1-1CSE-Network Management—Introduction © 1999, Cisco Systems, Inc. www.cisco.com
Understanding Routing
7-2CSE: Networking Fundamentals—Routing © 1999, Cisco Systems, Inc. www.cisco.com
Agenda
• What Is Routing?
• Routing Protocols
1-3CSE-Network Management—Introduction © 1999, Cisco Systems, Inc. www.cisco.comwww.cisco.com© 1999, Cisco Systems, Inc.
What is Routing?What is Routing?
7-4CSE: Networking Fundamentals—Routing © 1999, Cisco Systems, Inc. www.cisco.com
What Is Routing?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).
7-5CSE: Networking Fundamentals—Routing © 1999, Cisco Systems, Inc. www.cisco.com
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 3Routers—Layer 3
7-6CSE: Networking Fundamentals—Routing © 1999, Cisco Systems, Inc. www.cisco.com
• LAN-to-LAN connectivity
• LAN-to-WAN connectivity
• Remote access
Where are Routers Used?
7-7CSE: Networking Fundamentals—Routing © 1999, Cisco Systems, Inc. www.cisco.com
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
7-8CSE: Networking Fundamentals—Routing © 1999, Cisco Systems, Inc. www.cisco.com
Path Determination
1
2
3
4
5
6
7
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?
7-9CSE: Networking Fundamentals—Routing © 1999, Cisco Systems, Inc. www.cisco.com
Routing Algorithm GoalsRouting 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.
7-10CSE: Networking Fundamentals—Routing © 1999, Cisco Systems, Inc. www.cisco.com
Routing MetricsRouting 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
7-11CSE: Networking Fundamentals—Routing © 1999, Cisco Systems, Inc. www.cisco.com
Types of Routing.Types of Routing.
Static routing
Default routing
Dynamic routing.
7-12CSE: Networking Fundamentals—Routing © 1999, Cisco Systems, Inc. www.cisco.com
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
7-13CSE: Networking Fundamentals—Routing © 1999, Cisco Systems, Inc. www.cisco.com
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
1-14CSE-Network Management—Introduction © 1999, Cisco Systems, Inc. www.cisco.comwww.cisco.com© 1999, Cisco Systems, Inc.
Routing ProtocolsRouting Protocols
7-15CSE: Networking Fundamentals—Routing © 1999, Cisco Systems, Inc. www.cisco.com
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
7-16CSE: Networking Fundamentals—Routing © 1999, Cisco Systems, Inc. www.cisco.com
Routing Protocol EvolutionsRouting 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
7-17CSE: Networking Fundamentals—Routing © 1999, Cisco Systems, Inc. www.cisco.com
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
7-18CSE: Networking Fundamentals—Routing © 1999, Cisco Systems, Inc. www.cisco.com
SummarySummary
• 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