Data Description Packet) is used during the exchange protocol and includes summary information about link-state entries.
  • A hello packet is used during the hello process and includes information that enables routers to establish neighbor relationship.
  • An internal router is a router that resides within an area.

29. Important features of stub area are:

  • A stub area reduces the size of the link-state database to be maintained in an area, which in turn result in less overhead in terms of memory capacity, computational power, and convergence time.
  • The routing in Stub and totally Stubby areas is based on default gateway. A default route ( need to be configured to route traffic outside the area.
  • The stub areas suited for Hub-Spoke topology.
  • Area 0 is not configured as Stubby or totally Stubby. This is because stub areas are configured mainly to avoid carrying external routes, whereas Area 0 carries external routes.

Some of the important terms used in Enhanced IGRP are:

  • Successor: A route (or routes) selected as the primary route(s) used to transport packets to reach destination. Note that successor entries are kept in the routing table of the router.
  • Feasible successor: A route (or routes) selected as backup route(s) used to transport packets to reach destination. Note that feasible successor entries are kept in the topology table of a router. There can be up to 6 (six) feasible successors for IOS version 11.0 or later. The default is 4 feasible successors.
  • DUAL (Diffusing Update Algorithm): Enhanced IGRP uses DUAL algorithm to calculate the best route to a destination.

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