Download - Understanding Routing
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Understanding Routing
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Agenda
What Is Routing? Network Addressing Routing Protocols
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What is Routing?
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What Is Routing? Routing is:
Finding a path between a source and destination (path determination)
Moving information across an internetwork from a source to a destination (switching*)
Very complex in large networks because of the many potential intermediate nodes
A router is: A network layer device that forwards packets
from one network to another and determines the optimal path for forwarding network traffic
* The term “switching”, when used to describe a router’s function, is different from a switch (the network device).
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Routing TableRouting Table192.168.3.0192.168.3.0 Frame RelayFrame Relay192.168.1.0192.168.1.0 EthernetEthernet192.168.2.0192.168.2.0 FDDIFDDI
Network 192.168.2.0FDDI
Network 192.168.3.0Frame Relay
Remote Location
Network 192.168.1.0Ethernet
Main Site
Routers—Layer 3
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LAN-to-LAN connectivity LAN-to-WAN connectivity Remote access
Where are Routers Used?
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LAN-to-LAN Connectivity
• Routers encapsulate and de-encapsulate data packets as they are transferred from system X to system Y
X Y
AA
BB
CC
Presentation
Data LinkPhysical
Data LinkPhysical
A B C
Data LinkPhysical
Data LinkNetworkTransportSessionPresentationApplication
PhysicalData LinkNetworkTransportSession
Application
Physical
Network Network Network
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Path Determination
1
2
3
4
5
6
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8 9
10 11
Routers find the best path through the network Routing tables contain route information Network addresses represent the path
of media connections to a destination
Which Path?Which Path?
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Routing Algorithm Goals Optimality
Selecting the best route based on metrics and metric weightings used in the calculation
Simplicity and low overhead Efficient routing algorithm functionality with a
minimum of software and utilization overhead
Robustness and stability Correct performance in the face of unusual
or unforeseen circumstances (e.g., high load)
Rapid convergence Fast agreement, by all routers, on optimal routes
Flexibility Quick and accurate adaptation to changes in
router availability, bandwidth, queue size, etc.
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Routing Metrics
Path length Total hop count or sum of cost per network link
Reliability Dependability (bit error rate) of each network link
Delay Useful because it depends on bandwidth, queues,
network congestion, and physical distance
Communication cost Operating expenses of links (private versus public)
Bandwidth and load
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Network Addressing
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Network and Node Addresses
Network Node
1
2
123
1
3 1
1.1
2.1
3.1
1.2
1.3
• Network address—Path part used by the router
• Node address—Specific port or device on the network
1
2
3
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Addressing Examples
Network Node/HostProtocol Address Address
General 1. 4
TCP/IP 10. 8.2.48
Novell IPX 1aceb0b 0000.0c00.6e25
AppleTalk 10. 1.
X.25 DNIC NTN
NTN: National Terminal Number
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131.108.0.0INTERNET
131.108.1.0
131.108.2.0131.108.3.0
131.108.5.0
131.108.4.0
131.108.8.0131.108.6.0
131.108.7.0
131.108.10.0
131.108.9.0
Subnetwork Addressing
Manufacturing
R&D
Subdividing address space into smaller blocks
Helps organize network Security (keeps HR
separately addressable) Scalability—Keeps traffic to
appropriate segments Allows single, summarized
routing entry (131.108.0.0) to be advertised to external networks
Specific route entries (131.108.8.0) required only for routers in the subnetted block
HR
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Routing Algorithm Types
Single-path versus multi-path
Flat versus hierarchical
Host-intelligent versus router-intelligent
Intradomain versus interdomain
Static versus dynamic routing
Link state versus distance vector
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Static Routing
“Stub” Network
A
B
Manual table updates by a network administrator
Benefits Reflects administrator’s special
topology knowledge Private—Not conveyed to other
routers in updates Avoids the overhead of dynamic
routing
Stub network When a node is accessible by
only one path, a static route is sufficient
Point-to-point or circuit-switched connection
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Dynamic Routing
A network change blocks the established path...
A B
CD
XA B
CD
X
…and an alternate route is found dynamically.
• Most internetworks use dynamic routing
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Distance Vector versus Link State
Distance vector Sends routing table info only to neighbors, so change
communication may need one min/router Also called “routing by rumor” Easy to configure, but slow
Link state Floods routing information about itself to all nodes,
so changes are known immediately Efficient, but complex to configure
Cisco’s EIGRP hybrid Efficient and easy to configure
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Routing Protocols
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Routed versus Routing Protocols
• Routed protocols used between routers to direct user traffic; also called network protocols– Examples: IP, IPX,
DECnet, AppleTalk, NetWare, OSI, VINES
1.02.03.0
1.12.13.1
DestinationNetwork
NetworkProtocol
Protocol name
Exit Port to Use
• Routing protocols used between routers to maintain routing tables– Examples: RIP, IGRP,
OSPF, BGP, EIGRP
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Routing Protocol Evolutions
Distance Vector
Link State
RIP• Distance vector• Most common IGP• Uses hop count
RIP• Distance vector• Most common IGP• Uses hop count
IGRP• Distance vector• Developed by Cisco • Addresses problems in large, heterogeneous networks
IGRP• Distance vector• Developed by Cisco • Addresses problems in large, heterogeneous networks
OSPF• Link state, hierarchical • Successor to RIP • Uses least-cost routing,
multipath routing, and load balancing
• Derived from IS-IS
OSPF• Link state, hierarchical • Successor to RIP • Uses least-cost routing,
multipath routing, and load balancing
• Derived from IS-IS
EIGRP• Hybrid protocol• Developed by Cisco• Superior convergence
and operating efficiency• Merges benefits of link
state & distance vector
EIGRP• Hybrid protocol• Developed by Cisco• Superior convergence
and operating efficiency• Merges benefits of link
state & distance vector
Hybrid
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RIP and IGRP
RIP
Industry standard that selects the path with the
fewest hops
19.2 k
64k64k 64k
IGRP
Cisco protocol that selects the fastest path (using
load, distance, etc.)
19.2 k
64k64k 64k
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OSPF and EIGRP
Aspect OSPF EIGRPTopology Hierarchical Not restricted
Memory & CPUrequirements High Moderate
Routing table size Large Moderate
Controlling body Industry standard Cisco proprietary
Convergence Fast Fast
Supported protocols IP
IPIPX
AppleTalk
Configuration Difficult Easy
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Summary
Routers move data across networks from a source to a destination
Routers determine the optimal path for forwarding network traffic
Routing protocols communicate reachability information between routers