Routing protocols

Static Routing

Routes to destinations are set up manually Route may be up or down but static routes

will remain in the routing tables and traffic would still be sent towards the route

Not suitable for large networks Also known as Non-adaptive routing

Static/Default route entries

Dynamic Routing

Routes are learnt via an internal or external routing protocols

Network reachability is dependent on the existence and state of the network

Routing decisions change to reflect the changes in topology

Also known as Adaptive routing

Path Determination

A B

C

192.168.1.0

192.168.7.0

192.168.6.0

192.168.5.0

192.168.2.0

192.168.3.0

192.168.4.0

Router-ANetwork Next Hop Router192.168.1.0 Direct192.168.2.0 Direct192.168.3.0 Direct192.168.4.0 B,C192.168.5.0 B,C192.168.6.0 B,C192.168.7.0 B,C•Networks192.168.4.0 to 192.168.7.0 can be reached via either router B or C, which path is preferable? •Metrics are needed to rank the alternatives.

Metrics Hop Count Bandwidth Load Delay Reliability

Autonomous System Definition

An autonomous system is an internetwork under a common administration

AS numbers are assigned in blocks by the Internet Assigned Numbers Authority (IANA) to regional Internet registries (RIRs). The appropriate RIR then assigns AS numbers to entities within its designated area

AS No: 16 bits and 32 bits with Public and Private ranges like in IPv4http://en.wikipedia.org/wiki/Autonomous_System_(Internet)

Autonomous systems

Popular routing protocols

Link State

Link state protocols, such as IS-IS and OSPF, rely on each router in the network to advertise the state of each of their links to every other router within the local routing domain. In distance vector routing, each node shares the knowledge about the entire AS with its immediate neighbors periodically .

This result is a complete network topology map, called a shortest path tree, compiled by each router in the network.

It uses an algorithm, called the shortest path first algorithm, to build a tree with itself as the center of that tree.

Distance Vector Routers running distance vector algorithms advertise the vector (path) and distance (metric) for each destination reachable within the network to adjacent (directly connected) peers.

Once the best path is determined, these best paths are advertised to each directly connected adjacent router , not every one as in Link State

Two common algorithms used for determining the best path are Bellman-Ford, which is used by the Routing Information Protocol (RIP and RIPv2), and the Diffusing Update Algorithm (DUAL), used by the Enhanced Interior Gateway Protocol (EIGRP).

Path Vector A path vector protocol does not rely on the cost of reaching a given destination to determine whether each path available is loop free or not. Instead, path vector protocols rely on analysis of the path to reach the destination to learn if it is loop free or not.

This algorithm used is Bellman–Ford routing algorithms to avoid "Count to Infinity" problems.

http://www.informit.com/articles/article.aspx?p=331613&seqNum=2

Routing Updates

After exchanging 2 periodic updates, the network is converged.

10.1.2.010.1.1.0 10.1.3.0 10.1.4.0

.2.1 .2.1 .1.2

A B C

10.1.4.0 10.1.2.2 2 10.1.1.0 10.1.3.1 2

10.1.3.0 10.1.2.2 1 10.1.1.0 10.1.2.1 110.1.4.0 10.1.3.2 1

10.1.2.0 10.1.3.1 1

NW VIA HOP10.1.1.0 ---------- 010.1.2.0 ---------- 0

NW VIA HOP10.1.2.0 ---------- 010.1.3.0 ---------- 0

NW VIA HOP10.1.3.0 ---------- 010.1.4.0 ---------- 0

Routing Table-A Routing Table-B Routing Table-C

Fully adjacent router networkRA-RB-4RA-RC-10RA-RD-5

RD-RA-5RD-RB-3RD-RC-2

RC-RA-10RC-RD-2

RB-RA-4RB-RD-3

A

C

B

D

tn

RC-RA-10RC-RD-2

RC-RA-10 RC-RD-2

RD-RA-5RD-RB-3RD-RC-2

RD-RA-5RD-RB-3RD-RC-2RD-RA-5

RD-RB-3RD-RC-2

RB-RA-4RB-RD-3

RB-RA-4RB-RD-3

RA-RB-4RA-RC-10RA-RD-5

RA-RB-4RA-RC-10RA-RD-5

RA-RB-4RA-RC-10RA-RD-5

tn+1

RC-RA-10RC-RD-2

RB-RA-4RB-RD-3

tn+2

A B

C D

4 4

10

5

2 2

5 3

3

10

Shortest Path Tree of Router-A

A B

C D

4 4

10 5

2 2

5 3

3

A B

C D

4

5

2

10

PATH VECTOR ROUTING

Path vector routing is similar to distance Path vector routing is similar to distance vector routing. There is at least one node, vector routing. There is at least one node, called the speaker node, in each AS that called the speaker node, in each AS that creates a routing table and advertises it to creates a routing table and advertises it to speaker nodes in the neighboring ASs..speaker nodes in the neighboring ASs..

Initial routing tables in path vector routing

Stabilized tables for four autonomous systems

Administrative Distances

Diversity of metrics poses problems in routers running more than one routing protocol.

Router may learn a route to the same destination from each of the protocols

Administrative distances are the route sources to determine most preferred source

Administrative distance is a measure of believability

Administrative Distances

The administrative distance of various protocols is as below: Connected Interface - 0 Static Route - 1 OSPF - 110 RIP - 120 Unknown - 255 The lower the administrative distance, the more

believable the protocol

Packet Received

Received ARP Reply

Send ICMP error message

Discard original Packet

Header & Checksum Valid

Route Found

Route table lookup on Dest. Add.

YES

NO

Decrement TTL;TTL>=0

YES

NO

YES

NO

If route available, search MAC in ARP

cache

Default route available

NO

YES

Send ARP request and wait for a response

Build new packet with MAC address and route through

port found in routing table

MAC Address Found

YES

NO

Received ARP reply, insert MAC and IP address into

ARP table

YESNO

Flow Chart of a PacketFlow Chart of a Packet