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Week Ten. Attendance Announcements Data Center Design update Final exam 150 points Review Week Nine Information Current Week Information Upcoming Assignments Mimic Simulator Lab Assignment 4-1-2, Basic Routing and LAN Switching Configuration. Week Ten Topics. Routing Protocols

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Week ten
Week Ten

  • Attendance

  • Announcements

    Data Center Design update

    Final exam 150 points

  • Review Week Nine Information

  • Current Week Information

  • Upcoming Assignments

  • Mimic Simulator Lab Assignment 4-1-2, Basic Routing and LAN Switching Configuration

Week ten topics
Week Ten Topics

  • Routing Protocols

  • Routing Table

  • Static versus dynamic routing

  • Interior Versus Exterior Routing Protocols

  • What is convergence?

  • Autonomous Systems

  • Definitions

  • Loop Free Path

Routing protocols
Routing Protocols

  • One of the primary jobs of a router is to determine the best path to a given destination

  • A router learns paths, or routes, from the staticconfiguration entered by an administrator or dynamically from other routers, through routing protocols

Routing table principles
Routing Table Principles

Three principles regarding routing tables:

  • Every router makes its decisions alone, based on the information it has in its routing table.

  • Different routing table may contain different information

  • A routing table can tell how to get to a destination but not how to get back (Asymmetric Routing)

Routing table objective
Routing Table Objective

  • To forward a packet towards its destination network, which is the destination IP address of the packet.

  • To do this, a router needs to search the routing table for this stored information.

Routing tables
Routing Tables

  • Routers keep a routing table in RAM

  • A routing table is a list of the best known available routes

  • Routers use this table to make decisions about how to forward a packet

  • On a Cisco router, the show IP route command is used to view the TCP/IP routing table

  • A routing table maps network prefixes to an outbound interface.

Routing table
Routing Table

  • When RTA receives a packet destined for, it looks for the prefix in the routing table

  • RTA then forwards the packet out an interface, such as Ethernet0, as directed in the routing table

Routing loops
Routing Loops

  • A routing loop is a network problem in which packets continue to be routed in an endless circle

  • It is caused by a router or line failure, and the notification of the downed link has not yet reached all the other routers

  • It can also occur over time due to normal growth or when networks are merged together

  • Routing protocols utilize various techniques to lessen the chance of a routing loop

Routing table specifics1
Routing Table Specifics

  • Directly connected networks-this occurs when a device is connected to another router interface

  • Remotely connected networks-this is a network that is not directly connected to a particular router network/next hop associations-about the networks include source of information, network address & subnet mask, and Ip address of next-hop router

  • The show ip route command is used to view a routing table on a Cisco router

Routing protocols1
Routing Protocols

  • Directly Connected Routes-To visit a neighbor, you only have to go down the street on which you already live. This path is similar to a directly-connected route because the "destination" is available directly through your "connected interface," the street.

Static routing
Static Routing

  • Static Connected Routes-A train uses the same railroad tracks every time for a specified route. This path is similar to a static route because the path to the destination is always the same.

Application for static routing
Application For Static Routing

  • When network only consists of a few routers

    Using a dynamic routing protocol in such a case does not present any substantial benefit.

  • Network is connected to Internet only through one ISP

    There is no need to use a dynamic routing protocol across this link because the ISP represents the only exit point to the Internet

Application for static routing1
Application For Static Routing

  • Static routing is not suitable for large, complex networks that include redundant links, multiple protocols, and meshed topologies

    Routers in complex networks must adapt to topology changes quickly and select the best route from multiple candidates

  • When an interface goes down, all static routes mapped to that interface are removed from the IP routing table

Application for static routing2
Application For Static Routing

  • Hub and spoke topology is used on a large network

    A hub-and-spoke topology consists of a central location (the hub or switch) and multiple branch locations (spokes), with each spoke having only one connection to the hub or switch.

    Using dynamic routing would be unnecessary because each branch has only one path to a given destination-through the central location.

  • Static routing is useful in networks that have a single path to any destination network.

Static routes in routing table
Static Routes in Routing Table

  • Includes: network address and subnet mask and IP address of next hop router or exit interface

  • Denoted with the code S in the routing table

  • Routing tables must contain directly connected networks in order to connect remote networks, before static or dynamic routing can be used

Static routing2
Static Routing

  • The corporate network router has only one path to the network connected to RTY

  • A static route is entered on RTZ

Routing protocols2
Routing Protocols

  • Dynamic Routes-When driving a car, you can "dynamically" choose a different path based on traffic, weather, or other conditions. This path is similar to a dynamic route because you can choose a new path at many different points on your way to the destination.

Dynamic routing protocol
Dynamic Routing Protocol

  • Are used to add remote networks to a routing table

  • Are used to discover networks

  • Are used to update and maintain routing tables

Dynamic routing protocol1
Dynamic Routing Protocol

  • Automatic network discovery

  • Network discovery is the ability of a routing protocol to share information about the networks that it knows about with other routers that are also using the same routing protocol.

  • Dynamic routing protocols are used to share routing information with other routers and to maintain an up-to-date routing table.

  • Dynamic routing protocols not only make a best path determination to various networks, they will also determine a new best path if the initial path becomes unusable (or if the topology changes)

Configuring dynamic routing protocols
Configuring Dynamic Routing Protocols

Dynamic routing of TCP/IP can be implemented using one or more protocols which are often grouped according to where they are used.

  • Routing protocols designed to work inside an autonomous system are categorized as interior gateway protocols (IGPs).

  • Protocols that work between autonomous systems are classified as exterior gateway protocols (EGPs).

  • Protocols can be further categorized as either distance vector or link-state routing protocols, depending on their method of operation.

Interior versus exterior routing protocols
Interior Versus Exterior Routing Protocols

An interior gateway protocol (IGP) is a routing protocol that is used within an autonomous system (AS). Two types of IGP.

Distance-vector routing protocols each router does not possess information about the full network topology. It advertises its distances to other routers and receives similar advertisements from other routers. Using these routing advertisements each router populates its routing table. In the next advertisement cycle, a router advertises updated information from its routing table. This process continues until the routing tables of each router converge to stable values.

Interior versus exterior routing protocols1
Interior Versus Exterior Routing Protocols

Distance-vector routing protocols make routing decisions based on hop-by-hop. A distance vector router’s understanding of the network is based on its neighbors definition of the topology, which could be referred to as routing by RUMOR.

Route flapping is caused by pathological conditions (hardware errors, software errors, configuration errors, intermittent errors in communications links, unreliable connections, etc.) within the network which cause certain reach ability information to be repeatedly advertised and withdrawn.

Interior versus exterior routing protocols2
Interior Versus Exterior Routing Protocols

In networks, with distance vector routing protocols flapping routes can trigger routing updates with every state change.

Cisco trigger updates are sent when these state changes occur. Traditionally, distance vector protocols do not send triggered updates.

Interior versus exterior routing protocols3
Interior Versus Exterior Routing Protocols

Link-state routing protocols, each node possesses information about the complete network topology. Each node then independently calculates the best next hop from it for every possible destination in the network using local information of the topology. The collection of best next hops forms the routing table for the node.

This contrasts with distance-vector routing protocols, which work by having each node share its routing table with its neighbors. In a link-state protocol, the only information passed between the nodes is information used to construct the connectivity maps.

Routing protocols3
Routing Protocols

  • Interior routing protocols are designed for use in a network that is controlled by a single organization

  • RIPv1RIPv2, EIGRP, OSPF and IS-IS are all Interior Gateway Protocols

Link state analogy
Link State Analogy

  • Each router has a map of the network

  • Each router looks at itself as the center of the topology

  • Compare this to a “you are here” map at the mall

  • The map is the same, but the perspective depends on where you are at the time You

Link state routing protocol
Link State Routing Protocol

  • The link-state algorithm is also known as Dijkstra's algorithm or as the shortest path first (SPF) algorithm

  • The link-state routing algorithm maintains a complex database of topology information

  • The link-state routing algorithm maintains full knowledge of distant routers and how they interconnect. They have a complete picture of the network

Exterior gateway routing protocol
Exterior Gateway Routing Protocol

An exterior routing protocol is designed for use between different networks that are under the control of different organizations

  • An exterior routing routes traffic between autonomous systems

  • These are typically used between ISPs or between a company and an ISP

  • BGPv4is the Exterior Gateway Protocol used by all ISPs on the Internet

What is convergence
What is Convergence

  • Routers share information with each other, but must individually recalculate their own routing tables

  • For individual routing tables to be accurate, all routers must have a common view of the network topology

  • When all routers in a network agree on the topology they are considered to have converged

Why is quick convergence important
Why is Quick Convergence Important?

  • When routers are in the process of convergence, the network is susceptible to routing problems because some routers learn that a link is down while others incorrectly believe that the link is still up

  • It is virtually impossible for all routers in a network to simultaneously detect a topology change.

Convergence issues
Convergence Issues

Factors affecting the convergence time include the following:

  • Routing protocol used

  • Distance of the router, or the number of hops from the point of change

  • Number of routers in the network that use dynamic routing protocols

  • Bandwidth and traffic load on communications links

  • Load on the router

  • Traffic patterns in relation to the topology change

What are autonomous systems
What are Autonomous Systems?

  • An Autonomous System (AS) is a group of routers that share similar routing policies and operate within a single administrative domain.

  • An AS can be a collection of routers running a single IGP, or it can be a collection of routers running different protocols all belonging to one organization.

  • In either case, the outside world views the entire Autonomous System as a single entity.

Autonomous system
Autonomous System

AS Numbers

  • Each AS has an identifying number that is assigned by an Internet registry or a service provider.

  • This number is between 1 and 65,535.

  • AS numbers within the range of 64,512 through 65,535are reserved for private use.

  • This is similar to RFC 1918 IP addresses.

  • Because of the finite number of available AS numbers, an organization must present justification of its need before it will be assigned an AS number.

  • An organization will usually be a part of the AS of their ISP

Autonomous system2
Autonomous System

  • Each AS has its own set of rules and policies.

  • The AS number uniquely distinguish it from other ASs around the world.


Metric is a numeric value used by routing protocols to help determine the best path to a destination.

RIP uses the metric hop count number . The lower the numeric value, the closer the destination.

OSPF uses the metric bandwidth.

EIGRP uses bandwidth


  • Flat routing protocol is when all routing information is spread through the entire network.

  • Hierarchical routing protocol are typically classless link-state protocols. This means that classless means that routing updates include subnet masks in their routing updates. Draw diagram

  • Administrative distance is the measure used by Cisco routers to select the best path when there are two or more different routes to the same destination from two different routing protocols. Administrative distance defines the reliability of a routing protocol. Each routing protocol is prioritized in order of most to least reliable (believable) using an administrative distance value. A lower numerical value is preferred.

Eigrp characteristics
EIGRP Characteristics

EIGRP is an advanced distance vector protocol that employs the best features of link-state routing.

Ospf characteristics
OSPF Characteristics

OSPF is the standardized protocol for routing IPv4. Since it’s initial development, OSPF has been revised to be implemented with the latest router protocols.

  • Developed for large networks (50 routers or more)

  • Must be a backbone area

  • Routers that operate on boundaries between the backbone and non-backbone are called, Area Border Routers (ABR)

  • OSPF is a link state protocol

Ospf characteristics1
OSPF Characteristics

When the OSPF topology table is fully populated, the SPF algorithm calculates the shortest path to the destination. Triggered updates and metric calculation based on the cost of a specific link ensure quick selection of the shortest path to the destination.

Ospf characteristics2
OSPF Characteristics

OSPF is link-state routing protocol

RIP and EIGRP are distance-vector (routing by rumor) routing protocols, susceptible to routing loops, split-horizon, and other issues.

OSPF has fast convergence

RIP hold-down timers can cause slow convergence.

OSPF supports VLSM and CIDR

RIPv1 does not

Ospf characteristics3
OSPF Characteristics

  • Cisco’s OSPF metric is based on bandwidth

  • RIP is based on hop count

  • OSPF only sends out changes when they occur.

  • RIP sends entire routing table every 30 seconds, IGRP every 90 seconds

  • OSPF also uses the concept of areas to implement hierarchical routing

  • A large internetwork can be broken up into multiple areas for management and route summarization


  • Two open-standard routing protocols to choose from:

    RIP, simple but very limited, or

    OSPF, robust but more sophisticated to implement.

    EIGRP is Cisco proprietary


Autonomous System Boundary Router (ASBR) is an area border router placed between an OSPF autonomous system and a non-OSPH network that operates both OSPF and an additional routing protocol, such as RIP. ASBRs must be located in a non-stub OSPF area.

Ospf characteristics5
OSPF Characteristics

When all routers are configured into a single area, the convention is to use area 0(zero)

If OSPF has more than one area, it must have an area 0

Multi-area OSPF becomes more complicated to configure and understand

OSPF Routing Domain

  • Single Area OSPF uses only one area, usually Area 0

Ospf characteristics6
OSPF Characteristics

1. Flooding of link-state information

The first thing that happens is that each node, router, on the network announces its own piece of link-state information to other all other routers on the network. This includes who their neighboring routers are and the cost of the link between them.

Example: “Hi, I’m Router A, and I can reach Router B via a T1 link and I can reach Router C via an Ethernet link.”

Each router sends these announcements to all of the routers in the network.

Ospf characteristics8
OSPF Characteristics

2. Building a Topological Database

Each router collects all of this link-state information from other routers and puts it into a topological database.

3. Shortest-Path First (SPF), Dijkstra’s Algorithm

Using this information, the routers can recreate a topology graph of the network.

Believe it or not, this is actually a very simple algorithm and I highly suggest you look at it some time, or even better, take a class on algorithms.

Ospf characteristics9
OSPF Characteristics

4. Shortest Path First Tree

This algorithm creates an SPF tree, with the router making itself the root of the tree and the other routers and links to those routers, the various branches.

5. Routing Table

Using this information, the router creates a routing table.

Large ospf networks
Large OSPF Networks

Large link-state table

Each router maintains a LSDB for all links in the area

The LSDB requires the use of memory

Frequent SPF calculations

A topology change in an area causes each router to re-run SPF to rebuild the SPF tree and the routing table.

A flapping link will affect an entire area.

SPF re-calculations are done only for changes within that area.

Issues with large ospfnetworks
Issues with large OSPFNetworks

Large routing table

Typically, the larger the area the larger the routing table.

A larger routing table requires more memory and takes more time to perform the route look-ups.

Solution: Divide the network into multiple areas

Link-State Advertisement (LSA) is contained inside the link-state packets (LSPs), these advertisements are usually multicast packets, containing information about neighbors and path costs, that are employed be link-state protocols. Receiving routers use LSAs to maintain their link-state databases and, ultimately, routing tables.

Ospf uses areas
OSPF Uses “Areas”

Hierarchical routing enables you to separate large internetworks (autonomous systems) into smaller internetworks that are called areas.

With this technique, routing still occurs between the areas (called inter-area routing), but many of the smaller internal routing operations, such as recalculating the database –re-running the SPF algorithm, are restricted within an area

Ospf uses areas1
OSPF Uses “Areas”

Changes in one area are generally not propagated (spread) to another

Route summarization is extensively used in multi-area OSPF

Ospf router types1
OSPF Router Types

Internal: Routers with all their interfaces within the same area

Backbone: Routers with at least one interface connected to area 0

ASBR:(Autonomous System Boundary Router): Routers that have at least one interface connected to an external internetwork (another autonomous system)

ABR: (Area Border Router): Routers with interfaces attached to multiple areas.

Is is characteristics
IS - IS Characteristics

  • IS-IS is an Open System Interconnection (OSI) routing protocol originally specified by International Organization for Standardization (ISO)

  • IS-IS is a dynamic, link-state, intra-domain, interior gateway protocol (IGP)

  • IS-IS was designed to operate in an OSI Connectionless Network Service (CLNS) environment

  • It was not originally designed to work with the IP protocol

Is is characteristics1
IS - IS Characteristics

  • Extensions were added so that IS-IS can route IP packets

  • IS-IS operates at Layer 3 (Network) of the OSI model

  • IS-IS selects routes based upon a cost metric assigned to links in the IS-IS network

  • A two-level hierarchy is used to support large routing domains

  • A large domain can be administratively divided into areas

Ospf and is is similarities
OSPF and IS – IS Similarities

  • Classless

  • Link-state databases an Dijkstra’s algorithm

  • Hello packets to form and maintain adjacencies

  • Use areas to form hierarchical topologies

  • Support address summarization between areas

  • Link-state representation, aging, and metrics

  • Update, decision, and flooding processes

  • Convergence capabilities

  • Deployed on ISP backbones

Is is and the osi protocol suite
IS – IS and the OSI Protocol Suite

  • The OSI suite of protocols were never widely implemented at the Layers 3-7 because the TCP/IP Protocols at these layers became the de-facto standard.

  • Layers 1 and 2 Protocols are widely used: IEEE 802.3, FDDI, IEEE 802.5, etc.

Link state routing analogy
Link State Routing Analogy

The Link State Database is similar to a map in a shopping mall. All maps in the mall is the same, just as the LSDB is the same in all routers within an area. The only real physical difference in the maps is “you are at” what dot. By looking at your position in relation to where you want to go, you can determine the best way to get to your destination. Link state routers calculate the best path to every network within the area, from their own perspective.

Osi terminology
OSI Terminology

  • End system (ES) is any non-routing network node (host)

  • Intermediate system (IS) is a router

  • An area is a logical entity formed by a set of contiguous routers, hosts, and the data links that connect them

  • Domain is a collection of connected areas under a common administrative authority(think AS)

  • The areas are connected to form a backbone

Is is is designed to be hierarchical
IS – IS is Designed to be Hierarchical

An OSI network is a hierarchy of these entities:

  • Domain -any portion of an OSI network under a common administration

  • Area –a part of a domain, broken up for easier management

  • Backbone –areas connect to other areas through the backbone

Is is is hierarchical
IS – IS is Hierarchical

There are four levels of routing:

  • Level 0, routing between an ES and IS

  • Level 1, routing between ISs in the same area

  • Level 2, routing between different areas in the same domain

  • Level 3, routing between separate domains

Why use is is instead of ospf
Why use IS – IS instead of OSPF?

  • IS-IS is more scalable than OSPF because it uses smaller LSPs for advertisements

  • Up to 1000 routers can reside in an IS-IS area versus several hundred for OSPF

  • IS-IS is more efficient with its updates and requires less CPU power

  • IS-IS has more timers that can be fine-tuned to speed up convergence

Eigrp characteristics1
EIGRP Characteristics

  • Cisco proprietary, released in 1994

  • EIGRP is an advanced distance-vector routing protocol that relies on features commonly associated with link-state protocols. (sometimes called a hybrid routing protocol)

  • Supports VLSM and CIDR

  • Uses multicasts for communication –not broadcasts

  • Establishes adjacencies with its neighbor routers by using a Hello protocol

  • Keeps all routes in a topology table

  • Has speed and efficiency of routing updates like a link-state protocol

Eigrp metric calculation
EIGRP Metric Calculation

By default, EIGRP uses only these:

  • Bandwidth (carrying capacity)

  • Delay (end-to-end travel time)

    If these are the default:

  • Bandwidth (default)

  • Delay (default)

    When are these used?

  • load

  • Reliability

    These values are used when the administrator manually enters them

Eigrp terminology
EIGRP Terminology

  • EIGRP uses DUAL, the Diffusing Update Algorithm to calculate routes –not Bellman-Ford algorithm.

  • The lowest cost path to a destination is called the feasible distance (FD)

  • The cost of the route as advertised by the neighboring router, is called reported distance (RD)

  • The best (primary) route to a destination is called the successor route (successor)

  • The next best route, (backup), if there is one, is called the feasible successor (FS)

Eigrp tables
EIGRP Tables

The following three tables are maintained by EIGRP:

  • Neighbor table

  • Topology table

  • Routing table

Week ten

BGP is a path vector routing protocol.

Defined in RFC 1772

BGP is a distance vector routing protocol, in that it relies on downstream neighbors to pass along routes from their routing table.

BGP uses a list of AS numbers through which a packet must pass to reach a destination.

Bgp basics
BGP Basics

  • Exchange routing information between autonomous systems

  • Guarantee the selection of a loop free path.

  • BGP4 is the first version of BGP that supports CIDR and route aggregation.

  • Common IGPs such as RIP, OSPF, and EIGRP use technical metrics.

  • BGP does not use technical metrics.

  • BGP makes routing decisions based on network policies, or rules (later)

  • BGP does not show the details of topologies within each AS.

  • BGP sees only a tree of autonomous systems.

Bgp basics1
BGP Basics

  • BGP updates are carried using TCP on port 179.

    In contrast, RIP updates use UDP port 520

    OSPF, IGRP, EIGRP does not use a Layer 4 protocol

  • Because BGP requires TCP, IP connectivity must exist between BGP peers.

  • TCP connections must also be negotiated between them before updates can be exchanged.

  • Therefore, BGP inherits those reliable, connection-oriented properties from TCP.

Loop free path
Loop Free Path

To guarantee loop free path selection, BGP constructs a graph of autonomous systems based on the information exchanged between BGP neighbors.

BGP views the whole internetwork as a graph, or tree, of autonomous systems.

The connection between any two systems forms a path.

The collection of path information is expressed as a sequence of AS numbers called the AS Path.

This sequence forms a route to reach a specific destination

Bgp operation
BGP Operation

When two routers establish a TCP-enabled BGP connection between each other, they are called neighbors or peers.

Each router running BGP is called a BGP speaker.

Upcoming deadlines
Upcoming Deadlines

  • Assignment 1-4-3 Data Center Design ProjectPhase 3: Data Center Network Design is due December 8, 2010.

  • Assignement 10-1 Concept Questions 7 is due November 17,2010.