KB.15.15

Effect of downstream VLAN tagging on untagged, mirrored traffic

In a remote mirroring application, if mirrored traffic leaves the switch without 802.1Q VLAN

tagging, but is forwarded through a downstream device that adds 802.1Q VLAN tags, the MTU

for untagged mirrored frames leaving the source switch is reduced below the values shown in

Table 23 (page 364).

For example, if the MTU on the path to the destination is 1522 bytes, untagged mirrored frames

leaving the source switch cannot exceed 1518 bytes. Likewise, if the MTU on the path to the

destination is 9220 bytes, untagged mirrored frames leaving the source switch cannot exceed

9216 bytes.

Figure 193 Effect of downstream VLAN tagging on the MTU for mirrored traffic

Operating notes for traffic mirroring

• Mirroring dropped traffic

When an interface is configured to mirror traffic to a local or remote destination, packets are

mirrored regardless of whether the traffic is dropped while on the interface. For example, if

an ACL is configured on a VLAN with a deny ACE that eliminates packets from a Telnet

application, the switch still mirrors the Telnet packets that are received on the interface and

subsequently dropped.

• Mirroring and spanning tree

Mirroring is performed regardless of the STP state of a port or trunk. This means, for example,

that inbound traffic on a port blocked by STP can still be monitored for STP packets during

the STP setup phase.

• Tagged and untagged frames

For a frame entering or leaving the switch on a mirrored port, the mirrored copy retains the

tagged or untagged state the original frame carried when it entered into or exited from the

switch. (The tagged or untagged VLAN membership of ports in the path leading to the mirroring

destination does not affect the tagged or untagged status of the mirrored copy itself.)

Thus, if a tagged frame arrives on a mirrored port, the mirrored copy is also tagged, regardless

of the status of ports in the destination path. If a frame exits from the switch on a mirrored port

that is a tagged member of a VLAN, the mirrored copy is also tagged for the same reason.

To prevent a VLAN tag from being added to the mirrored copy of an outbound packet sent

to a mirroring destination, you must enter the no-tag-added parameter when you configure

a port, trunk, or mesh interface to select mirrored traffic.

Effect of downstream VLAN tagging on untagged, mirrored traffic 365