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8/2/2019 3 - Routers and Routing
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Understanding
Routing
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7-2
Agenda
Routing and a Router? Network Addressing
Routing Protocols
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Router and Routing
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7-4
What Is Routing, Router?
Routing is:
Finding a path between a source anddestination (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 packetsfrom one network to another and determinesthe optimal path for forwarding network traffic
* The term switching, when used to describe a routers
function, is different from a switch (the network device).
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7-5
Routers
A router is a more sophisticated device than ahub or a switch. It determines the appropriatenetwork path to send the packet along bykeeping an up-to-date network topology inmemory, its routing table.
Routers keep track of each others routes byalternately listening, and periodicallysending,
route information
Bridging is faster than Routing, but unlikeBridging, Routing provides LAN Segmentation,Broadcast Control, Security and Scalability.
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Routing Table
192.168.3.0 Frame Relay192.168.1.0 Ethernet192.168.2.0 FDDI
Network 192.168.2.0FDDI
Network 192.168.3.0Frame Relay
RemoteLocation
Network 192.168.1.0
Ethernet
Main Site
RoutersLayer 3
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Generalized Router Hardware
System Bus
Interface
Network
Controller
Interface:
Network
Controller
Interface:
Network
Controller
Flash
NVRAM
ROM
CPU
RAM
Bus
Interface
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Software Components
# ROM Monitor: # Device Micro code:
Startup Diagnostic code Part of IOS that deals with network
Contains exception handling controllers. Also deals with
modular interface processor.
# RxBoot: # Configuration Register:
Host mode Operating System 16 Bits, Specifies router startup
Used for downloading full parameters.
CISCO IOS
# CISCO IOS: # Configuration Files:
Contains process scheduler, memory Startup-config: contains
manager, parser configuration information
Also contains protocol specific code Running-config: contains
for packet handling active configurations
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Startup Procedure
ROM MonitorDiagnostic, Console Setup, Memory Sizing
Config Register Check.
Loads RxBoot, or stays in ROMMON
RxBoot
Builds Basic Data Structures, Interface Setup,
Host Mode Functionality, Startup-config Check.
Loads CISCO IOS or Stays in RxBoot
Prompt [router(boot)]
IOS
Interface Setup, Router Functionality,Allocate Buffers, Loads
Startup-config.
Boot Process Completes. [router>] Prompts Appears
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LAN-to-LAN connectivity
WAN-to-WAN connectivity
Remote access
Where are Routers Used?
Popular WAN service options include ISDN(Integrated Services Digital Network),Leased Lines, Frame Relay and X.25
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Packet Flow in a Routed Network
Routers encapsulate and de-encapsulate data packetsas 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 Link
NetworkTransport
Session
Presentation
Application
PhysicalData Link
Network
Transport
Session
Application
Physical
Network Network Network
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From the OSI model reference point of view -
The router de-encapsulates and examinesthe frame to determine what type of network
layer data is being carried. The network layerdata is sent to the appropriate network layerprocess, and the frame itself is discarded.
The network layer process examines the
header to determine the destination networkand then references the routing table thatassociates networks to outgoing interfaces.
The packet is again encapsulated in the linkframe for the selected interface and sent on.
Packet Flow in a Routed Network
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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 pathof media connections to a destination
Which Path?
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Multiprotocol Routing
IP 15.17.132.6
IP 15.16.42.8
IP 15.16.50.3
Routing Tables
IP
Token
Ring
TokenRing
AppleTalk 200.167
AppleTalk 100.110
Apple
IPX 4b.0800.0121.ab13
IPX 3a.0800.5678.12ab
Novell
DECnet 5.8
DECnet 10.1
DEC
VAX
VAX
Routers can support multiple independent routing algorithms & maintain
associated routing tables for several routed protocols
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Routing Tables
Routing algorithms
Initialize and maintainrouting tablesto helpwith path determination
Route information types
Destination/next-hop associations
Path desirability
Vary depending on routing algorithm
Message = Routing tablemaintenance communications
Routing update messages
Link-state advertisement
To Reach SendNetwork: To:
27 Node A
57 Node B
17 Node C
24 Node B
11 Node B
72 Node A
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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 aminimum of software and utilization overhead
Robustness and stability
Correct performance in the face of unusualor 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, networkcongestion, 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 addressPath part used by the router
Node addressSpecific 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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131.108.0.0INTERNET131.108.1.0
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 addressspace into smaller blocks
Helps organize network
Security (keeps HRseparately addressable)
ScalabilityKeeps traffic toappropriate segments
Allows single, summarizedrouting entry (131.108.0.0) tobe advertised to externalnetworks
Specific route entries(131.108.8.0) required onlyfor 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 bya network administrator
Benefits
Reflects administrators special
topology knowledge PrivateNot conveyed to other
routers in updates
Avoids the overhead of dynamicrouting
Stub network When a node is accessible by
only one path, a static route issufficient
Point-to-point or circuit-switched
connection
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Dynamic Routing
A network change blocksthe 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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Routing Protocols
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Routed versus Routing Protocols
Routed protocolsused betweenrouters to direct usertraffic; also callednetwork protocols
Examples: IP, IPX,DECnet, AppleTalk,NetWare, OSI, VINES
1.02.03.0
1.12.13.1
DestinationNetworkNetworkProtocol
Protocol name
Exit Portto Use
Routing protocols
used betweenrouters to maintainrouting tables
Examples: RIP, IGRP,
OSPF, BGP, EIGRP
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Routing Protocols
Distance vector
Sends routing table info only to neighbors, sochange communication may need one min/router
Also called routing by rumor Easy to configure, but slow
Link state (shortest path first)
Floods routing information about itself to all nodes,
so changes are known immediately Efficient, but complex to configure
Ciscos EIGRP hybrid
Efficient and easy to configure
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Routing Protocol Evolutions
Distance Vector
Link State
RIPDistance vector
Most common IGP
Uses hop count
IGRPDistance vector
Developed by Cisco
Addresses problems in
large, heterogeneousnetworks
OSPF Link state, hierarchical
Successor to RIP
Uses least-cost routing,multipath routing, andload balancing
Derived from IS-IS
EIGRP Hybrid protocol Developed by Cisco
Superior convergenceand operating efficiency
Merges benefits of link
state & distance vector
Hybrid
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RIP and IGRP
RIP
Industry standard thatselects the path with the
fewest hops
19.2 k
64k64k 64k
IGRP
Cisco protocol that selectsthe fastest path (using
load, distance, etc.)
19.2 k
64k64k 64k
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OSPF and EIGRP
Aspect OSPF EIGRP
Topology Hierarchical Not restricted
Memory & CPUrequirements
High Moderate
Routing table size Large Moderate
Controlling body Industry standard Cisco proprietary
Convergence Fast Fast
Supportedprotocols
IPIP
IPXAppleTalk
Configuration Difficult Easy
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Summary
Routers move data across networks from asource to a destination
Routers determine the optimal path forforwarding network traffic
Routing protocols communicatereachability information between routers