Semester 3 v3'1'1: Switching Basics and Intermediate Routing

1
Semester 3 v3.1.1Switching Basics and
Intermediate Routing

• MODULE 2
• Single-Area OSPF

2
Link-State Routing

• Distance vector protocols have limitations
• Slow to converge prone to routing loops high
  network overheads
• Link-state protocols utilise shortest path based
  on several metrics
• Quicker convergence high hardware overheads
• Hello packets and Link State Advertisements
  (LSAs) help build network topology database
• Routing update LSAs triggered by link changes
  keep all routing tables synchronised

3
Link-State Routing

• LSAs build a routers network topology database
• Topological database crunched by Shortest Path
  First (SPF) algorithm to calculate route costs
• Calculated route costs applied to routing table

4
The OSPF Routing Protocol

• Described in several IETF standards, OSPF is more
  scalable than RIP
• Not limited to 15 hops
• Uses metrics that matter, including bandwidth
• Lower network overheads
• Routing updates sent on link changes
• Routing updates contain only link change
  information
• Routing updates use multicast within an
  administrative domain a defined area
• Supports VLSM and classless routing

5
How does OSPF work?

• Each router has a copy of the same topological
  database...
• ... But each router has a unique adjacency table
• To help maintain a master copy of the
  topological datbase, a Designated Router (DR) and
  Backup Designated Router (BDR) are elected per
  OSPF Area.

6
Itll cost you...

• The SPF algorithm assigns a cost to each link in
  the topological database.
• Cost 108/Bandwidth (bps)
• The link costs help build a loop-free topology,
  installed in the routing table.

7
OSPF in the Network

• OSPF-enabled routers have an adjacency table of
  neighbour routers
• Which routers are my OSPF neighbours?
• Depends upon OSPF Area, and OSPF network type
• Broadcast multi-access
• Point-to-point
• Non-broadcast multi-access
• Point-to-multipoint
• Only Broadcast and NBMA have OSPF DR/BDR elections

8
Calling all DRs...

• For OSPF routers to route packets between each
  other, adjacency is required
• For OSPF routers to receive routing updates from
  each other, a DR is required
• OSPF Hello packets carry adjacency information
• Hellos sent on router startup and then regularly
• Adjacency state determines a routers OSPF status
• Who are my neighbours?
• Who is my DR and BDR?

9
Configuring OSPF

• OSPF areas limit administrative domain
• All multi-area OSPF networks require Area 0, the
  backbone area.
• Then assign networks to OSPF areas
• Router(config) router ospf process-id
• Router(config) network ip-address wildcard-mask
  area area-id

10
Further OSPF Configuration

• OSPF routers do not route without a router ID,
  based up on highest active local IP address
• Ensure OSPF routers are always active by
  activating an always-on software IP interface a
  loopback interface

11
Welcome to my world )

• Daniel Comarmond
• CCNP, CCDP, CCSP, CCAI
• Cisco Networking Academy Instructor
• Systems Engineer Cisco Systems
• E-Mail dcom_at_it.uts.edu.au
• MSN dcom82_at_dcom82.com
• Phone [phone]
• Website http//www-staff.it.uts.edu.au/dcom
• Take care, and SMILE!!! )