Distance Vector vs. Link State docx

Distance Vector vs. Link State
B

A

C

D
X

E

Distance Vector
Link State
Hybrid

Routing by rumor
Route table
Topology

Periodic Update

Incremental Update

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Distance Vector vs. Link State
Distance Vector
• Updates frequently
• Each router is

"aware" only of its
immediate neighbors
• Slow convergence
• Prone to routing loops
• Easy to configure

Link State
• Updates are event
triggered
• Each router is
"aware" of all other
routers in the "area"
• Fast convergence
• Less subject to
routing loops
• More difficult to
configure
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Comparison Continued
Distance Vector
• Fewer router resources
required
• Updates require more
bandwidth
• Does not "understand"
the topology of the
network

Link State
• More router resource
intensive
• Updates require less
bandwidth
• Has detailed knowledge
of distant networks and
routers

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Link State
Example
OSPF
IS-IS

OSPF is used for corporate networks
IS-IS is used for ISP’s

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Open Shortest Path First (OSPF)
 OSPF is an open standards routing protocol
 This works by using the Dijkstra algorithm
 OSPF provides the following features:

 Minimizes routing update traffic
 Allows scalability (e.g. RIP is limited to 15 hops)
 Has unlimited hop count
 Supports VLSM/CIDR
 Allows multi-vendor deployment (open standard)

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Link State
There are two types of Packets
Hello
LSA’s

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OSPF Hello
A

B

•
•
•
•

C

When router A starts it send Hello packet – uses 224.0.0.5

Hello packets are received by all neighbors
B will write A’s name in its neighbor table
C also process the same way

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"Hello" Packets
•
•
•
•

Small frequently issued packets
Discover neighbours and negotiate "adjacencies"
Verify continued availability of adjacent neighbours
Hello packets and Link State Advertisements (LSAs)
build and maintain the topological database
• Hello packets are addressed to 224.0.0.5.

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Link State Advertisement
(LSA)
 An OSPF data packet containing link state and routing
information that is shared among OSPF routers
 LSAs are shared only with routers with whom it has
formed adjacencies
 LSA packets are used to update and maintain the

topology database.

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Link State
There are three type of tables
Neighbor
Topology
Routing

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Tables
 Neighbor
 Contain information about the neighbors
 Neighbor is a router which shares a link on same
network
 Another relationship is adjacency
 Not necessarily all neighbors
 LSA updates are only when adjacency is established

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Tables
 Topology
 Contain information about all network and path to
reach any network

 All LSA’s are entered in to topology table
 When topology changes LSA’s are generated and send
new LSA’s
 On topology table an algorithm is run to create a
shortest path, this algorithm is known as SPF or
dijkstra algorithm

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Tables
 Routing Table
 Also knows as forwarding database
 Generated when an algorithm is run on the topology
database
 Routing table for each router is unique

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OSPF Terms
 Link
 Router ID
 Neighbours
 Adjacency
 OSPF Area

 Backbone area
 Internal routers
 Area Border Router

(ABR)
 Autonomous System
Boundary Router
(ASBR)

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Link
 A network or router interface assigned to a given
network
 Link (interface) will have "state" information
associated with it
Status (up or down)
IP Address
Network type (e.g. Fast Ethernet)
Bandwidth
Addresses of other routers attached to this
interface
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OSPF Term: Link

A link is a network or router interface assigned to any given
network
This link, or interface, will have state information associated
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with it (up or down) as well as one or more IP addresses

OSPF Term: Link State

Status of a link between two routers
Information is shared between directly connected routers.
This information propagates throughout the network unchanged and
is also used to create a shortest path first (SPF) tree.
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Router ID
 The Router ID (RID) is an IP address used to identify the router
 Cisco chooses the Router ID by using the highest IP address of all
configured loopback interfaces
 If no loopback interfaces are configured with addresses, OSPF will
choose the highest IP address of all active physical interfaces.
 You can manually assign the router ID.
 The RID interface MUST always be up, therefore loopbacks are
preferred

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Neighbours
 Neighbours are two or more routers that
have an interface on a common network
E.g. two routers connected on a serial link
E.g. several routers connected on a common
Ethernet or Frame relay network

 Communication takes place between /
among neighbours
 neighbours form "adjacencies"
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Adjacency
 A relationship between two routers that
permits the direct exchange of route
updates
 Not all neighbours will form adjacencies
This is done for reasons of efficiency – more
later

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OSPF Design

Each router connects to the backbone called area 0, or the backbone area.
Routers that connect other areas to the backbone within an AS are called Area Border Routers (ABRs).
One interface must be in area 0.
OSPF runs inside an autonomous system, but can also connect multiple autonomous systems together.
The router that connects these ASes together is called an Autonomous System Boundary Router (ASBR).

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OSPF Areas
 An OSPF area is a grouping of contiguous networks and

routers
 Share a common area ID
 A router can be a member of more than one area (area
border router)
 All routers in the same area have the same topology
database
 When multiple areas exist, there must always be an area
0 (the backbone) to which other areas connect

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Why areas?
 Decreases routing overhead
 Compare to multiple smaller broadcast domains
instead of one large one
 Speeds convergence
 Confines network instability (e.g. route "flapping") to
single area of the network
 Adds considerably to the complexity of setting up OSPF
 CCNA certification deals only with single-area OSPF
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Area Terminology

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