OSPF

Introduction

OSPF uses SPF to find the shortest path to the destination. In other to do that, OSPF routers need to understand the full topology of the network. It's not scalable to use all information in the network because it will generate too many informations. OSPF introduces 7 LSAs to depict the full topology.

OSPF routes have an AD of 110

OSPF uses IP protocol 89, not TCP or UDP

The dead timer is 4x the Hello interval

Adjacency Requirements

• Same area
• Authentication match
• Subnet mask match
• Hello and dead timers match
• Stub flags match
• MTU match

State

• Down
• Attempt
• Init
• 2-Way
• Exstart
• Exchange
• Loading
• Full

Metric Calculation

(config)# auto-cost reference-bandwidth <bandwidth in Mbps>

LSA

There are 6 LSA types:

http://blog.xuite.net/juilin77/happy/5702604-OSPF+LSA+Types%E6%95%B4%E7%90%86

Network Area

There are 6 area types in OSPF:

• Backbone area (area 0)
• Standard area
• Stub area
• Totally stubby area
• Not-so-stubby area (NSSA)
• Totally Not-so-stubby area

Network Type

Stub network is serial link(point to point), loopback

Transit network is Ethernet link(multi-access?)

Route Summarization

Since OSPF is a link state routing protocol, it is impossible to filter or summarize the routes in any router. Instead, the summarization need to be applied in the ABR or ASBR and advertised via a network summary LSA(Type 3 LSA).

on ABR

R1(config-router)# area 18 range 8.0.0.0 255.0.0.0

This will summarize the subnet of 8.0.0.0/8 into only 1 summary route.

Typically set on ABR for summarizing the routes that are added using network command.

This summary route will not be advertised if there is no subnet of 8.0.0.0/8 exists in R1's routing table.

on ASBR

R1(config-router)# summary-address 1.0.0.0 255.0.0.0

This will summarize the subnet of 1.0.0.0/8 into only 1 summary route.

Typically set on ASBR for summarizing the redistributed routes.

This summary route will not be advertised if there is no subnet of 1.0.0.0/8 exists in R1's routing table.