Cisco CCNP ROUTE Implementing a Scalable Multiarea Network OSPF Based Solution


Cisco CCNP ROUTE Objectives


Cisco CCNP ROUTE Open Shortest Path First


If you remember, we discussed link state routing protocols and identified that they require more administration.  This design administration requires that all OSFP routers are configured in a hierarchical area design.

RIP & IGRP use the Bellman-Ford algorithm to find their best route, OSPF uses the Dijkstra algorithm to find the shortest path to remote networks.

OSPF uses protocol value 89 in the IP packet.

OSPF communicates through the use of multicast packets. 

All OSPF routers participate in multicast group

All OSPF designated routers (DR & BDR) participate in as well as



OSPF was developed to combat the limitation of RIP.

OSPF is a LINK STATE Routing protocol that has these following characteristics

Speed of convergence

Faster than RIP with minimum of traffic exchanged


Supports summarization at area boarders


No hop count limits, approx 40-50 routers per area

Bandwidth Utilization

Incremental updates, topology data kept within single area

Path selection

Metric based on bandwidth, equal cost load balancing over up to six paths



  • • Identified by a 32 bit field
  • • Manually configured
  • • If only one area exists, it’s ID can be ANY number
  • • If more than one area is defined, area 0 must exist and is the backbone
  • • Area 0 must be connected to all other areas 
  • • There is only one area 0

Cisco CCNP ROUTE OSPF Terminology


Link – Interface on a router

Link-state – Status of a link between two routers

Includes a router’s interface and its relationship to its neighboring routers

Cost – Indication of the overhead required to send packets across an interface

High bandwidth =  low cost

Area – A contiguous grouping of networks and routers sharing the same Area ID and topology database

DR (Designated Router) – An elected router in Broadcast and Non-Broadcast environments that is adjacent to all other OSPF routers

BDR (Backup Designated Router) – An elected router in Broadcast and Non-Broadcast environments that is adjacent to all other OSPF routers

 that will promote it self if the Designated Router is no longer available

Neighborship Database  – Listing of all the neighbors to which a router has established bi-directional communication 

Link State/Topological Database  – List of information about all other routers in the same area

Routers in the same area have identical link state databases  

Routing Table  – Generated when the SPF algorithm is run on the link-state database

Each router has a unique routing table


Cisco CCNP ROUTE OSPF Terminology



Two routers that have an interface on a common network

Usually discovered by hello’s but can also be configured administratively


Relationship formed between selected neighbors in which routing information is exchanged.  Not all neighbors are adjacent

Only Broadcast and Non-Broadcast network types have Designated and Backup Designated Routers!!!



  • •Elected routers that coordinate the flow of route updates within a subnet
  • • All routers on a sub-net form adjacencies with the DR and BDR
  • • Clients send updates to the DR and BDR only
  • • The DR informs all other clients of changes in topology
  • • BDR assumes DR role if DR fails but performs no DR functions if DR remains available
  • • It is generally not a good idea to select the same router to be designated router on many LANs simultaneously.
  • • ONLY Broadcast and Non-Broadcast Network types have Designated and Backup Designated Routers.

Cisco CCNP ROUTE Electing the DR and BDR


The following outlines the process OSPF takes and rules that are followed when electing a Designated Router:

  • • Routers elect a DR and BDR per network
  • • All routers set by default to priority 1 (0-255)
  • • Priority of zero (0) means router can not be elected as a DR
  • • Router with highest priority wins BDR (1 – 255), if no other router has a higher priority the BDR will then become the DR
  • • RouterID breaks tie, Router ID is either the Highest Loopback or Highest Configured IP address on any given active interface
  • • If DR fails, BDR promoted to DR and a new BDR is elected
  • • Existing DR will not be overthrown if “better” router is turned on after initial election
  • • DRs and BDRs listen to multicast traffic on both multicast address and is exclusively listed to by DRs


Cisco CCNP ROUTE Router ID (RID)


Each router that is participating in OSPF needs to be uniquely identified. The method of identification that OSPF uses is Router IDs (RID).

32 bits that uniquely identifies an OSPF router

Highest IP address in router is RouterID

Overridden by Loopback interface if present

Even if Loopback address has lower value

Recommended to use loopback interface

Easier to manipulate this number

Always up

Interface loopback 0

Ip address

You can also Statically assign the Router ID in the OSPF router configuration mode:

(config)# router ospf 1

(config-router)# router-id

Do NOT use same loopback address on different routers

Cisco CCNP ROUTE  Modifying OSPF operation


Router(config-if)# ip ospf priority {0 – 255}

Change the priority of a router on an interface

0 means to not participate in election

1 is default, 255 is highest priority

Router(config-if)# ip ospf cost

Change the cost of an interface

Default cost is 100,000,000/bandwidth

10Mbps Ethernet = 10

100 Mbps Ethernet = 1

Can be used to ensure that two routers use the same cost to refer to the same interface. Ex. Nortel’s default cost for 10MBPS link is 1

r3(config-if)# ip ospf priority 10

r2# sho ip ospf neighbor detail

 Neighbor, interface address

    In the area 0 via interface Ethernet0

    Neighbor priority is 10, State is FULL, 6 state changes

    DR is BDR is

    Options 2

    Dead timer due in 00:00:37

r2(config)# int s0

r2(config-if)# ip ospf cost 1000

r2# show ip route is variably subnetted, 5 subnets, 3 masks


[110/1001] via, 00:00:13, Serial0

C is directly connected, Serial0

O [110/11] via, 00:00:13, Ethernet0

C is directly connected, Ethernet0

C is directly connected, Loopback0


Cisco CCNP ROUTE ip ospf cost


COST  is 1 to 65535.  In general, the path cost in Cisco routers is calculated using the formula 108/bandwidth. The following are the cost associated with OSPF using the default cost metrics.

56 kbps / 64 kbps        =          1785 / 1563

T1 (1.544)                   =          64

10 Mbps                      =          10

100 Mbps                    =          1

16 Mbps                      =          6

FDDI              =          1

1 Gbps             =          1

Cisco CCNP ROUTE Types of OSPF Routers


Internal Routers:

All interfaces reside inside same area

Area Border Routers:

One interface in each of two areas

Backbone Routers:

At least one interface resides in Area 0

AS Boundary Routers:

One interface resides in a non-OSPF area

Cisco CCNP ROUTE Planning for OSPF


Since OSPF is hierarchical in nature, coming up with an OSPF design to include not only an IP Address plan but also defining OSPF areas to include types in key.  As such, OSPF implementation depends on the specific needs and topology.  When planning an OSPF design you must gather information  about your topology.  Examples are as follows:

  • • IP Address usage.
  • • Network topology (e.g. Connected links)
  • • Design of multiple OSPF areas as it makes sense.

Cisco CCNP ROUTE OSPF Implementation Plan


Once you has compiled your requirements, it is time to create an Implementation Plan.  Key pieces developing an Implementation Plan are as follows:

  • • Define IP Networks that need to be included (advertised) by OSPF.
  • • Enable OSPF on the correct interfaces  or with the correct network statement under the OSPF routing process
  • • Verify OSPF is enabled within the correct area
  • • Specify any metrics if you want to change the default behavior of OSPF.
  • •After implementation is complete, verify everything is working as expected.