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Chapter 9
Routing
Contents
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•
•
•
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Definition
Differences from switching
Autonomous systems
Routing tables
Viewing routes
Routing protocols
Route aggregation
2
Defn
Vs
switching
AS
Routing Viewing
tables
routes
Routing
protocols
Routing
• Definition
– Moving information across networks from the
source network to the destination network
• In LANs, the source and destination are in the
same network
• Routing is done by devices called Routers
3
Route
aggregation
MPLS
Defn
Vs
switching
AS
Routing Viewing
tables
routes
Routing
protocols
Switching vs. Routing
• There can be multiple paths between source
and destination in larger networks (routed
networks)
• One of the most important tasks of a router is
to send packets to the destination using the
best available path
4
Route
aggregation
MPLS
Defn
Vs
switching
AS
Routing Viewing
tables
routes
Routing
protocols
Switching vs. Routing
Source host
Source network
Router 1
Switch
Router 2
Router 3
Router 4
Destination network
Destination host
5
Route
aggregation
MPLS
Defn
Vs
switching
AS
Routing Viewing
tables
routes
Routing
protocols
Routers in networks
Router interface facing USF
Router interface facing Bright House
IP address: 131.247.254.182
(from USF pool)
IP address: 65.32.8.150
(from Bright House pool)
Net 1: USF
131.247.0.0/16
Net 2: Bright House
65.32.0.0/15
Router
Home
network
port
WAN port
802.11 wireless LAN
To home
computers
To ISP
Switch
Router
Home router
6
Route
aggregation
MPLS
Defn
Vs
switching
AS
Routing Viewing
tables
routes
Routing
protocols
Autonomous systems
• Autonomous Systems (AS) are the unit of
Internet routing
• RFC 1930
– An AS is a connected group of one or more IP
prefixes which has a SINGLE and CLEARLY DEFINED
routing policy
– Each AS has a globally unique AS number
– Routes are advertised as a chain of AS
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Route
aggregation
MPLS
Defn
Vs
switching
AS
Routing Viewing
tables
routes
Routing
protocols
View autonomous systems
• BGPlay
– Try 131.247.0.0/16
– Start date: 20 days before today
– End date: yesterday
• AS number to name mapping
– http://www.cidr-report.org/as2.0/autnums.html
• Search for a specific AS. E.g. 5661 (USF)
8
Route
aggregation
MPLS
Defn
Vs
switching
AS
Routing Viewing
tables
routes
Routing
protocols
AS around 131.247.0.0/16
9
Route
aggregation
MPLS
Defn
Vs
switching
AS
Routing Viewing
tables
routes
Routing
protocols
Visualizing network routes
• http://bgplay.routeviews.org/bgplay/
• http://www.arin.net
– Maintains a database of ownership of IP address
blocks
• E.g. Search for 131.247.100.1 (USF)
10
Route
aggregation
MPLS
Defn
Vs
switching
AS
Routing Viewing
tables
routes
Routing
protocols
Routing tables
• For each known path to a destination, the
router records the next hop in routing table
• A router is only responsible for sending the
packet to the next router
• When multiple paths are known, the metric
and next hop associated with each path is
recorded
– Note for next slide – all paths to USF (AS 5661)
pass through AS 174 or AS 11096
11
Route
aggregation
MPLS
Defn
Vs
switching
AS
Routing Viewing
tables
routes
Routing
protocols
Example routes to 131.247.0.0/16
• Uncompressed and extracted from
http://archive.routeviews.org/oix-route-views/2009.08/
Network
131.247.0.0/16
131.247.0.0/16
131.247.0.0/16
131.247.0.0/16
131.247.0.0/16
131.247.0.0/16
131.247.0.0/16
131.247.0.0/16
131.247.0.0/16
131.247.0.0/16
131.247.0.0/16
Next Hop
64.71.255.61
66.185.128.1
217.75.96.60
208.51.134.246
12.0.1.63
67.17.82.114
192.203.116.253
203.181.248.168
64.57.28.241
216.18.31.102
216.218.252.164
Metric
0
563
0
13186
0
2503
0
0
1045
0
0
Path
0 812 174 5661 5661 i
0 1668 174 5661 5661 i
0 16150 3549 174 5661 5661 i
0 3549 174 5661 5661 i
0 7018 174 5661 5661 i
0 3549 174 5661 5661 i
0 22388 11537 11096 11096 5661 i
0 7660 22388 11537 11096 11096 5661 i
0 11537 11096 11096 5661 i
0 6539 11164 11096 5661 i
0 6939 11096 5661 i
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Route
aggregation
MPLS
Defn
Vs
switching
AS
Routing Viewing
tables
routes
Routing
protocols
Route selection
• Routers keep information on all announced
routes
– Routers need a measure to compare alternate
paths to the same destination
– These measures are called routing metrics
– When alternate paths are available, path with the
lowest metric is chosen
13
Route
aggregation
MPLS
Defn
Vs
switching
AS
Routing Viewing
tables
routes
Routing
protocols
Route
aggregation
– Easiest to use is
tracert
– In Windows,
Start > Run >
cmd
– tracert
<domain>
Sprint
• Many utilities
are available to
see Internet
routes
Cog
ent USF
Viewing routes
14
MPLS
Defn
Vs
switching
AS
Routing Viewing
tables
routes
Routing
protocols
Routing protocols
• There are two kinds of routing protocols used
on the Internet
– Exterior routing protocols
• connect autonomous systems to each other
• E.g. BGP (Border Gateway Protocol)
• Discussion so far
– Interior routing protocols
• used within an autonomous system
• E.g. OSPF (Open Shortest Path First)
• To reach internal networks
15
Route
aggregation
MPLS
Defn
Vs
switching
AS
Routing Viewing
tables
routes
Routing
protocols
Simplifying Routing Tables
• As more and more organizations join the
Internet, routing tables keep getting larger to
accommodate the newer routes
– 900 MB uncompressed snapshot on July 24, 2008
from routeviews.org
• Route aggregation is used to simplify routing
tables
– RFC 1518 for address allocation with CIDR
16
Route
aggregation
MPLS
Defn
Vs
switching
AS
Routing Viewing
tables
routes
Routing
protocols
Route aggregation in CIDR
• CIDR also includes route aggregation
– Instead of assigning address blocks to end-user
organizations, assign larger blocks of addresses to
large network service providers
– Organizations acquire addresses from these
network service providers
– Routers in the rest of the world only maintain one
entry to the ISP’s larger address block
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Route
aggregation
MPLS
Defn
Vs
switching
AS
Routing Viewing
tables
routes
Routing
protocols
Routing table without aggregation
131.244.*.*/ 16
65.148.*.*/ 16
131.245.*.*/ 16
65.149.*.*/ 16
131.246.*.*/ 16
65.150.*.*/ 16
Router A
Router B
131.247.*.*/ 16
65.151.*.*/ 16
Router A’s routing table
Network
Next hop
Router B’s routing table
Network
Next hop
65.148.0.0/ 16
B
131.244.0.0/ 16
A
65.149.0.0/ 16
B
131.245.0.0/ 16
A
65.150.0.0/ 16
B
131.246.0.0/ 16
A
65.151.0.0/ 16
B
131.247.0.0/ 16
A
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Route
aggregation
MPLS
Defn
Vs
switching
AS
Routing Viewing
tables
routes
Routing
protocols
Routing table with aggregation
131.244.*.*/ 16
65.148.*.*/ 16
131.245.*.*/ 16
65.149.*.*/ 16
131.246.*.*/ 16
65.150.*.*/ 16
Router A
Router B
131.247.*.*/ 16
65.151.*.*/ 16
Router A’s routing table
Router B’s routing table
Network
Next hop
Network
Next hop
65.148.0.0 / 14
B
131.244.0.0 / 14
A
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Route
aggregation
MPLS
Defn
Vs
switching
AS
Routing Viewing
tables
routes
Routing
protocols
Route
aggregation
Routing table aggregation
Route advertised
to other networks:
38.0.0.0 174
38.0.0.0-38.255.255.255
(38.0.0.0/8)
AS 174
Other networks in
38.0.0.0/8
38.14.192.0/24
38.14.224.0/15
38.14.193.0/24
Interior structure of
38.0.0.0/8
38.14.224.0/16
20
MPLS
Defn
Vs
switching
AS
Routing Viewing
tables
routes
Routing
protocols
Route aggregation status
• http://www.cidr-report.org/
– http://www.cidrreport.org/as2.0/#General_Status
• Route aggregation status
–
–
–
–
Networks added to routing table
Networks that should consolidate
ISPs decreasing announced routes
ISPs increasing announced routes
21
Route
aggregation
MPLS
Defn
Vs
switching
AS
Routing Viewing
tables
routes
Routing
protocols
MPLS
• Multi-protocol label switching
• Defined in RFC 3031 in 2001
• Not a different kind of WAN, but simplifies
network layer equipment on any WAN
• From RFC 3031:
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Route
aggregation
MPLS
Defn
Vs
switching
AS
Routing Viewing
tables
routes
Routing
protocols
MPLS
• Consider 2 packets traveling from Lansing to
San Diego
– E.g. 2 users at MSU, one visiting sandiego.edu,
another visiting sdsu.edu
– Both packets take the same path from source to
destination
• But, in traditional routing, each router on the
path will independently make a routing
decision on each packet
23
Route
aggregation
MPLS
Defn
Vs
switching
AS
Routing Viewing
tables
routes
Routing
protocols
Networks around sandiego.edu
24
Route
aggregation
MPLS
Defn
Vs
switching
AS
Routing Viewing
tables
routes
Routing
protocols
MPLS
• Packets to be treated similarly are said to be in
the same forwarding equivalence class (FEC)
• When the 2 packets enter a network, both
packets will be assigned the same FEC
• FEC is called the label and is added to packet
• Routers determine next hop from the label
25
Route
aggregation
MPLS
Defn
Vs
switching
AS
Routing Viewing
tables
routes
Routing
protocols
MPLS
• In MPLS, routers only know how to forward incoming
packets with a known set of labels
– In traditional routing, routers can route packets to any
destination in the world
– At each router, administrators create a forwarding table
– Labeling decision only done once per packet
– Label removed when packet leaves network
• MPLS simplifies routing in 2 ways
– Eliminate processing of unnecessary header fields
– Routing decision only made once per network per packet
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Route
aggregation
MPLS
Summary
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How routing is different from switching
How routers interface between networks
What are autonomous systems
How routers select routes for packets
What do routing tables look like
What is route aggregation
What is MPLS
Case study – network resilience
following Katrina and 9/11
• The superior reliability of packet networks was
demonstrated after Hurricane Katrina and
9/11
– Cell phones and landlines did not work, but voice
over IP did
• But effects can be widespread
– The greatest impact of 9/11 on Internet
connectivity occurred in South Africa
• DNS resolution done in NYC
• CNN web page fit on one IP packet
Hands-on exercise
• Bgplay
– Obtain school IP address using tracert
– Obtain CIDR address block from ARIN
– View network neighborhood using bgplay
Network design
• Failover