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Discussion of Dynamic Routing
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Dynamic Routing Protocols
A Brief Overview
March 22, 2001Professor Tom Mavroidis
Why Dynamic Routing
• For small networks static routing is fine
• As the network grows it becomes impossible to keep the routing tables consistent
• Dynamic Routing lets the network keep itself up to date
Routing Server or Routing Daemon
• On the internet gateway systems maintain up to date information about routes.
• Each router propagates its routing information to other routers in the network
• They also update their routing tables from information sent by other routers
• This process is known as the routing server or routing daemon
Dynamic Routing...
• Determines the information placed placed in the routing table
• If multiple routes are found the best route will be inserted into the routing table (choice is made according to a routing policy in the router)
Routing Protocols
• Two Classifications– link-state protocols– vector-distance protocols
ICMP Redirect Messages
• Simplest form of dynamic routing
• Host systems are initialized with minimal routing information
• If the router discovers better route to a particular host it send an ICMP redirect message to the router closer to the source who updates its routing table.
Vector-Distance Protocols
• Many vector-distance Protocols have been superseded by link-state Protocols (such as RIP and Hello)
• Vector-distance Protocols forward routes across the network form router to router
• Routes are sent as a table (Destination/Distance)
• If the destination does not exist in the routing table a new route is added
Vector-Distance continued
• If a route already exists it is updated– both longer and shorter routes are updated since
routes may have changed by a router coming off line or going on line
• Rip is the most popular vector-distance protocol– was made available through the 4.2 BSD version of
Unix– Limits hops to 16 (therefore limits size of network)
Link-State Protocols
• OSPF - Open Shortest Path First– Uses a graph searching algorithm– Updates don’t contain routes only status of
directly connected links– When status changes the router rebuilds its
graph and re-computes the routes
Advantages to Link-state Routing
• Each router re-computes routes independently
• Routers are guaranteed to converge - (agree between each other)
• Link Status messages pass across the network unchanged (easier to debug)
• Scaleable - message size doesn’t depend on the number of networks
• Generates less traffic on the network
Other Interior Gateway Protocols
• ICMP Router Discovery Protocol– host selects the most appropriate default route
– work on an advertise/solicit mechanism
– host broadcasts a router solicitation message on startup
– Receiving routers send back an advertisement packet containing its IP address and a preference value
– host will select and update is routing table based on the best preference value
Other Interior Gateway Protocols
• Hello – uses a time delay measurement to choose the
best route
• IDRP - InterDomain Routing Protocol
• IS-IS - Intermediate system to Intermediate system
Exterior Gateways
• A collection of networks and routers under a single administrative control is known as an Autonomous System
• Central core network provides a method of connecting constituent networks
• Admin of the subsections is delegated to the core network
• Packets destined out of the interior network are forwarded to the exterior gateway
Exterior Gateways
• Greatly increase the scalability of the Internet backbone
• Alleviate bandwidth problems
• Keeps the size of of the routing tables to a minimum
Linux
• The routed daemon implemented dynamic routing based on RIP
• A modern replacement for routed is gated– supports more protocols
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