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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter9 Creating and Maintaining VLANs
Using the VLAN Trunking Protocol
VTP Version 2
If you use VTP in your network, you must decide whether to u se versi on 1 or v ers ion 2.
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 “VLANs in the VTP Database” section on page 9-15.
•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 trans par ent switc h inspe cts VTP
messages for the domain name and version and forwards a messag e o nly 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 Cluste r
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 trun k links
that the traffic must use to reach the destination devices. Without VTP p run ing, a swit ch f l oods
broadcast, multicast, and unknown unicast traffic across all t run k link s w ithi n a VTP domai n eve n
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 defaul t,
VLANs 2 through 1001 are pruning eligible on Catal yst 3550 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 9-2 shows a switched network without VTP pruning enabled. Port 1 on Switc h 1 a nd 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 re cei ves i t, even tho ugh Sw itch es 3, 5,
and 6 have no ports in the Red VLAN.