Chapter 9

Routing

Contents

• Definition• Differences from switching• Autonomous systems• Routing tables• Viewing routes• Routing protocols• Route aggregation

2

Vs switching

Routing tables

Viewing routes

ASDefn Routing protocols

Route aggregation MPLS

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

Vs switching

Routing tables

Viewing routes

ASDefn Routing protocols

Route aggregation MPLS

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

Vs switching

Routing tables

Viewing routes

ASDefn Routing protocols

Route aggregation MPLS

Switching vs. Routing

Switch

Source host

Destination host

Source network

Destination network

Router 1

Router 3Router 2

Router 4

5

Vs switching

Routing tables

Viewing routes

ASDefn Routing protocols

Route aggregation MPLS

Routers in networks

6

Router

Net 1: USF131.247.0.0/16

Net 2: Bright House65.32.0.0/15

Router interface facing USFIP address: 131.247.254.182

(from USF pool)

Router interface facing Bright HouseIP address: 65.32.8.150 (from Bright House pool)

To ISPTo home

computers

RouterSwitch

WA

N p

ort

Ho

me

ne

two

rk

po

rt

Home router

802.11 wireless LAN

Vs switching

Routing tables

Viewing routes

ASDefn Routing protocols

Route aggregation MPLS

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

7

Vs switching

Routing tables

Viewing routes

ASDefn Routing protocols

Route aggregation MPLS

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

Vs switching

Routing tables

Viewing routes

ASDefn Routing protocols

Route aggregation MPLS

AS around 131.247.0.0/16

9

Vs switching

Routing tables

Viewing routes

ASDefn Routing protocols

Route aggregation MPLS

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

Vs switching

Routing tables

Viewing routes

ASDefn Routing protocols

Route aggregation MPLS

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 1109611

Vs switching

Routing tables

Viewing routes

ASDefn Routing protocols

Route aggregation MPLS

Example routes to 131.247.0.0/16

• Uncompressed and extracted fromhttp://archive.routeviews.org/oix-route-views/2009.08/

Network Next Hop Metric Path 131.247.0.0/16 64.71.255.61 0 0 812 174 5661 5661 i131.247.0.0/16 66.185.128.1 563 0 1668 174 5661 5661 i131.247.0.0/16 217.75.96.60 0 0 16150 3549 174 5661 5661 i131.247.0.0/16 208.51.134.246 13186 0 3549 174 5661 5661 i131.247.0.0/16 12.0.1.63 0 0 7018 174 5661 5661 i131.247.0.0/16 67.17.82.114 2503 0 3549 174 5661 5661 i131.247.0.0/16 192.203.116.253 0 0 22388 11537 11096 11096 5661 i131.247.0.0/16 203.181.248.168 0 0 7660 22388 11537 11096 11096 5661 i131.247.0.0/16 64.57.28.241 1045 0 11537 11096 11096 5661 i131.247.0.0/16 216.18.31.102 0 0 6539 11164 11096 5661 i131.247.0.0/16 216.218.252.164 0 0 6939 11096 5661 i

12

Vs switching

Routing tables

Viewing routes

ASDefn Routing protocols

Route aggregation MPLS

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

Vs switching

Routing tables

Viewing routes

ASDefn Routing protocols

Route aggregation MPLS

Viewing routes

• Many utilities are available to see Internet routes– Easiest to use is

tracert– In Windows,

Start > Run > cmd

– tracert <domain>

14

U SF

Co ge ntSp

rint

Vs switching

Routing tables

Viewing routes

ASDefn Routing protocols

Route aggregation MPLS

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

Vs switching

Routing tables

Viewing routes

ASDefn Routing protocols

Route aggregation MPLS

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

Vs switching

Routing tables

Viewing routes

ASDefn Routing protocols

Route aggregation MPLS

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

17

Vs switching

Routing tables

Viewing routes

ASDefn Routing protocols

Route aggregation MPLS

Routing table without aggregation

Router A

131.244.*.*/ 16

131.245.*.*/ 16

131.246.*.*/ 16

131.247.*.*/ 16

Router B

65.148.*.*/ 16

65.149.*.*/ 16

65.150.*.*/ 16

65.151.*.*/ 16

131.244.0.0/ 16

131.245.0.0/ 16

131.246.0.0/ 16

131.247.0.0/ 16

A

A

A

A

Router B’s routing table

Network Next hop

65.148.0.0/ 16

65.149.0.0/ 16

65.150.0.0/ 16

65.151.0.0/ 16

B

B

B

B

Router A’s routing table

Network Next hop

18

Vs switching

Routing tables

Viewing routes

ASDefn Routing protocols

Route aggregation MPLS

Routing table with aggregation

Router A

131.244.*.*/ 16

131.245.*.*/ 16

131.246.*.*/ 16

131.247.*.*/ 16

Router B

65.148.*.*/ 16

65.149.*.*/ 16

65.150.*.*/ 16

65.151.*.*/ 16

131.244.0.0 / 14 A

Router B’s routing table

Network Next hop

65.148.0.0 / 14 B

Router A’s routing table

Network Next hop

19

Vs switching

Routing tables

Viewing routes

ASDefn Routing protocols

Route aggregation MPLS

Routing table aggregation

38.14.192.0/24 38.14.193.0/24

Other networks in 38.0.0.0/8

Route advertised to other networks:

38.0.0.0 174

Interior structure of 38.0.0.0/8

38.14.224.0/15

38.14.224.0/16

38.0.0.0-38.255.255.255 (38.0.0.0/8)

AS 174

20

Vs switching

Routing tables

Viewing routes

ASDefn Routing protocols

Route aggregation MPLS

Route aggregation status

• http://www.cidr-report.org/– http://www.cidr-report.org/as2.0/#General_Statu

s• Route aggregation status

– Networks added to routing table– Networks that should consolidate– ISPs decreasing announced routes– ISPs increasing announced routes

21

Vs switching

Routing tables

Viewing routes

ASDefn Routing protocols

Route aggregation MPLS

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:

22

Vs switching

Routing tables

Viewing routes

ASDefn Routing protocols

Route aggregation MPLS

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

Vs switching

Routing tables

Viewing routes

ASDefn Routing protocols

Route aggregation MPLS

Networks around sandiego.edu

24

Vs switching

Routing tables

Viewing routes

ASDefn Routing protocols

Route aggregation MPLS

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

Vs switching

Routing tables

Viewing routes

ASDefn Routing protocols

Route aggregation MPLS

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

26

Summary

• 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