13 September 2014

Quick Notes - VTP Modes and Features


[-] VTPv1 and VTPv2 use four types of messages:

Summary Advertisement: This message is originated by VTP
Server and Client switches every 5 minutes and, in addition,
after each modification to the VLAN database. This message
carries information about VTP domain name, revision number,
identity of the last updater, time stamp of the last update, MD5
sum computed over the contents of the VLAN database and the
VTP password (if configured), and the number of Subset Advert-
isement messages that optionally follow this Summary Advert-
isement. Summary Advertisement messages do not carry
VLAN database contents.

Subset Advertisement: This message is originated by VTP
Server and Client switches after modifying the VLAN database.
Subset Advertisements carry full contents of the VLAN database.
One Subset Advertisement can hold multiple VLAN database
entries. However, multiple Subset Advertisements might be
required if the VLAN database is large.

Advertisement Request: This message is originated by
VTP Server and Client switches to request their neighbors
send the complete VLAN database or a part of it. Advertisement
requests are sent when a VTP Client switch is restarted, when
a switch enters the Client mode, or when a Server or Client switch
receives a Summary Advertisement with a higher revision
number than its own.

Join: This message is originated by each VTP Server and Client
switch periodically every 6 seconds if VTP Pruning is active.
Join messages contain a bit field that, for each VLAN in the
normal range, indicates whether it is active or unused (that is, pruned).


Note In any VTP version, VTP messages are transmitted and
accepted only on trunk ports. Access ports neither send nor
accept VTP messages. For two switches to communicate in VTP,
they must first be interconnected through a working trunk link.


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07 September 2014

Quick Notes - Switching Work Method

Store-and-forward

The switch fully receives all bits in the frame (store) before
forwarding the frame (forward). This allows the switch to
check the frame check sequence (FCS) before forwarding
the frame, thus ensuring that errored frames are not forwarded.

Cut-through

The switch performs the address table lookup as soon as
the Destination Address field in the header is received.
The first bits in the frame can be sent out the outbound
port before the final bits in the incoming frame are received.
This does not allow the switch to discard frames that fail
the FCS check, but the forwarding action is faster, resulting
in lower latency.

Fragment-free

This performs like cut-through switching, but the switch
waits for 64 bytes to be received before forwarding the
first bytes of the outgoing frame. According to Ethernet
specifications, collisions should be detected during the
first 64 bytes of the frame, so frames that are in error
because of a collision will not be forwarded.


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27 August 2014

Quick Notes - STP Port States and Times


IEEE 802.1d Spec.

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