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Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
OL-8978-04
Chapter 11 Configuring VLANs
Configuring VLAN Mappings on a Per-Port or Per-ASIC Basis
Global VLAN mapping supports a maximum of eight VLANs. If VLAN X is mapped to VLAN Y,
VLAN Y is mapped to a discarded VLAN internally. Per-port/per-ASIC VLAN mapping does not
work that way. If VLAN X is mapped to VLAN Y, all the internally switched traffic to a port on
VLAN Y is mapped to VLAN X.
•
VLAN mapping is applied in both directions. For example, if port P has a VLAN mapping of
VLAN x to VLAN y, all the traffic received by port P on VLAN X is mapped and processed in
VLAN Y, and all the traffic internally tagged with VLAN Y that leaves port P, is tagged with
VLAN X.
•
EtherChannel
VLAN mapping is supported on EtherChannels, both PAgP and LACP. If you enable or disable
VLAN mapping on one port of the channel, the feature is enabled or disabled on all the ports in the
channel. Similarly, if you configure a VLAN mapping on one port in the channel, the mapping is
applied to all ports in the channel.
All the ports in the EtherChannel must have the same port ASIC capability in terms of VLAN
mapping. If you try to configure a VLAN mapping on an EtherChannel where some of the ports in
the channel do not have the same port ASIC capabilities, the command is rejected.
•
SPAN and RSPAN
If per-port VLAN mapping is enabled on a port, the port ASIC changes the source VLAN to the
translated VLAN. Any SPAN configuration works on the translated VLAN.
The RSPAN VLAN cannot be translated; you must not configure the RSPAN VLAN to be mapped
to any VLAN. Similarly, the translated VLAN cannot be used as an RSPAN VLAN.
•
Spanning tree
In the PVST+ implementation, spanning-tree BPDUs are tagged with a TLV of “VLAN ID” on each
trunk port. This TLV helps spanning tree in determining the port VLAN ID consistency. In PVST+
and Rapid-PVST+, this VLAN ID is equal to the spanning-tree instance number (the VLAN ID).
With Shared Spanning Tree Protocol (SSTP), be careful when per-port/per-ASIC VLAN mapping
is enabled on a port. For example, in
Figure 11-2
, switch 1 and switch 2 are connected using trunk
T that carries VLAN 101. On switch 2, per-port/per-ASIC VLAN mapping is enabled on trunk port
P and one of the mappings is VLAN 101 to VLAN 202. As shown in
Figure 11-2
, on the trunk link,
the BPDU has the 802.1Q VLAN and the TLV VLAN as VLAN 101. When this BPDU reaches
port P, its 802.1Q VLAN is changed to VLAN 202 because of the mapping but the TLV VLAN still
remains VLAN 101. When the BPDU reaches the spanning-tree process, spanning tree concludes
that the VLAN 101 BPDU is received on VLAN 202 and thinks that it is inconsistent and reports
this as an inconsistent port.
To correct this problem, the spanning tree processes this BPDU in VLAN 202 and the TLV VLAN
is mapped to the translated VLAN and checked for consistency. When that occurs, the spanning-tree
instance 101 of switch 1 is merged with the spanning-tree instance 202 of switch 2. This process is
also done on the transmit side.
Figure 11-2
Understanding VLAN Mapping and Spanning Tree
Switch 1
Switch 2
802.1Q tag
(VLAN 101)
TLV
(VLAN 101)
BPDU
Trunk T
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