playing with cisco ios

draft – tested on old Catalysts 2950

understanding the defaults


ip subnet-zero

historically allows first .0 and last subnets .255


spanning-tree mode pvst
no spanning-tree optimize bpdu transmission
spanning-tree extend system-id

keepalive loop

you will notice LOOP packets in wireshark, as keepalive is the default. keep it as such

Consquences of “no keepalive” on switch port

Error - “%ETHCNTR-3-LOOP_BACK_DETECTED” Catalyst switch that runs Cisco IOSĀ® Software

Disabling keepalive in switch


interface fa0/X
switchport access vlan XXX
switchport mode access
spanning-tree portfast

eventually find out what NIC is connected to that port

trunk & vtp

pass the VTP adverts through to trunks only,

vtp mode transparent

more about discovery protocols

interface gi0/X
switchport access vlan XXX
switchport mode trunk
do show interface gi0/1 trunk

defining an access vlan right in front of the trunk helps to fallback there in case of troubles

ready to go

do write memory
!do copy running-config startup-config



Introduction to Cisco IOS CLI (Command-Line Interface)

User Mode and Privileged Mode Security

IOS and Configuration Basics

Cisco IOS Configuration Fundamentals Command Reference

Cisco IOS Configuration Fundamentals Command Reference, Release 12.2

IOS and Configuration Basics

howto save Running Config File from CLI ?

Cisco Courses

Self-Paced Lessons


What is IP Subnet Zero? - Cisco Articles & Tips

What does ip subnet-zero command do?


vlan internal allocation policy ascending

Internal Vlan usage for routed port

Catalyst 3560 Software Configuration Guide, Release 12.2(58)SE

VLAN analysis With Wireshark

Wireshark capture VLAN IDs

VLAN range 1006-1024

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