Transcript Dynamic Routing Chapter 6

Dynamic Routing
Chapter 6
Dynamic Routing
• Relies on routing protocols to maintain routing
tables
• Routes for remote networks are learned from
routing protocol traffic and are added to or
removed from IP routing tables
• Routing protocols can be class-based or classless
• Two most common IP dynamic routing protocols
are RIP and OSPF
• Windows 2000 supports both these protocols
Dynamic routing – Cont.
• Routers share routing information automatically
• Routing tables are built dynamically
• Minimal configuration is required by a network
administrator
• Typically used on large networks because of the
large amount of work that would be required in
maintaining the routing tables of static routers
RIP Routing Protocol
• RIP stands for Routing Information Protocol
• This protocol periodically exchanges routes to
known networks among the dynamic routers
• RIP discovers the cost of a given route in terms of
hops and stores that information in the routing
table
• IP then uses this information in selecting the most
efficient, low-cost route to the destination
• It works by watching for routing table broadcasts
by other routers and updating its own routing table
in the event of a change
RIP cont.
• When a router comes on line, it broadcasts a
request for routing table information from all the
routers it can see
• Once the table is built, RIP routers broadcast
routing table advertisements every 30 seconds,
containing its known destinations and the cost in
hops to get to each
• On large networks with lots of routers, a large
amount of bandwidth can be used by these RIP
broadcasts
RIP Routing Table Fields
• IP Destination address
• Metric (1-15) indicating the total cost in
hops of the route
• IP address of the router a datagram would
reach next on the path to its destination
• Marker signaling recent changes to a route
• Timer
A Problem with RIP
• Problem is know as “counting to infinity”
• In some configurations, an endless loop between
routers can occur if one of the networks becomes
unavailable
• RIP keeps counting hops each time the broadcast
reaches a router in hopes of finding a new route to
the formerly available network.
• To prevent this, a hop-count limit of 1-15 is used.
• This imposes a size restriction on networks.
• RIP can’t be used on a network with an area
consisting of more than 15 hops
A Problem with RIP – Cont.
• See Figure 3.3 for an example