OSPF

1
OSPF

• Semester 5, Chapter 4
• Allan Johnson
• Modified by Sean He

2
OSPF Overview
3
OSPF Terminology

• Linksnetworks a router knows about
• Areaa group of routers identified with a unique
  ID all routers in the same area share the same
  link-state database.
• Costis the speed of the media can be manually
  configured
• SPF Algorithm (Dijkstra)calculated by each
  router to choose the lowest-cost path
• link-stateis a link up or down
• LSAa link state advertisement
• Adjacencies Databasekeeps track of all directly
  connected neighbors
• Link-State Databasealso known as the Topology
  database picture of who is connected to what
  all routers should have the same L-S DB
• Forwarding Databaseknown as the Routing table
  where the lowest-cost paths are installed
• Designated Router/Backup Designated Router
  (DR/BDR) routers that are elected on multiaccess
  networks to be the focal point for routing
  updates.

4
OSPF Packet Types

• OSPF uses a variety of packets to communicate
  with neighbors and the DR/BDR.
• Type 1 Hello packet a packet sent at regular
  intervals to keep a link alive
• Type 2 DBD (Database Description) summary
  contents of a routers link-state database sent
  to a newly discovered neighbor
• Type 3 LSR (Link-State Request) requests more
  specific information about a link from a
  neighbors link-state database
• Type 4 LSU (Link-State Update) transports LSAs
  to neighbor routers for example, a reply to an
  LSR
• Type 5 LSAck (Link-State Acknowledgement)
  acknowledges receipt of a LSA OSPFs routing
  updates are connection-oriented

5
OSPF States

• OSPF uses neighbor adjacencies to gain full
  knowledge of the network.
• These adjacencies (relationships) are established
  through a seven step process.
• Each step is called a state. Briefly, they are
• Down
• Init
• 2Way
• Ex Start
• Exchange
• Loading
• Full

6
Steps in the OSPF Operation

• Routers move through five distinct steps of
  operation.
• Step 1 Establish Router Adjacencies
• Done after the 2Way state
• Step 2 Elect a DR and BDR
• Done on multiaccess networks only
• Step 3 Discover Routes
• Done in the Ex Start and Exchange States
• Step 4 Select Appropriate Routes
• Done through the calculation of the SPF algorithm
• Step 5 Maintain Routing Information
• Done through the regular exchange of Hellos

7
Configuring OSPF
8
Basic OSPF Configuration

• Below is the basic commands to configure OSPF.
• Router(config)router ospf process_id
• Router(config-router)network address
  wildcard-mask area area_id
• The process_id is used to identify the OSPF
  process
• The process id field is 16 bits can be a number
  from 1 to 65,535
• You can have multiple instances of OSPF running
  on the same router
• Just like RIP and IGRP, you enter a network
  command for each network you want OSPF to
  advertise.
• The wildcard-mask argument is simply the mirror
  of the networks subnet mask and is used to
  associate the network with its area.
• For example, 255.255.224.0 would yield a wildcard
  of 0.0.31.255
• The area command must be entered. In single area
  OSPF configuration, the area_id is always zero.

9
Router Priority Loopbacks

• To configure which router will become the DR and
  BDR, use the priority command on the interface
  participating in the multiaccess network.
• Router priority is 1 by default. A priority of 0
  means never DR.
• Router(config-if)ip ospf priority number
• If the highest priority is shared by more than
  one router, then router ID determines DR/BDR.
• To add stability to router IDs, use a configured
  loopback.
• The highest loopback address will become the
  routers ID instead of the highest configured IP
  address on an interface.
• When configuring loopbacks, use a /32 mask to
  avoid potential routing problems.
• Router(config)interface loopback0
• Router(config-if)ip address 1.1.16.172
  255.255.255.255

10
Cost Calculation

• OSPF calculates the cost of a link based upon the
  following formula
• 108/bps Cost (Note bps for 1K 1000, not
  1024)
• 56K dial-up link 108/56,000 bps 1784
• T1 dedicated link 108/1,544,000 64
• 100Mbs Ethernet 108/100,000,000 bps 1
• Routers must agree on the cost of a shared link.
  Otherwise, the link will be considered down.
• A Cisco routers default cost for a serial link
  is 1784 (1.544Mbps).
• To change the cost of a link in a multivendor
  environment, use
• Router(config-if)ip ospf cost number
• In addition, if a links default bandwidth is not
  1.544Mbps, you must change it. For example, when
  your link is a partial T1.
• Router(config-if)bandwidth Kbps

11
Authentication

• OSPF Authentication is used to insure only
  desired OSPF routers join the area.
• Authentication commands must be entered on all
  routers in the area.
• Step 1 Configure authentication in ospf process
• Router(config-router)area area_id
  authentication message-digest
• The message-digest option is recommended.
  Otherwise, passwords are sent in clear text.
• Step 2 Configure each interfaces key and
  password
• Router(config-if)ip ospf message-digest-key
  key-id md5 encryption-type password
• key-id identifies which md5 process is running
  since a router can have multiple instances of
  authentication
• Encryption-types are 0 7 0 is default 7 is
  Cisco proprietary.

12
Configuring Hello Interval

• All routers in an OSPF area must agree on the
  same hello interval.
• This is necessary to insure that each router
  knows when to consider any link to be down (four
  times the hello).
• The default hello intervals are
• 10 seconds for links with T1 speeds or greater
  and
• 30 seconds for links with less than T1 speeds.
• Changing the hello interval.
• Router(config-if)ip ospf hello-interval seconds
• Note The dead interval will automatically adjust
  to four times the new hello interval. However, to
  configure it
• Router(config-if)ip ospf dead-interval seconds

13
Configuring NBMA to Broadcast

• Each participating interface must be configured
  to broadcast and
• Neighbor ip addresses must be mapped to their
  DLCIs.
• Router(config-if)encapsulation frame-relay
• Router(config-if)frame-relay map ip address
  dlci broadcast
• (Enter a frame-relay map command for each
  neighbor)