Week Ten Attendance Announcements Data Center Design update Final exam 150 points Review Week Nine Information Current Week Information Upcoming Assignments

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

1. Routing Protocols

2. Routing Table

3. Static versus dynamic routing

4. Interior Versus Exterior Routing Protocols5. What is convergence?

6. Autonomous Systems

7. Definitions

8. Loop Free Path

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 static configuration entered by an administrator or dynamically from other routers, through routing protocols

Routing Table PrinciplesThree principles regarding routing tables:

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

2. Different routing table may contain different information

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

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• 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 Specifics

Routing Table

• When RTA receives a packet destined for 192.168.4.46, it looks for the prefix 192.168.4.0/24 in the routing table

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

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


• 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 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 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 • 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 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 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

• 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 Routing

Static Routing

• The corporate network router has only one path to the network 172.24.4.0 connected to RTY

• A static route is entered on RTZ

Static Routing

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

• 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 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)

Dynamic Routing Protocol

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

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.


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.


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.


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 Protocols

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

• RIPv1 RIPv2, EIGRP, OSPF and IS-IS are all Interior Gateway Protocols

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• 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

Link State Analogy

Distant Vector Versus Link State

Distant Vectors Routing Protocols Link State Routing Protocols

RIP (v1 and v2) OSPF

EIGRP (hybrid) IS - IS

Exterior Gateway Routing ProtocolAn 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

EGI and EGP Routing Protocol

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?

• 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

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?• 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 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 System

Autonomous System

• Each AS has its own set of rules and policies.• The AS number uniquely distinguish it from

other ASs around the world.

Definitions

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

Definitions• 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.

Administrative Distance

EIGRP Characteristics

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

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


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 CharacteristicsOSPF is link-state routing protocolRIP and EIGRP are distance-vector (routing by rumor) routing protocols, susceptible to routing loops, split-horizon, and other issues.OSPF has fast convergenceRIP hold-down timers can cause slow convergence.OSPF supports VLSM and CIDRRIPv1 does not


• 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

OSPFCharacteristics

• 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

OSPFCharacteristics

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 CharacteristicsWhen 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 Characteristics1. 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 Characteristics2. 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 Characteristics4. 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 NetworksLarge link-state table

Each router maintains a LSDB for all links in the areaThe LSDB requires the use of memory

Frequent SPF calculationsA 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 OSPFNetworksLarge 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” 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 “Areas”

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

Route summarization is extensively used in multi-area OSPF

OSPF Router Types

OSPF Router TypesInternal: 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 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 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• 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• 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 AnalogyThe 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• 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

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 HierarchicalThere 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

IS – IS is Hierarchical

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 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

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• ReliabilityThese values are used when the administrator manually

enters them

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 TablesThe following three tables are maintained by

EIGRP:• Neighbor table • Topology table • Routing table

BGP

BGP is a path vector routing protocol.Defined in RFC 1772BGP 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

• 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 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

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

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• Assignment 1-4-3 Data Center Design Project

Phase 3: Data Center Network Design is due December 8, 2010.

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

Documents

Week Ten Attendance Announcements Data Center Design update Final exam 150 points Review Week Nine Information Current Week Information Upcoming Assignments