Sunday, January 21, 2018

IAM Search

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
0
...
written by ASHISH, March 27, 2014
good site

Write comment

security code
Write the displayed characters


busy

Promotions

  • ...more

Disclaimer

Important: All images are copyrighted to their respective owners. All content cited is derived from their respective sources.

Contact us for information and your inquiries. IAMechatronics is open to link exchanges.

IAMechatronics Login