3Com 4210 PWR Configuration Manual page 323

All devices use NDP to collect the information about their neighbors, including
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software version, host name, MAC address, and port name.
The management device uses NTDP to collect the information about the
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devices within specific hops and the topology information about the devices. It
also determines the candidate devices according to the information collected.
The management device adds the candidate devices to the cluster or removes
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member devices from the cluster according to the candidate device information
collected through NTDP.
Introduction to NDP
NDP is a protocol used to discover adjacent devices and provide information about
them. NDP operates on the data link layer, and therefore it supports different
network layer protocols.
NDP is able to discover directly connected neighbors and provide the following
neighbor information: device type, software/hardware version, and connecting
port. In addition, it may provide the following neighbor information: device ID,
port full/half duplex mode, product version, the Boot ROM version and so on.
An NDP-enabled device maintains an NDP neighbor table. Each entry in the
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NDP table can automatically ages out. You can also clear the current NDP
information manually to have neighbor information collected again.
An NDP-enabled device regularly broadcasts NDP packet through all its active
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ports. An NDP packet carries a holdtime field, which indicates how long the
receiving devices will keep the NDP packet data. The receiving devices store the
information carried in the NDP packet into the NDP table but do not forward
the NDP packet. When they receive another NDP packet, if the information
carried in the packet is different from the stored one, the corresponding entry
in the NDP table is updated, otherwise only the holdtime of the entry is
updated.
Introduction to NTDP
NTDP is a protocol used to collect network topology information. NTDP provides
information required for cluster management: it collects topology information
about the switches within the specified hop count, so as to provide the
information of which devices can be added to a cluster.
Based on the neighbor information stored in the neighbor table maintained by
NDP, NTDP on the management device advertises NTDP topology collection
requests to collect the NDP information of each device in a specific network range
as well as the connection information of all its neighbors. The information
collected will be used by the management device or the network management
software to implement required functions.
When a member device detects a change on its neighbors through its NDP table, it
informs the management device through handshake packets, and the
management device triggers its NTDP to perform specific topology collection, so
that its NTDP can discover topology changes timely.
The management device collects the topology information periodically. You can
also launch an operation of topology information collection by executing related
commands. The process of topology information collection is as follows.
Cluster Overview 321