OSPF notes

This is part of the Cisco Notes series on Mike’s World News.

Open Shortest Path First (OSPF) is a Link State Routing protocol.

It is a Classless protocol, so it supports Variable Length Subnet Masks (VLSM) and CIDR.

OSPF does not use Split Horizon nor does it use Auto Summarization.

It uses Dijkstra SPF algorithm.

Uses Multicast address 224.0.0.5 for updates (as well as 224.0.0.6).

OSPF uses multicast 224.0.0.5 and 224.0.0.6

OSPF sends updates every 30-minutes
has Administrative Distance of 110.

Metric is a cost basis.
cost=bandwidth

Router ID (RID)

Each individual router is identified by a RID.

You can manually configuring a manual Router ID
If no router ID has been configured the highest loopback address assigned becomes the Router ID.
If not Lookback address has been assigned, the highest IP address of any working interface becomes the Router ID.

OSPF creates a neighbor Link State Dabaase (LSDB) table. It sends out a hello with its Router ID and all the Router IDs that it knows about.

Every network must have an Area 0. All other areas must have a connection back to area 0.

Area Border Routers sit between areas, with at least one interface in each area.

Designated Router (DR)
Backup Designated Router (BDR)
DROTHER

Changes are sent only to the DR and BDR.

The Router with the highest OSPF priority wins the Election process and becomes the DR.

By default all routers have a priority of 1. If there is a tie it goes to the router with the highest Router ID (according to above directions).

A lookback address is preferred over an IP address because then you can control which router becomes the DR and a lookback interface never goes down.

Supports load balancing over equal costs paths. By default, will load balance over 4, can be configured for upto 16.

OSPF only sends out updates, not the entire routing table.

OSPF creates a neighbor Link State Database (LSDB) table. It sends out a hello with its Router ID and all the Router IDs that it knows about.  OSPF only sends out updates, not the entire routing table

Every network must have an Area 0. All other areas must have a connection back to area 0.

Area Border Routers sit between areas, with at least one interface in each area.

OSPF can only become neighbors and share information with adjacent routers. To become adjacent the routers must:

  • be on the same subnet, with the same subnet mask.
  • must have the same hellow and dead intervals.
  • must be in the same OSPF area.
  • if used, authentication must match
  • Stub area flag must match.

OSPF network statements must include Wildcard mask and area ID.

Print Friendly, PDF & Email

Leave a Reply

Your email address will not be published. Required fields are marked *