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1. © 2002, Cisco Systems, Inc. All rights reserved. ICND v2.0—5-1
OPEN SHORTEST PATH
FIRST (OSPF)
Points to remember:
 OSPF is open protocol & is link state protocol.
 OSPF supports for VLSM & SUBNETTING.
 OSPF uses Dijkstra Algorithm to calculate the best paths.
 OSPF uses Cost (inverse of Bandwidth) as a metric.
 Sends updates when topology changes.
 OSPF is divided into hierarchical designs to minimize the routing
table.
 It is divided in Areas & Autonomous System.
 Hello packets mechanism is used as well as link state
advertisements packets are used to maintain the topology database.
 OSPF supports load balancing of up to six equal-cost paths to a single
destination

2. © 2002, Cisco Systems, Inc. All rights reserved. ICND v2.0—5-2
OSPF metric
 OSPF uses cost. Cost is actually the inverse of the bandwidth of a link: the
faster the speed of the connection, the lower the cost. The most preferred
path is the one with the lowest cost.
 Cost = 10(power 8 ) /bandwidth (bps)
 OSPF supports load balancing of up to six equal-cost
paths to a single destination.

3. © 2002, Cisco Systems, Inc. All rights reserved. ICND v2.0—5-3
OSPF Hierarchical Routing

4. © 2002, Cisco Systems, Inc. All rights reserved. ICND v2.0—5-4
REASONS FOR HEIRACHIAL
DESIGNS
 To minimize the routing table.
 To speed up convergence.
 To confine the LSA flooding to the particular area only.
 Minimizes routing updates traffic.

5. © 2002, Cisco Systems, Inc. All rights reserved. ICND v2.0—5-5
Configuring Single Area OSPF
g-router)#network net add wildcard mask area area-id
• Assigns networks to a specific OSPF area
Router(config)#router ospf process-id
• Defines OSPF as the IP routing protocol

6. © 2002, Cisco Systems, Inc. All rights reserved. ICND v2.0—5-6
OSPF Configuration Example

7. © 2002, Cisco Systems, Inc. All rights reserved. ICND v2.0—5-7
Router#show ip ospf interface
Verifying the OSPF Configuration
• Displays area-ID and adjacency information
Router#show ip protocols
• Verifies that OSPF is configured
Router#show ip route
• Displays all the routes learned by the router
Router#show ip ospf neighbor
• Displays OSPF-neighbor information on a per-interface basis

8. © 2002, Cisco Systems, Inc. All rights reserved. ICND v2.0—5-8
OSPF debug commands
Router#debug ip ospf events
OSPF:hello with invalid timers on interface Ethernet0
hello interval received 10 configured 10
net mask received 255.255.255.0 configured 255.255.255.0
dead interval received 40 configured 30
Router# debug ip ospf packet
OSPF: rcv. v:2 t:1 l:48 rid:200.0.0.117
aid:0.0.0.0 chk:6AB2 aut:0 auk:
Router#debug ip ospf packet
OSPF: rcv. v:2 t:1 l:48 rid:200.0.0.116
aid:0.0.0.0 chk:0 aut:2 keyid:1 seq:0x0

9. © 2002, Cisco Systems, Inc. All rights reserved. ICND v2.0—5-9
ELECTION OF DR
& BDR
The OSPF router with the highest priority becomes the DR
for the segment. If there is a tie, the router with the highest
router ID will become the DR. By default, all routers have a
priority of 1 (priorities can range 0–255). If the DR fails, the
BDR is promoted to DR and another router is elected as the
BDR.
When an OSPF router comes up, it forms adjacencies with
the DR and the BDR on each multi-access segment that it is
connected to. Any exchange of routing information is
between these DR/BDR routers and the other OSPF
neighbors on a segment (and vice versa). An OSPF router
talks to a DR using the IP multicast address of 224.0.0.6. The
DR and the BDR talk to all routers using the 224.0.0.5
multicast IP address.

10. © 2002, Cisco Systems, Inc. All rights reserved. ICND v2.0—5-10
Loopback interfaces
A loopback interface is a logical, virtual interface on a
router. By default, the router doesn’t have any
loopback interfaces, but they can easily be created
from 0 to to 2147483647.
Reasons to create a loopback interface:
1) To assign a router ID to an OSPF router
2) To use for testing purposes, since this interface is
always up

11. © 2002, Cisco Systems, Inc. All rights reserved. ICND v2.0—5-11