I am getting the following error using ns2 help to debug. can't read "udp1": no such variable while executing "$cbr1 attach-agent $udp1"

Question

set ns [ new Simulator ]
set trf [ open 1.tr w ]
$ns trace-all $trf
set namf [ open 1.nam w ]
$ns namtrace-all $namf

# The below code is used to create the nodes.

set n0 [$ns node]
set n1 [$ns node]
set n2 [$ns node]

This is used to give color to the flow of packets.

$ns color 1 "red"
$ns color 2 "green" $n0 label "Source/udp0"
$n1 label "Source/udp1"
$n2 label “destination”

providing the link

$ns duplex-link $n0 $n2 10Kb 100ms DropTail
$ns duplex-link $n1 $n2 5Mb 200ms DropTail

set the queue size b/w the nodes

$ns set queue-limit $n0 $n2 10
set cbr1 [new Application/Traffic/CBR]
$cbr1 attach-agent $udp1

The below code sets the udp0 packets to red and udp1 packets to blue color

$udp0 set class_ 1 $udp1 set class_ 2

The below code is used to connect the agents.

$ns connect $udp0 $null3 $ns connect $udp1 $null3

The below code is used to set the packet size to 500

$cbr1 set packetSize_ 500Mb

The below code is used to set the interval of the packets,

$cbr1 set interval_ 0.001
proc finish { } {
global ns namf trf
$ns flush-trace
exec nam 1.nam &
close $trf close $namf
exit 0
}
$ns at 0.1 "$cbr0 start"
$ns at 0.1 "$cbr1 start"
$ns at 10.0 "finish"
$ns run

Answer 1

You need to set an agent and then attach it to a node

set udp0 [new Agent/UDP]
$ns attach-agent $n0 $udp0