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Switching Basics and Intermediate Routing - Single Area OSPF

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

• LSAs build a router’s network topology database

• Topological database crunched by Shortest Path First (SPF) algorithm to calculate route costs

• Calculated route costs applied to routing table

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

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.

It’ll 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.

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

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 router’s OSPF status

– Who are my neighbours?

– Who is my DR and BDR?

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 ospfprocess-id

• Router(config)# networkip-address wildcard-maskarea area-id

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

Comments (1)Add Comment
written by ASHISH, March 27, 2014
good site

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