Chapter 8 Routing. Introduction Look at: –Routing Basics (8.1) –Address Resolution (8.2)...
-
Upload
regina-goodman -
Category
Documents
-
view
225 -
download
4
Transcript of Chapter 8 Routing. Introduction Look at: –Routing Basics (8.1) –Address Resolution (8.2)...
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