How to set the time on routers or switches

Either on a switch or a router use clock set HH:MM:SS day MON YEAR:

Device#clock set 14:20:00 12 JUN 2014

To view the time use show clock:

Device#show clock
14:22:43.531 UTC Thu Jun 12 2014

If you want to be sure that the time is approximatively exact use a syslog server (tftpd 32) to check it. I have ~2 seconds difference between the router’s time and a computer’s time.

Sys Log Time

Sys Log Time

here you will see it much clearer:

SysLog Time 02

SysLog Time 02

Using CCP (Cisco Configuration Professional) I have used the PC to synchronize the time on the deive.
These are the commands:

clock timezone PCTime 2 0
clock summer-time PCTime date Mar 30 2003 3:00 Oct 26 2003 4:00

On reload it will synchronize with the PCTime, which has the summer hours:

  • on march, 30th 2003 it is 3am, and
  • on octomber, 26th 2003 it is 4am.


R2#show clock
*21:11:57.991 PCTime Sun Nov 23 2014

STP: IEEE instead of PVST

When changing STP modes back to default it shows “Spanning tree enabled protocol ieee“.
The thing is that the ping between two hosts in different vlans with router on a stick configurations is not working anymore.

Off course, then I have checked the ping from the router to the hosts and they worked. I have turned to the first host, re-pinged the second host, and it worked.
The ping from second host to the first host was working since I have changed the spanning-tree to pvst.

SW3(config-if)#spa mode pvst
SW3(config)#do sh spa vl 20

  Spanning tree enabled protocol ieee
  Root ID    Priority    32768
             Address     001f.c958.1a80
             Cost        19
             Port        22 (FastEthernet0/22)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32788  (priority 32768 sys-id-ext 20)
             Address     0019.563a.9580
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/3            Desg LRN 19        128.3    P2p
Fa0/13           Desg LRN 19        128.13   P2p
Fa0/22           Root LRN 19        128.22   P2p

read here:

Access Router shortcuts

[Special telnet escape help]
^^B sends telnet BREAK
^^C sends telnet IP
^^H sends telnet EC
^^O sends telnet AO
^^T sends telnet AYT
^^U sends telnet EL

VTP Advertisements

Summary Advertisements
Subset Advertisements

Request Advertisements – When a request advertisement is sent to a VTP server in the same VTP domain, the VTP server responds by sending a summary advertisement and then a subset advertisement.
VTP Client:
In addition, the VLAN configuration information that a VTP client switch receives from a VTP server switch is stored in a VLAN database, not in NVRAM.
VTP Transparent:
In transparent mode, VLAN configurations are saved in NVRAM (but not advertised to other switches), so the configuration is available after a switch reload.

Configuration Revision

Each time a VLAN is added or removed, the configuration revision number is incremented.

A VTP domain name change does not increment the revision number. Instead, it resets the revision number to zero.

The configuration revision number determines whether the configuration information received from another VTP-enabled switch is more recent than the version stored on the switch.

Trunk and VLANs

A trunk is a point-to-point link between two network devices that carries more than one VLAN. A VLAN trunk allows you to extend the VLANs across an entire network.
A VLAN trunk does not belong to a specific VLAN, rather it is a conduit for VLANs between switches and routers.

Data VLAN = data VLAN is sometimes referred to as a user VLAN.

Default VLAN
The default VLAN for Cisco switches is VLAN 1. VLAN 1 has all the features of any VLAN, except that you cannot rename it and you can not delete it.
It is a security best practice to change the default VLAN to a VLAN other than VLAN 1; this entails configuring all the ports on the switch to be associated with a default VLAN other than VLAN 1.

Native VLAN=A native VLAN is assigned to an 802.1Q trunk port.

An 802.1Q trunk port supports traffic coming from many VLANs (tagged traffic) as well as traffic that does not come from a VLAN (untagged traffic).

Management VLAN=A management VLAN is any VLAN you configure to access the management capabilities of a switch.
VLAN 1 would serve as the management VLAN if you did not proactively define a unique VLAN to serve as the management VLAN. You assign the management VLAN an IP address and subnet mask.

Voice VLANs
Tagging refers to the addition of bytes to a field in the data frame which is used by the switch to identify which VLAN the data frame should be sent to.
Static VLAN
Dynamic VLAN

Voice VLAN – Before you configure a voice VLAN on the port, you need to first configure a VLAN for voice and a VLAN for data.
1-1005, and an extended range, 1006-4094
inter-switch link (ISL),
two types of trunk ports, IEEE 802.1Q and ISL, today only 802.1Q is used.

Trunking Modes
On (default)
Dynamic auto If both ports on the switches are set to auto, they do not negotiate to be in a trunking state. They negotiate to be in the access (non-trunk) mode state.
Dynamic desirable

Collision Domains

What is a collision domain ?

A collision domain represents the end devices that are interconnected via a hub or a series of hubs. A collision domain is also referred to as a network segment. Hubs and repeaters therefore have the effect of increasing the size of the collision domain.

Switches allow the segmentation of the LAN into separate collision domains, which means that one port on the switch is one collision domain; the number of ports on the switch gives the number of collision domains, but if you have in the network L1, L2 and L3 devices you have to analyze the network and compute the number of collision domains.

I have a table that you can see (the source table is from here:

Hub/Repeater Doesn’t Split Doesn’t Split
Switch/Bridge Does Split Doesn’t Split (By default)
Can be done through VLAN
Router Does Split Does Split

To have a better understanding watch Dan’s explication from

If the video does not work, click here:

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