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15-4
Catalyst 2950 Desktop Switch Software Configuration Guide
78-14982-01
Chapter 15 Configuring VTP
Understanding VTP
• MD5 digest VLAN configuration, including maximum transmission unit (MTU) size for each
VLAN.
• Frame format
VTP advertisements distribute this VLAN information for each configured VLAN:
• VLAN IDs
• VLAN name
• VLAN type
• VLAN state
• Additional VLAN configuration information specific to the VLAN type
VTP Version 2
If you use VTP in your network, you must decide whether to use version 1 or version 2. By default, VTP
operates in version 1.
VTP version 2 supports these features not supported in version 1:
• Token Ring support—VTP version 2 supports Token Ring Bridge Relay Function (TrBRF) and
Token Ring Concentrator Relay Function (TrCRF) VLANs. For more information about Token Ring
VLANs, see the “Configuring Normal-Range VLANs” section on page 14-4.
• Unrecognized Type-Length-Value (TLV) support—A VTP server or client propagates configuration
changes to its other trunks, even for TLVs it is not able to parse. The unrecognized TLV is saved in
NVRAM when the switch is operating in VTP server mode.
• Version-Dependent Transparent Mode—In VTP version 1, a VTP transparent switch inspects VTP
messages for the domain name and version and forwards a message only if the version and domain
name match. Because VTP version 2 supports only one domain, it forwards VTP messages in
transparent mode without inspecting the version and domain name.
• Consistency Checks—In VTP version 2, VLAN consistency checks (such as VLAN names and
values) are performed only when you enter new information through the CLI, the Cluster
Management Software (CMS), or SNMP. Consistency checks are not performed when new
information is obtained from a VTP message or when information is read from NVRAM. If the MD5
digest on a received VTP message is correct, its information is accepted.
VTP Pruning
VTP pruning increases network available bandwidth by restricting flooded traffic to those trunk links
that the traffic must use to reach the destination devices. Without VTP pruning, a switch floods
broadcast, multicast, and unknown unicast traffic across all trunk links within a VTP domain even
though receiving switches might discard them. VTP pruning is disabled by default.
VTP pruning blocks unneeded flooded traffic to VLANs on trunk ports that are included in the
pruning-eligible list. Only VLANs included in the pruning-eligible list can be pruned. By default,
VLANs 2 through 1001 are pruning eligible switch trunk ports. If the VLANs are configured as
pruning-ineligible, the flooding continues. VTP pruning is supported with VTP version 1 and version 2.
Figure 15-1 shows a switched network without VTP pruning enabled. Port 1 on Switch 1 and Port 2 on
Switch 4 are assigned to the Red VLAN. If a broadcast is sent from the host connected to Switch 1,
Switch 1 floods the broadcast and every switch in the network receives it, even though Switches 3, 5,
and 6 have no ports in the Red VLAN.