Download - IP Routing Principles
IP Routing Principles
Network-Layer Protocol Operations
• Each router provides network layer (routing) services
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ApplicationPresentationSessionTransportNetworkData LinkPhysical
ApplicationPresentationSessionTransportNetworkData LinkPhysical
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NetworkData LinkPhysical
NetworkData LinkPhysical
NetworkData LinkPhysical
Static versus Dynamic Routes
• Static route
Uses a protocol route that a network administrator enters into the router
• Dynamic route
Uses a route that a network routing protocol adjusts automatically for topology or traffic changes
Static Route Example
• Fixed route to address reflects administrator’s knowledge
Point-to-point orcircuit-switched
connection
“Stub” Network
Only a single network connection with no need
for routing updates BB
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Default (static) Route Example
• Use if next hop is not explicitly listed in the routing table
Company X Internet
10.0.0.0
172.34.56.0
Routing TableRouting Table
No entry for destination netTry router B default route
No entry for destination netTry router B default route
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Dynamic routing
Adapting to Topology Change
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Adapting to Topology Change
• Can an alternate route substitute for a failed route?
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Adapting to Topology Change
• Can an alternate route substitute for a failed route?
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Dynamic Routing Operations
• Routing protocol maintains and distributes routing information
A router passes routing information to its neighbors
RoutingTable
RoutingTable
NetworkNetwork Routing ProtocolRouting Protocol
Dynamic Routing Operations
• Routing protocol maintains and distributes routing information
A router passes routing information to its neighbors
RoutingTable
RoutingTable
RoutingTable
RoutingTable
NetworkNetwork Routing ProtocolRouting Protocol Routing ProtocolRouting Protocol
Representing Distance with Metrics
• Information used to select the best path for routing
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Hop count
Ticks
Cost
Hop count
Ticks
Cost
Representing Distance with Metrics
• Information used to select the best path for routing
56T1
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B
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T1
Bandwidth
Delay
Load
Reliability
MTU
Bandwidth
Delay
Load
Reliability
MTU
Hop count
Ticks
Cost
Hop count
Ticks
Cost
Classes of Routing Protocols
Distance VectorDistance Vector
Hybrid RoutingHybrid Routing
Link StateLink StateCC
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• Convergence occurs when all routers use a consistent perspective of network topology
• After a topology changes, routers must recompute routes, which disrupts routing
• The process and time required for router reconvergence varies in routing protocols
One Issue: Time to Convergence
Distance Vector Concept
• Pass periodic copies of routing table to neighbor routers and accumulate distance vectors
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RoutingTable
RoutingTable
RoutingTable
RoutingTable
RoutingTable
RoutingTable
RoutingTable
RoutingTable
Link-State Concept
• After initial flood, pass small event-triggered link-state updates to all other routers
Link-State Packets
SPFAlgorithm
TopologicalDatabase
Shortest Path First Tree
RoutingTable
RoutingTable
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Comparing Distance Vector Routing to Link-State Routing
Link-State
Gets common view of entire network topology
Calculates the shortest path to other routers
Event-triggered updates: faster convergence
Passes link-state routing updates to other routers
Distance Vector
Views net topology from neighbor’s perspective
Adds hops from router to router
Frequent, periodic updates: slow convergence
Passes copies of routing table to neighbor routers
• Share attributes of both distance-vector and link-state routing
Hybrid Routing
Choose arouting path based on distance vectors
Converge rapidly using change-based
updates
Choose arouting path based on distance vectors
Converge rapidly using change-based
updates
Balanced Hybrid RoutingBalanced Hybrid Routing
Static Routing
• Advantages:predictable
no additional CPU overhead
no additional network overhead
easy to configure
• Disadvantages:does not scale
hard to maintain
does not adapt to network changes
Dynamic Routing
• Advantages:scalability
adaptability
• Disadvantages:increased complexity
increased bandwidth overhead
increased resource usage
Dynamic Routing
• Moral of the story:
Use static routing where you can,use dynamic routing only where you must
Autonomous Systems
• Internetwork
• The big “I” Internet
• AS = a group of routers and their networks (administered by the same owner)
• AS peering
Routing Protocols
• Where is it used?
Interior Gateway Protocols (IGP)
RIP
OSPF
EIGRP
(IS-IS)
Exterior Gateway Protocols (EGP)
BGP
Routing Protocols
• How does it work?
Distance-Vector Protocols
RIP
EIGRP
BGP
Link State
OSPF
Protocol Features
• RIPv2
hop count metric
unreliable transport
passive RIP
simple
Protocol Features
• OSPF
full CIDR support
trust
route injection
hierarchical routing
a smarter protocol
Protocol Features
• EIGRP
more information in advertisements
improved convergence properties
partial and incremental updates
no support for areas
Protocol Features
• BGP
BGP-4: RFC 1771
created to support CIDR
even more information
Policy routing
Reliable transport
can also be used as an IGP (IBGP)
high overhead
Real World Routing
• multiple routing protocols are generally necessary
• interior routing is a LOT different than exterior routing
• multi-protocol issues
Implementation Considerations
172.16
RIP
172.16
EIGRP
RIP
172.16.0.0
AS 300EIGRP
ASBR
ASBR
172.16
RIP
172.16
EIGRP
Routing feedbackSuboptimal path selectionRouting loops
Incompatible routing informationInconsistent convergence time
Controlling Routing Update Traffic
How can we preventrouting update traffic from crossing some of these links?
172.16.7.1
172.16.7.2
172.16.6.1172.16.6.2
172.16.5.2
172.16.5.1172.16.4.1
172.16.4.2
172.16.1.1 172.16.1.2
172.16.3.1
172.16.3.2172.16.2.2
172.16.2.1
172.16.9.1 172.16.10.1
172.16.11.1
TransTrans
R200R200 CenCen
R300R300 R100R100
RemRem
172.16.12.1
64Kb
64Kb
T-1
T-1Frame Relay
•Different protocols use different metrics
•Metrics are difficult to compare algorithmically
Defining Distance
•Different protocols use different metrics
•Metrics are difficult to compare algorithmically
•Therefore, need a selection process:
1—Which protocol do you believe the most?
2—Then decide which metric is the best
Defining Distance
What Protocol to Believe?
Connected Interface Connected Interface 00
Static RouteStatic Route 11
Enhanced IGRP Summary RouteEnhanced IGRP Summary Route 55
External BGPExternal BGP 2020
Internal Enhanced IGRPInternal Enhanced IGRP 9090
IGRPIGRP 100100
OSPFOSPF 110110
IS-IS IS-IS 115115
RIP RIP 120120
EGPEGP 140140
External Enhanced IGRPExternal Enhanced IGRP 170170
Internal BGPInternal BGP 200200
UnknownUnknown 255255
Route SourceRoute Source Default DistanceDefault Distance
Routers!
• Cisco CRS-1 IP/MPLS Router92 Tbps total switching capacity
1152 x OC-768c/STM-256c (40Gbps) Line cards
707 Kg, 15 KW per chassis
IPv4, IPv6, MPLS
Routers!
• Juniper T640 Internet Routing Node640 Gbps total switching capacity
770 Million packet per second forwarding
40 Gbps per slot (4 * OC-192c, 1 * OC-768c)
8 slots per rack
Routers!
• Foundry NetIron 1500 Internet Router
480 Gbps total switching capacity
178 Million packet per second forwarding
10 Gbps per slot (1 * 10G Ethernet)
15 slots per rack