Chapter 8 Routing. Introduction Look at: –Routing Basics (8.1) –Address Resolution (8.2) –Routing Protocols (8.3) –Administrative Classification (8.4)

Chapter 8

Routing

Introduction

• Look at:– Routing Basics (8.1)– Address Resolution (8.2)– Routing Protocols (8.3)– Administrative Classification (8.4)– Hierarchical Routing (8.5)

Introduction

• When networking was first introduced, only a small number of devices were interconnected

• As networks grew, broadcasts and collisions caused significant issues

• In order to reduce broadcasts, Layer 3 devices, protocols, and addresses are used to segment the network

Routing Basics

• Reasons to segment a network at Layer 3 include:– Creation of small troubleshooting areas– Creation of small administrator

management areas– Interconnection of remote offices using

WAN technologies– Grouping clients together with similar

network resources

Routing Basics

• A router is required to properly forward data between clients on different segments.

• A router is a device that forwards data based on a logical Layer 3 address

• Many routers support the use of different protocols

Routing Basics

• The routing process usually occurs between physical network interfaces but can also be accomplished between logical network interfaces

• When a single physical network interface has more than one address assigned, it is said to have a logical interface

• A router can be any network device with the proper software to make routing decisions

Routing Basics

• Two key pieces of information are required for any device to route packets:– A route to one or more networks– A destination Layer 3 address

• The routes to a given network are stored in the memory of the router and are referred to as a routing table

Routing Basics

• On a Router there is an entry for each network and its corresponding network interface kept in memory

• The table allows the router to properly forward frames out the correct interface

• This occurs once the frame is received and the destination address has been processed

Routing Basics

• In standard routing, the route table is consulted every time a frame is received and so it plays a fundamental role in the proper delivery of data

• A routing table only maintains the best possible route to a destination, not all possible routes

Routing Basics

• Routing table entries have the following functions:– Network destination– Netmask– Gateway– Interface – Metric

Routing Basics

• The task of populating the routing table is accomplished by using either dynamic routing or static routing

• Dynamic routing uses routing protocols to build route tables automatically

• Static routing requires manual route table entries and updates to different networks

Routing Basics

• Layer 3 addressing is critical for end-to-end reachability and does not change throughout the routing process

• The Layer 2  address moves the packet from one hand-off point or hop to the next

• The router’s Layer 2 address is the frame’s destination

• The Layer 3 address remains constant through each hop

Address Resolution

• Address resolution is the mapping of one address to another

• It is generally a mapping between a Layer 3 network address (logical) and a Layer 2 hardware address (physical)

• The reverse process is also address resolution

Address Resolution

• Address resolution is accomplished in one of the following ways: – Table lookup is a rarely used method of

address resolution  – Closed-form computation is only used in

very specific networks and is time consuming to configure

– Dynamic message exchange is the most common and involves an exchange of information between two hosts

Address Resolution

• Address Resolution Protocol (ARP) is used when an IP host has a known destination IP address (Layer 3) and it needs to retrieve the corresponding Layer 2 MAC address from the destination host

• The ARP cache is used to further reduce the need for broadcasts by storing the IP-to- MAC mapping in memory for a specified duration

Address Resolution

• The ARP Process:– Client A sends out an ARP broadcast – All clients receive and process the

broadcast frame but only Machine B responds

– Client A receives the response and places Machine B’s MAC address in its ARP cache

Address Resolution

• ARP locates the Layer 2 address when the Layer 3 address is known

• Reverse Address Resolution Protocol (RARP) finds the Layer 3 address when the Layer 2 address is known

• A good example of RARP is found in TCP/IP address reservations and the Boot Protocol (BootP)

• Using BootP, IP hosts are automatically assigned their configuration information through a BootP server

Routing Protocols

• Dynamic routing using routing protocols

• Purpose of routing protocols is to build a routing table with the best routes

• Routing protocols are categorized into two types:– Distance Vector– Link State

Routing Protocols

• Distance vector routing protocols are simple • Generally they are easy to configure• They use simple logic to determine the best

path to a given destination• The term metric refers to the method or

measurement used by the routing protocol logic to determine the best path to a given network

Routing Protocols

• A distance vector routing protocol usually uses hop count as its metric

• A distance vector routing protocol is characterized by how it communicates with other routing devices

• Distance vector routing protocols use broadcasts to advertise their entire routing table to directly connected peer routers

Routing Protocols

• Convergence is the time it takes for a given set of routers to learn routes to all networks

• Convergence describes the time it takes a set of routers to learn of a change in the network

• Distance vector routing protocols generally take longer to converge than link state protocols because they use a periodic route advertisement schedule

Routing Protocols

• A routing loop occurs when routers get confused during update operations, causing frames to bounce back and forth between a set of interfaces

• Two easy methods to identify routing loops:– Tracert or traceroute TCP/IP utilities– View the routing table and the metric

associated with the network

Routing Protocols

• Prevent routing loops by using the following software based methods:– Split horizon – Hold-down timers – Triggered updates – Hop count limits – Poisoning

Routing Protocols

• Link state routing protocols are more intelligent than distance vector protocols

• The metric used by most link state protocols is bandwidth allowing more complex routing configurations

• Routing protocols capable of making complex decisions use a mathematical formula or algorithm for deriving the best path or route to a given network

Routing Protocols

• Some link state protocols  are capable of determining the best route to a destination network based on the following:– Delay – Load – Reliability – MTU

Routing Protocols

• When more than one metric is used it is referred to as a composite metric

• Link state protocols only send updates when changes occur, and they only send the changes, not the entire route table

• Link state protocols use multicast and unicast traffic instead of broadcast traffic

• Link state routers also develop an overall picture of the networks available by establishing neighbor relationships

Administrative Classification

• Routing protocols are also separated by an administrative classification based on where they are used in the networking environment:– Interior routing protocols or interior

gateway protocols– Exterior routing protocols or exterior

gateway protocols

Administrative Classification

• Interior gateway protocols (IGPs) are used within a company’s network infrastructure to maintain routing tables and policies set by the network administrators

• The two industry standard IGPs are: – Routing Information Protocol– Open Shortest Path First

Administrative Classification

• RIP is a distance vector protocol that uses hop count for its metric when determining the best route to a given network

• In most implementations, RIP uses split horizon, hop count limit, and poisoning for routing loop prevention

• RIP is a classful routing protocol

Administrative Classification

• The shortest path as measured by Open Shortest Path First (OSPF) is actually the fastest path based on bandwidth

• Shortest refers to the shortest time• OSPF is used in large networks and

ones requiring more intelligence than distance vector routing protocols

Administrative Classification

• OSPF communicates using unicast and multicast packets

• It only transmits changes or updates to the routing table when they occur

• It uses hello packets to determine the current state of a link between itself and its neighbors

• It utilizes a link state database to maintain a local view of the entire routing environment

Administrative Classification

• The configuration possibilities using OSPF– Areas– Autonomous system (AS) – Backbone router – Area border router (ABR)– Autonomous system boundary router

(ASBR)

Administrative Classification

• The decision making process of EGPs is far more complex than that of internal protocols

• The power and routing flexibility associated with EGPs requires knowledge and understanding of the complex nature of your network and its traffic

• EGPs can let you influence and manage traffic only as it enters or leaves your AS

Administrative Classification

• One member of EGPs is the Exterior Gateway Protocol (EGP)

• EGP was the first protocol developed that allowed isolation of autonomous systems

• EPG is not used today and is replaced by the Border Gateway Protocol

Administrative Classification

• Border Gateway Protocol (BGP) version 4 is the most widely used exterior protocol in the world

• BGP is a well established standard and commonly used by ISPs and in very large companies

• there are actually two different classifications of BGP – internal– external

Administrative Classification

• iBGP is used for internal routing

• eBGP is used for external routing

• iBGP functions under different rules than eBGP

• If two routers running BGP are in the same AS, they are running an iBGP connection

Administrative Classification

• BGP communication starts by establishing peers

• Once the peers have been established, BGP routing information is exchanged and updated as necessary

• BGP is an advanced distance vector protocol that uses triggered updates for communicating changes in the routing environment

• Routing loops in BGP are avoided by using the AS-path attribute

Administrative Classification

• BGP uses active TCP sessions that are setup and continuously maintained. Convergence in the routing environment is very fast

• BGP has features that you can use to help speed the convergence of the network routes under your control

Hierarchical Routing

• Hierarchical routing depends on hierarchical addressing

• It is a routing technique originally designed to help reduce the size of the routing tables on the Internet as well as speed up the overall routing process

• The concept uses an address block or blocks to represent different sections of a network

Hierarchical Routing

• Summarizing routes is often referred to as supernetting networks

• The process of summarization is built around the binary bit patterns just as in subnetting

• The difference is that rather than extending the subnet mask by adding bits, we remove bits

Hierarchical Routing

• By using summarization, you reduce the routing tables on each router

• To accomplish the summarization, you need to determine how many bits to unmask or un-subnet in order to make the networks appear as one big address block

• The routing protocol must transmit the network prefix along with the network address during route advertisements