Download Network-Layer Protocol Operations

IP Routing Principles
Network-Layer Protocol
Operations
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Application
Presentation
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Transport
Network
Data Link
Physical
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Network
Data Link
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Network
Data Link
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Network
Data Link
Physical
Application
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Transport
Network
Data Link
Physical
• Each router provides network layer (routing) services
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
A
Point-to-point or
circuit-switched
connection
Only a single network
connection with no need
for routing updates
B
“Stub” Network
• Fixed route to address reflects
administrator’s knowledge
Default (static) Route Example
Company X
A
Internet
172.34.56.0
B
C
10.0.0.0
Routing Table
No entry for destination net
Try router B default route
• Use if next hop is not explicitly listed in the routing table
Dynamic routing
Adapting to Topology Change
A
B
D
C
Adapting to Topology Change
A
B
X
D
C
• Can an alternate route substitute for a failed route?
Adapting to Topology Change
A
B
X
D
C
• Can an alternate route substitute for a failed route?
Dynamic Routing Operations
Network
Routing Protocol
Routing
Table
A router passes
routing information
to its neighbors
• Routing protocol maintains and distributes routing information
Dynamic Routing Operations
Network
Routing Protocol
Routing
Table
Routing Protocol
A router passes
routing information
to its neighbors
Routing
Table
• Routing protocol maintains and distributes routing information
Representing Distance with
Metrics
A
56
Hop count
Ticks
T1
56
Cost
T1
B
• Information used to select the best path for routing
Representing Distance with
Metrics
A
Bandwidth
56
Delay
Hop count
Ticks
Load
T1
56
Cost
Reliability
MTU
T1
B
• Information used to select the best path for routing
Classes of Routing Protocols
B
Distance Vector
A
C
D
Hybrid Routing
B
A
C
D
Link State
One Issue: Time to
Convergence
• 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
Distance Vector Concept
B
A
C
D
D
C
B
A
Routing
Table
Routing
Table
Routing
Table
Routing
Table
• Pass periodic copies of routing table to neighbor
routers and accumulate distance vectors
Link-State Concept
B
C
A
D
Link-State Packets
Topological
Database
Routing
Table
SPF
Algorithm
Shortest Path First Tree
• After initial flood, pass small event-triggered link-state
updates to all other routers
Comparing Distance Vector
Routing to Link-State Routing
Distance Vector
Link-State
Views net topology from
neighbor’s perspective
Gets common view of
entire network topology
Adds hops from router to
router
Calculates the shortest
path to other routers
Frequent, periodic updates:
slow convergence
Event-triggered updates:
faster convergence
Passes copies of routing
table to neighbor routers
Passes link-state routing
updates to other routers
Hybrid Routing
Choose a
routing path based
on distance vectors
Balanced Hybrid Routing
Converge rapidly using
change-based
updates
• Share attributes of both distance-vector
and link-state 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
EIGRP
RIP
172.16
172.16
RIP
172.16.0.0
AS 300
EIGRP
ASBR
RIP
172.16
ASBR
EIGRP
172.16
Routing feedback
Suboptimal path selection
Routing loops
Incompatible routing information
Inconsistent convergence time
Controlling Routing Update
Traffic
172.16.12.1
How can we prevent
routing update traffic
from crossing some
of these links?
172.16.3.2
Trans
172.16.2.2
172.16.2.1
T-1
172.16.3.1
172.16.1.1
172.16.1.2
Cen
R200
172.16.4.1
172.16.5.1
172.16.7.2
172.16.4.2
T-1
Frame Relay
64Kb
Rem
172.16.11.1
172.16.7.1
64Kb
R300
172.16.9.1
172.16.5.2
172.16.6.1
R100
172.16.6.2
172.16.10.1
Defining Distance
• 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
What Protocol to Believe?
Route Source
Default Distance
Connected Interface
Static Route
Enhanced IGRP Summary Route
External BGP
Internal Enhanced IGRP
IGRP
OSPF
IS-IS
RIP
EGP
External Enhanced IGRP
Internal BGP
Unknown
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1
5
20
90
100
110
115
120
140
170
200
255
Routers!
• Cisco CRS-1 IP/MPLS Router
92 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 Node
640 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