Networking II (MODULE #1)

Open Shortest Path First (OSPF) is designed to

find the fastest available path for a packet from source to destination

OSPVFv2 is used for

IPv4 networks

OSPVFv3 is used for

IPv6 networks

OSPF (link-state) protocol developed as an

alternate for Routing Information Protocol (RIP)

RIP used

hop count which does not scale well

A link is known as an interface on a

router

a link is also a network segment that connects

two routers/stub networks

Info about the state of a link is known as a

link-state

all link-state information includes the

network prefix, prefix length, and cost

routing protocols use messages to

exchange route information

the messages bulid data structures, which are processed using a

routing algorithm

3 OSPF databases:

Adjancy database, Link-state database, Forwarding database

Adjancy database definition

creates the neighbor table

Link-state database (LSDB) definition

creates the topology table

Forwarding database definition

creates the routing table

these tables that are created are maintained in

RAM

What does a topology table in networking represent?

It represents the network topology.

What information does a topology table list?

It lists information about other routers in the network.

What do routers within the same area have in common regarding LSDB?

Routers within an area have the same Link-State Database (LSDB).

Which command is used to show the OSPF database?

The command is 'show ip ospf database'.

router bulids topology table using calculations based on the

Dijkstra shortest-path first (SPF) algorithm

SPF is based on the cost to

reach a destination

What does the SPF algorithm create?

An SPF tree

Where is each router placed in the SPF tree?

At the root of the tree

What does the SPF algorithm calculate for each node?

The shortest path

OSPF places the best routes into the

forwarding database, which makes routing table

OSPF router sends hello packets out of all the enabled interfaces to see if

neighbors are present on those links

if a neighbor is present

the OSPF enabled router attempts to establish a neighbor adjacency

after adjancies are established, routers exchange link-state advertisements (LSAs) which

routers flood LSAs to adjacent neighbors until all routers have LSAs

after LSAs are recieved,

OSPF enabled routers bulid topology table (LSDB) based on the recieved LSAs

the topology table (LSDB) holds all the information about

the topology of the area

after the topology table is built,

the SPF algorithm is executed

the SPF algorithm creates the

SPF tree

after the SPF tree is built,

the best paths to the network are updated in IP routing table

the route will be in the routing table unless there is a

static route

static route definition

route source to the same network with lower administrative distance

routing decisions are made based on the

entries in the routing table

OSPF supports

hierarchical routing

OSPF area is a

group of routers sharing the same link-state information in their LSDBs

first way OSPF is implemented

single-area OSPF

second way OSPF is implemented

multi area OSPF

single-area OSPF definition:

all routers in one area (USE AREA 0)

multi area OSPF definition

routers in multiple areas in hierarchy

all area of multi area OSPF must connect to

area 0 (backbone)

routers interconnecting the areas are referred to as

area border routers (ABRs)

with OSPF, routing between the areas occured is called

interarea routing

the processor routing operations (ex. recalculating DB) are kept within

the area

too many routers in the area would make the

LSDBs too large and increase CPU load

OSPFv3 uses the SPF algorithm to

determine the best paths in routing

OSPFv3 routing processes and operations run

independently

in OSPF, cost is used to determine the best path to

the destination

Dijkstra's algorithm is responsible for

the cost of each route

OSPF component associated with routing table is

forwarding database

OSPF component associated with topology table is

link-state database

routers exchange...

type 2 DBD packets used by receiving routers to check against LSDB

type 3 LSR packet is used by receiving routers to

request more information about an entry in DBD

LSUs forward OSPF routing updates, such as

link changes

LSU packet contains 11 different types of

OSPFv2 LSAs

OSPFv3 contains two additional

LSA's

an LSU contains one or more

LSAs

point-to-point links DO NOT require

DR or BDR

What is the nature of communication between two routers in a two-way state description?

Communication is bidirectional.

What do routers elect on multiaccess links?

Routers elect a Designated Router (DR) and a Backup Designated Router (BDR).

In what type of network do routers elect a DR and BDR?

In a common Ethernet network.

What is the next state after the election of DR and BDR in a two-way state description?

Transition to ExStart state.

What is the ExStart state in PTP networks?

It is the state where two routers decide which router initiates DBD packet exchange.

What do routers decide during the ExStart state regarding DBD packets?

They decide the initial DBD packet sequence number.

What are LSRs used for in a loading state?

LSRs are used to gain more route information.

What are LSUs used for in a loading state?

LSUs are used to gain more route information.

What algorithm do routes use during the loading state?

Routes use the SPF (Shortest Path First) algorithm.

What is the transition that occurs in a loading state?

The transition is to full state.

full state definition

link-state database of router is fully synchronized

What state do routers enter to decide who sends packets first?

Exstart state

In the exstart state, how do routers determine which one sends packets first?

The router with the higher ID sends first.

If R2 has a higher ID than R1, which router will send packets first during the exchange state?

R2

DR & DBR creates two challenges:

creation of multiple adjacencies

extensive flooding of LSAs