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Cisco S3 C5
Routing Protocols
Network Design Characteristics
• Reliable – provides mechanisms for error
detection and correction
• Connectivity – incorporate variety of hard
ware and software
• Easy to use – don’t need to know structure
• Easy to modify – can evolve and adapt
• Easy to implement – follow standards &
allow for variety of configurations
Path Determination
• Frame received on router interface and data
link layer frame is removed and discarded
• Network layer handled by appropriate
network layer process
• Network protocol header examined to
determine destination
• Refers to routing table to determine which
port to send packet back out
IP Routing Table
• Consists of destination network address and next
hop pairs
• When no match is found, datagram is sent to
default router; drops packet if no default router
• Routers communicate to build tables and update at
intervals or by event
• Routing updates can help build detailed picture of
network topology
Routing Metrics
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Hop Count
Bandwidth
Delay
Load
Reliability (administrative number)
Ticks
Cost
Routed and Routing
• Routed Protocols
– IP, IPX, Appletalk, Vines, DecNet
• Routing Protocols
– IGRP EIGRP
– RIP OSPF
– EGP BGP
OSI routing
Advanced Peer-to-Peer
Multi-protocol Routing
• Route multiple routed protocols – IP, IPX,
AppleTalk, etc.
• Can use multiple routing protocols – RIP,
IGRP
• Routers pass traffic from all routed
protocols over inter-network
Routing Protocol Differences
• Goals of protocol designer affect operation
of resulting routing protocol
• Various types of routing protocols – each
protocol has different effect on network and
router resources
• Routing protocols use variety of metrics to
identify best routes
• Two broad classes – Exterior and Interior
Routing Protocol Goals
• Optimal Route – select best route – depends on
metrics and metric weightings
• Simplicity and Efficiency – important when there
are limited resources
• Robustness – perform correctly in difficult
circumstances – high loads
• Rapid convergence – all routers see same topology
• Flexibility – adapt to variety of network
circumstances
Routing Loops
• Maximum defined by protocol
– IGRP has 255 maximum; defaults to 100;
usually set for 50
Static vs Dynamic
• Static
– Uses a protocol route that network
administrator enters into router
• Dynamic Route
– Uses route that network routing protocol
adjusts automatically for topology or traffic
changes
• Supplement with static routes and router of last
resort – gateway router
Routing Classes
• Distance vector – metrics – IGRP and RIP
• Link State – SPF – re-creates exact
topology – OSPF , IS-IS and NLSP
• Hybrid – combines aspects of distance
vector and hybrid -- EIGRP
Configuring Protocols
• Each routing protocol must be configured
separately
– Create the routing process with a router
command
– Configure protocol specifics
• Both RIP and IGRP must have list of networks
specified before routing activities begin
• IGRP requires an autonomous system AS number
Choosing a Routing Protocol
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Network size and complexity
Network traffic levels
Security needs
Reliability needs
Network delay characteristics
Organization policies
Organization tolerance for change
Distance Vector Metrics
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Reliability
Load
Bandwidth
Delay
1 to 255
1 to 255
1200 bps to 10 Gbps
1 to 224
• Default values related to weightings for IGRP give
most importance to bandwidth making it superior
to RIP which uses hop count
How IGRP Works
• Used only on autonomous systems
• If network is not sub-netted, it does not advertise
interior routes and subnet information is not
included in IGRP updates
• System routes to other major networks in AS do
not include sub-netting information
• Exterior routes are considered when identifying
gateway of last resort
• Sends up-date every 90 seconds
• Maximum hop count is 255
Router Commands
• Router (config)#router igrp 202.22.22.0 120
• Router (config-router)# network 222.22.22.0
• Sample Router Statement
– Router(config)# router igrp 46
– timers basic 15 45 0 60
–
network 128.6.0.0
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no metric holddown
–
metric maximum-hop 50
Routing Problems
• Holddown – used when router learns that a
network is a greater distance than
previously known or a network is down –
advertisements from other routers are
ignored – helps avoid routing loop
• Holddown period is greater than period of
time necessary to update entire network
Routing Problems
• Split Horizons provide extra protocol stability;
Keeps router from sending information about a
route back in the direction from which it came;
defeats loops between adjacent routers
• Poison Reverse – intended to defeat larger routing
loops; increase in metrics indicate loops. Poison
reverse updates sent to remove route and place it
in holddown – sends metric of infinity – helps
speed convergence