Layer 3 Routing Protocols - Rab Nawaz Jadoon · 2013-12-09 · Department of Computer Science 3 Routing Delivery Delivery refers to the way a packet is handled by the underlying networks

Department of Computer Science

DCS

COMSATS Institute of Information Technology

Layer 3 Routing Protocols

Rab Nawaz JadoonAssistant Professor

COMSATS IIT, Abbottabad

Pakistan

Telecom Network Design

Department of Computer Science 2

Network Layer


Routing

Delivery

Delivery refers to the way a packet is handled by the underlying networks under the control of the network layer.

Forwarding

Forwarding refers to the way a packet is delivered to the next station.

Routing

Routing refers to the way routing tables are created to help in forwarding.


Routing

Direct and indirect delivery

Direct Delivery Indirect and Direct Delivery


Routing at Network Layer

Forwarding Forwarding requires a host or a router to have a

routing table.

When a host has a packet to send or when a router has received a packet to be forwarded, it looks at this table to find the route to the final destination.

Forwarding Techniques Several techniques can make the size of the routing

table manageable, Next-Hop Method vs Route Method

Network-Specific vs Host specific method





Host-specific versus network-specific method



Default Method

Host A is connected to a network with two routers.

Router R1 routes the packets to hosts connected to network N2.

So instead of listing all networks in the entire Internet, host A can just have one entry called the default



Routing Table

Static Routing Table

(A static routing table contains information entered manually)

Dynamic Routing Table

A dynamic routing table is updated periodically by using one of the dynamic routing protocols such as RIP, OSPF, or BGP.

Automatic updating of routing tables.


Unicast routing protocols

Routing tables need to be updated when a router is down, and they need to be updated whenever a better route has been found. Routing protocols have been created in response to

the demand for dynamic routing tables.

A routing protocol is a combination of rules and procedures that lets routers in the internet inform each other of changes.

It allows routers to share whatever they know about the internet or their neighborhood.

The routing protocols also include procedures for combining information received from other routers.



Optimization

A router is usually attached to several networks.

When it receives a packet, to which network should it pass the packet?

The decision is based on optimization: optimum route selection.

One approach is to assign a cost for passing through a network, called metric.

The metric assigned to each network depends on the type of protocol.


Routing

Intra- and Interdomain Routing

Today, an internet can be so large that one routing protocol cannot handle the task of updating the routing tables of all routers.

For this reason, an internet is divided into autonomous systems (AS).

An autonomous system (AS) is a group of networks and routers under the authority of a single administration.



Intradomain

Routing inside an autonomous system is referred to as intradomain routing.

Interdomain

Routing between autonomous systems is referred to as interdomain routing.


Autonomous system


Routing Protocols

Popular routing protocols


Distance Vector Routing


In distance vector routing, the least-cost route between any two nodes is the route with minimum distance.

Each node maintains a vector (table) of minimum distances to every node.

Routing tables maintains the next hop entry to forward the packet.



----C

---A-

-AA-A

---A-

C--C-



In distance vector routing, each node shares its routing table with its immediate neighbors periodically and when there is a change.

Initialization



Table Updating



When to Share?

Periodic Update

A node sends its routing table, normally every 30 s, in a periodic update.

Triggered Update

Anytime there is a change in its routing table.



Two-Node Loop Instability

X 6 AX-6-AX-2-

X-6-AX-ὲ-

X-10-A X-6-A

X-10-A X-14-A

X-ὲ- X-ὲ-



Some solution

Defining infinity

Split Horizon

Split Horizon and Poison Reverse


RIP

RIP (Routing Information Protocol)

An Interdomain routing protocol used inside an autonomous system.

Based on distance vector routing.

RIP implements distance vector routing directly with some consideration

In an autonomous system, we are dealing with routers and networks (links). The routers have routing tables; networks do not.

The destination in a routing table is a network, which means the first column defines a network address.


RIP

The metric in RIP is called a hop count.

Infinity is defined as 16, which means that any route in an autonomous system using RIP cannot have more than 15 hops.

The next-node column defines the address of the router to which the packet is to be sent to reach its destination.


RIP

Example of a domain using RIP


Link State Routing

In link state routing, each node in the domain has the entire topology of the domain. The node can use Dijkstra's algorithm to build a

routing table.


Link State Routing

Each node uses the same topology to create a routing table, but the routing table for each node is unique because the calculations are based on different interpretations of the topology.

This is analogous to a city map.

Each person may have the same map, each needs to take a different route to reach her specific destination.


Link State Routing

If there are changes in any point in the network, The topology must be updated for each node.

Building Routing Tables In link state routing, four sets of actions are required

Routing table showing the least-cost node to every other node. Creation of the link state packet (LSP).

Flooding of LSPs

Formation of a shortest path tree for each node.

Calculation of a routing table based on the shortest path tree.


Link State Routing

LSPs are generated on two occasions:

When there is a change in the topology of the domain.

On a periodic basis.

The period is much longer than Distance vector.

Flooding of LSPs

The creating node sends a copy of the LSP out of each interface.

Older LSPs are discarded, new are kept. This is done through sequence numbers included in LSP.

Flood the information out of each interface except the one from which the packet arrives.

This ensures that flooding stop somewhere in domain.


Link State Routing

Formation of Shortest Path Tree: Dijkstra Algorithm

After receiving all LSPs, each node will have a copy of the whole topology.

A shortest path tree is needed.

A tree is a graph of nodes and links; one node is called the root.

A shortest path tree is a tree in which the path between the root and every other node is the shortest.


Link State Routing


Link State Routing

Calculation of Routing Table from Shortest Path Tree

Each node uses the shortest path tree protocol to construct its routing table.

The routing table shows the cost of reaching each node from the root.


OSPF

Open shortest Path First An intradomain routing protocol based on link state

routing.

Areas An area is a collection of networks, hosts, and routers all

contained within an autonomous system.

An autonomous system can be divided into many different areas.

Routers inside an area flood the area with routing information.

At the border of an area, special routers called area border routers summarize the information about the area and send it to other areas.


OSPF

Among the areas inside an autonomous system is a special area called the backbone;

All the areas inside an autonomous system must be connected to the backbone.

the backbone serves as a primary area.

The routers inside the backbone are called the backbone routers.

A backbone router can also be an area border router.

Each area has an area identification.

The area identification of the backbone is zero.


OSPF

Area in Autonomous system


OSPF

Metric The OSPF protocol allows the administrator to assign

a cost, called the metric, to each route.

The metric can be based on a type of service (minimum delay, maximum throughput, and so on).

Types of Links In OSPF terminology, a connection is called a link.

Four types of links, Point to point

Transient

Stub

virtual


OSPF

Point to point link

4 4

Transient link

A

C

B

E D


OSPF

If we want to show the neighborhood of previous figure the relationship in this situation,

A

C

B

E

D

Unrealistic representation


OSPF

Realistic representation

A

C

B

E

D

Designated Router


OSPF

Stub link

A stub link is a network that is connected to only one router.

The data packets enter the network through this single router and leave the network through this same router.

This is a special case of the transient network. We can show this situation using the router as a node and using the designated router for the network.


OSPF

Virtual Link

When the link between two routers is broken, the administration may create a virtual link between them, using a longer path that probably goes through several routers.


Choosing Between Distance-Vector and Link-State Protocols

According to Cisco design documents, you can use the following guidelines to help you decide which type of routing protocol to deploy?

Choose distance-vector protocols when

The network uses a simple, flat topology and does not require a hierarchical design.

The network uses a simple hub-and-spoke topology.

The administrators do not have enough knowledge to operate and troubleshoot link state protocols.

Worst-case convergence times in the network are not a concern.

42


Cont…

Choose link-state protocols when

The network design is hierarchical, which is usually the case for large networks.

The administrators are knowledgeable about the selected link-state protocol.

Fast convergence of the network is crucial.

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Interdomain Routing

Path Vector Routing

In path vector routing, we assume that there is one node called speaker node in each autonomous system that acts on behalf of the entire autonomous system.

The speaker node in an AS creates a routing table and advertises it to speaker nodes in the neighboring ASs.

The idea is the same as for distance vector routing except that only speaker nodes in each AS can communicate with each other.


Path Vector Routing

The advertisement is also different in this case,

A speaker node advertises the path, not the metric of the nodes, in its autonomous system or other autonomous systems.

Initialization

At the beginning, each speaker node can know only the reachability of nodes inside its autonomous system.


Path Vector Routing

Initial routing tables in path vector routing

Speaker NodeA1 for AS1B1 for AS2C1 for AS3D1 for As4


Path Vector Routing

Sharing In path vector routing, a speaker in an autonomous

system shares its table with immediate neighbors.

When a speaker node receives a two-column table from a neighbor, it updates its own table by adding the nodes that are not in its routing table and adding its own autonomous system and the autonomous system that sent the table.

According to the figure, if router A1 receives a packet for nodes A3, it knows that the path is in AS1 (the packet is at home);

But if it receives a packet for D1, it knows that the packet should go from AS 1, to AS2, and then to AS3.


Path Vector Routing

Stabilized tables for three autonomous systems


BGP

Border Gateway Protocol

Border Gateway Protocol (BGP) is an interdomainrouting protocol using path vector routing.

It first appeared in 1989 and has gone through four versions.

We can divide autonomous systems into three categories:

Stub

Multihomed

Transit.


BGP

Stub AS A stub AS has only one connection to another AS.

Multihomed AS. A multihomed AS has more than one connection to

other Ass.

It does not allow data coming from one AS and going to another AS to pass through.

It does not allow transient traffic.

Transient AS A transit AS is a multihomed AS that also allows

transient traffic.

E.g. national and international ISPs (Internet backbones).


BGP Session

External and Internal BGP

BGP can have two types of sessions:

External BGP (E-BGP)

The E-BGP session is used to exchange information between two speaker nodes belonging to two different autonomous systems.

Internal BGP (I-BGP)

It is used to exchange routing information between two routers inside an autonomous system.

A1B1AS1 AS2

--- I-BGPE-BGP



Switch

A switch refers to a device that operates at Layers 1 and 2 of the OSI reference model.

The term switch does have a more generic meaning, though.

Switching means to move something to a different position.

An internetworking device moves data that comes in one interface to a different interface.

A switch permits or interrupts the flow of data and, although it doesn’t turn a locomotive, it does turn bits, frames, and packets.

53


Switch vs bridge

Switches take advantage of fast integrated circuits to offer low latency.

Bridges were much slower than switches and had fewer ports and a higher cost per port.

For these reasons, switches have replaced bridges these days.

Switches have the capability to do store-and-forward processing or cut-through processing.

A disadvantage with cut-through processing is that it forwards illegal frames (for example, Ethernet runts) and frames with CRC errors.

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Cont…

On a network that is prone to runts and errors, cut-through processing should not be used.

Some switches have the capability to automatically move from cut-through mode to store-and-forward mode when an error threshold is reached.

This feature is called adaptive cut-through switching by some vendors.

A switch also supports parallel forwarding, whereas bridges usually do not.

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Cont…

There is only one forwarding path in a bridge.

A switch, on the other hand, allows multiple, parallel forwarding paths, which means a switch can handle a high volume of traffic more quickly than a bridge.

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Documents

Layer 3 Routing Protocols - Rab Nawaz Jadoon · 2013-12-09 · Department of Computer Science 3 Routing Delivery Delivery refers to the way a packet is handled by the underlying networks