Open Shortest Path First (OSPF)

Introduction

OSPF is an open source Routing protocol that handles routing for IP traffic standardized in IETF RFC2328.

OSPF is classified as an Interior Gateway Protocol (IGP). This means that it distributes routing information between routers belonging to a single Autonomous System. The OSPF protocol is based on link-state or SPF technology.

It was designed mainly for the TCP/IP internet environment. Main focus was to produce a protocol that responds quickly to topology changes, with small amounts of control traffic. OSPF is a dynamic routing protocol.  It quickly detects topological changes in the AS (such as router interface failures) and calculates new loop-free routes after a period of convergence. This period of convergence is short and involves a minimum of routing traffic.

OSPF routes IP packets based solely on the destination IP address found in the IP packet header.  IP packets are routed “as is” — they are not encapsulated in any further protocol headers as they transit the Autonomous System.

OSPF uses Port 89 (IP) for Transport (own transport, not TCP/UDP).

OSPF Attributes (Summary)

OSPF Router IDs

Every Person/machine/protocol needs an ID in this world & so is the OSPF.

OSPF Router ID (aka Router Identifier) is a 32-bit number (like an IP address) that uniquely identifies this router in the Autonomous System (AS is like a whole world for OSPF).

If a router’s OSPF Router ID is changed, the router’s OSPF software should be restarted before the new Router ID takes effect.

Most of Routers (including Cisco) derive the Router ID in one of three ways and with the following precedence order:

1. Manually configured Router ID
2. Highest Loopback address
3. Highest Physical IP address on the router

OSPF Router types

When whole AS is one area then there Internal & External Routers only.

But when the AS is divided into areas then the routers are further divided according to function into the following four overlapping categories:

i. Internal router (IR)

A router that has all its interfaces belonging to the same OSPF area

ii. Area border router (ABR)

A router that connects one or more areas to the main backbone network

iii. Backbone router (BR)

A backbone router has an interface to the backbone area

iv. Autonomous system boundary router (ASBR)

An autonomous system boundary router is a router that is connected by using more than one routing protocol and that exchanges routing information with routers autonomous systems.

DR & BDR in OSPF

Problems (without DR): When multiple OSPF routers are connected to a multi-access medium the networks, there are two problems that arise:

Problem1: High Number of Adjacencies (A large number of adjecancies is required as in fig below)

Problem2: LSA flooding (Traffic increases exponentially because every router needs to flood its information to all other routers in the OSPF domain & for every LSA sent out there must be an acknowledgement sent back. It wastes a lot of BW)

Solution (with DR): To resolve these problems & control the un-necessary flooding of information, DR/BDR solution was proposed. With DR/BDR solution,

•    Adjacencies are formed with DR/BDR only.
•    All routers send LSA’s to DR only, DR sends out LSA’s to all other Routers.

Designated Router (DR) reduces the network traffic significantly. DR’s reduce network traffic as only they maintain the complete ospf database and then send updates to the other routers on the shared network segment.

Backup Designated Router (BDR) is simply a backup/standby of the DR. If DR fails then BDR takes its position.

DR/BDR Election Process

DR is chosen in below order:

 – Highest OSPF priority (default priority is 1), if all are equal then:

 – Highest OSPF Router ID, then

 – Highest loopback interface, then

 – Highest physical interface (should be up/up)

Then Router with the second preference in above list becomes the BDR.

Important Points about DR/BDR:

DR/BDR elections DO NOT occur in point to point networks

In DR/BDR, there is no preemption. Meaning that when a DR goes ofline, the BDR takes his place and when it comes back online it wont be the DR again, for it to be considered for another election process in a network segment, all DR/BDR must go offline so that one router can become the DR again for that segment.

OSPF Metric (Cost)

Every protocol uses some calculations (metrics) to judge a path’s feasibility (when multiple paths exist to for same destination). OSPF uses cost as metric for this purpose.

Logically a packet will face more overhead in crossing a 56Kbps serial link than crossing a 100Mbps Ethernet link. Respectively it will take less time in crossing a higher bandwidth link than a lower bandwidth link. OSPF uses this logic to calculate the cost. Cost is the inverse proportional of bandwidth. Higher bandwidth has a lower cost. Lower bandwidth has a higher cost.

To calculate cost, we have to use some reference Bandwidth to calculate cost. Reference Bandwidth for OSPF is 100 Mbps for cost calculation. The best route will have the lowest cost. Reference bandwidth defaults to 100Mbps (10^8) & can be modified with command.

Formula to calculate OSPF Cost:

Below is the table for reference Bandwidth: