Routing Overview
• Two main functions of a router:
– Maintain routing tables
• Communicate topology changes to other routers.
• Use routing protocols to communicate network information with other routers.
– Use the routing table to determine where to send a packet.
• Switches the packets to the appropriate interface
• Adds the frame information for the interface, and then transmits the frame
• Routable protocols:
– IP
– IPX/SPX (Novell)
– AppleTalk
• Non-routable protocol – NetBEUI; peer-to-peer networking
Routed or Routable Protocols
• A routed protocol allows the router to forward data between nodes on different networks.
• A routable protocol must provide the ability to assign a network number and a host number to each device.
• With IP, the network address is determined by ANDing the address with the network mask.
ANDing Determines the Network
IP – Routed Protocol
• The most widely used network-addressing scheme
– Hierarchical
– Connectionless – no dedicated circuit connection
– Unreliable – no error checking
– Best-effort delivery – no acknowledgements
• IP does not verify that data sent on the network reaches its destination.
Network Layer Data Flow
IP – Routed Protocol
• Layer 3
• Finds the most efficient (quickest) path form one device to another
• Makes logical decisions about the best path for the delivery of data.
• Directs packets to the appropriate output port to be encapsulated for transmission.
Routed or Routable Protocols
• Layer 3 protocols used to transfer data from source to destination
• Rules for how data is packaged to enable it to be sent across networks
Routing Protocols
• Two families of routing protocols are Interior Gateway Protocols (IGPs) and Exterior Gateway Protocols (EGPs)
– IGP – route data within an AS
• RIP and RIPv2
• IGRP
• EIGRP
• OSPF
• Intermediate System-to-Intermediate System (IS-IS)
– EGP – route data between ASs
• BGP (border gateway protocol)
Routing Protocols
• Routers use routing protocols to exchange routing tables and share routing information.
– routers communicate with other routers to update and maintain the routing tables
• Routing protocols include:
– RIP
– IGRP
– OSPF
– BGP
– EIGRP
Routing Protocols
• Allow routers to choose the best path for data from a source to a destination
• Enable routers to route routed protocols
Interior and Exterior Gateway Protocols
Autonomous System
• An autonomous system (AS) is a network or set of networks under common administrative control
– cisco.com domain.
– microsoft.com
• Routers in an autonomous system have a consistent view of the entire network.
• This is due to the routers communicating with one another with routing update messages to maintain their routing tables.
Routing Tables
• Routers use routing protocols to build and maintain routing tables.
• Routing tables are used to determine the best path to a destination.
• The routing tables include:
– Protocol type
– Next-hop associations – indicates the destination is directly connected or can be reached via another router
– Routing metric – Routing metrics are used to determine the desirability of a route.
– Outbound interfaces – The interface used to reach the final destination.
Routing Metrics
• Routing metrics determine the optimal path to forward packets.
• Routing metrics are values that are used to determine the advantage of one route over another.
• Routing protocols use various combinations of metrics to determine the best path for data.
– Metrics include hop count, bandwidth, delay, reliability, load and cost.
• Routers use metrics to determine the most efficient path for each packet. These paths can change from one packet to the next as the metrics change.
Routing Metrics
• Routing metrics are the values used to choose the port to which a packet should be sent .
• Routing algorithms use different metrics to determine the best route.
• A routing algorithm generates a number called a metric value for each path through a network.
• Smaller metric values indicate preferred paths.
Routing Metrics
• Bandwidth – the data capacity of a link.
• Delay – the length of time required to move a packet along each link from source to destination.
• Load – the amount of activity on a router or a link.
• Reliability – the error rate of a network link.
• Hop count – the number of routers that a packet must travel through before reaching its destination. One router = one hop. The path with the least number of hops is preferred.
• Ticks – The delay on a data link using clock ticks. One tick = 1/18 second.
• Cost – an arbitrary value, based on bandwidth, money expense, or other measurement, that is assigned by a network administrator.
Path Determination
• What is the best path from A to F?
– Routers make decisions based on the:
• load
• bandwidth
• reliability of a network link
• Delay
• Cost
How routers determine the path
• The router compares the IP address of the packet that it received to the IP tables that it has.
• The destination address is obtained from the packet.
• The mask of the first entry in the routing table is applied to the destination address.
• The masked destination and the routing table entry are compared.
• If there is a match, the packet is forwarded to the port that is associated with that table entry.
• If there is not a match, the next entry in the table is checked.
• If the packet does not match any entries in the table, the router checks to see if a default route has been set.
• If a default route has been set, the packet is forwarded to the associated port. A default route is a route that is configured by the network administrator as the route to use if there are no matches in the routing table.
• If there is no default route, the packet is discarded. A message is often sent back to the device that sent the data to indicate that the destination was unreachable.
Routing vs Switching
• Routing
– Layer 3
– IP addresses
– Slower
– Block broadcasts
– Higher security
– ARP table and
-Routing Table
• Switching
– Layer 2
– MAC addresses
– Faster
– Forward broadcasts
– Lower security
– ARP table
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