Ex 10 OSPF

8/3/2019 Ex 10 OSPF

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8/3/2019 Ex 10 OSPF


8/3/2019 Ex 10 OSPF


Distance Vector vs. Link State Routing

With distance vector routing, each node has

information only about the next hop: Node A: to reach F go to B

Node B: to reach F go to D

Node D: to reach F go to E

Node E: go directly to F

Distance vector routing makes

poor routing decisions if

directions are not completely

correct

(e.g., because a node is down).

If parts of the directions incorrect, the routing may be incorrect until therouting algorithms has re-converged.

A B C

D E F

8/3/2019 Ex 10 OSPF


Distance Vector vs. Link State Routing

In link state routing, each node has a complete map of the

topology

If a node fails, each

node can calculate

the new route

Difficulty: All nodes need to

have a consistent view of the

network

A B C

D E F

A B C

D E F

A B C

D E F

A B C

D E F

A B C

D E F

A B C

D E F

A B C

D E F

8/3/2019 Ex 10 OSPF


Link State Routing: Properties

Each node requires complete topology information

Link state information must be flooded to all nodes

Guaranteed to converge

8/3/2019 Ex 10 OSPF


Link State Algorithm

Each router contains a database containing a map ofthe whole topology Links

Their state (including cost) All routers have the same information

All routers calculate the best path to every destination

Any link state changes are flooded across the

network ´Global spread of local knowledgeµ

8/3/2019 Ex 10 OSPF


Link State Routing

Each router Distributes LSA (Link StateAnnouncement/Advertisement) to other routers

Each router computes its best path to everydestination

On network failure

New LSAs are flooded

All routers recompute routing table Only changes are propagated

R1

LSA

X

LSA

8/3/2019 Ex 10 OSPF


OSPF: How it works

Hello Protocol

Responsible for establishing and maintaining neighbour

relationships

Hello Packets sent periodically on all OSPF enabledinterfaces

Contains information like

Hello Interval

A list of known neighbours Router Dead Interval

Hello

HelloHello

8/3/2019 Ex 10 OSPF


1 ± Flooding of link-state

information

2 ± Building a

Topological

Database

3 ± SPF Algorithm

4 ± SPF Tree

5 ± Routing Table

8/3/2019 Ex 10 OSPF


OSPF: How it works

Trade Information using LSAs

Each router builds an identical link state database

SPF algorithm run on the database Forwarding table built from the SPF tree

When change occurs:

Announce the change to all OSPF neighbours

All routers run the SPF algorithm on the revised

database

Install any change in the forwarding table

8/3/2019 Ex 10 OSPF


OSPF (Open Shortest Path First)

An interior routing protocol

Dividing an AS into areas

to handle routing efficiently and in a timely manner

8/3/2019 Ex 10 OSPF


OSPF

Areas

is a collection of networks, hosts, and routers in AS

AS can be divided into many different areas.

Routers inside an area flood the area with routing information.

Area Border Router

summarizing the information about the area and send it to otherareas

Backbone

All of the areas inside an AS must be connected to the backbone

serving as a primary area

consisting of backbone routers

8/3/2019 Ex 10 OSPF


OSPF

Areas in an AS

8/3/2019 Ex 10 OSPF


OSPF Cost

Cost is the OSPF metric used in path selection

Cost is based on bandwidth

Default bandwidth is 1.544Mbps ² cost is 64

Cost can be 1 (100Mbps) to 65535

Cost is 108÷ bandwidth

8/3/2019 Ex 10 OSPF


OSPF

Advantages

Changes in an OSPF network arepropagated quickly.

OSPF is hierarchical, using area 0 as thetop as the hierarchy.

OSPF is a Link State Algorithm.

OSPF supports Variable Length SubnetMasks (VLSM).

OSPF uses multicasting within areas.

After initialization, OSPF only sendsupdates on routing table sections whichhave changed; it does not send the entirerouting table.

Using areas, OSPF networks can belogically segmented to decrease the sizeof routing tables. Table size can befurther reduced by using routesummarization.

OSPF is an open standard, not related toany particular vendor.

Disadvantages

OSPF is very processor intensive.

OSPF maintains multiple copies of routinginformation, increasing the amount ofmemory needed.

OSPF is not as easy to learn as someother protocols.

In the case where an entire network isrunning OSPF, and one link within it is"bouncing" every few seconds, OSPFupdates would dominate the network byinforming every other router every timethe link changed state

8/3/2019 Ex 10 OSPF


8/3/2019 Ex 10 OSPF


Wildcard Mask

A wildcard mask is a mask of bits that indicates which parts of an IPaddress can assume any value.

In the Cisco IOS, they are used in several places, for example:

To indicate the size of a network for some routing protocols, such as OSPF.

To indicate what IP addresses should be permitted or denied in accesscontrol lists (ACLs).

A wildcard mask can be thought of as a subnet mask, with ones andzeros inverted

For example, a wildcard mask of 0.0.0.255 corresponds to a subnetmask of 255.255.255.0.

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Ex 10 OSPF