As Ron Trunk pointed out, bridging VLANs can be problematic in the results. Cisco offers SPAN for network monitoring. You can mirror the traffic from on or more interfaces or VLANs to an interface to which you connect your monitoring equipment. There is also RSPAN that lets you transport the the mirrored traffic across layer-2 to a different switch, and ERSPAN (for select equipment) that will encapsulate the mirrored traffic so that it can cross layer-3.
Understanding SPAN,RSPAN,and ERSPAN
Local SPAN: Mirrors traffic from one or more interface on the switch
to one or more interfaces on the same switch.
Remote SPAN (RSPAN): An extension of SPAN called remote SPAN or RSPAN.
RSPAN allows you to monitor traffic from source ports distributed over
multiple switches, which means that you can centralize your network
capture devices. RSPAN works by mirroring the traffic from the source
ports of an RSPAN session onto a VLAN that is dedicated for the RSPAN
session. This VLAN is then trunked to other switches, allowing the
RSPAN session traffic to be transported across multiple switches. On
the switch that contains the destination port for the session, traffic
from the RSPAN session VLAN is simply mirrored out the destination
port.
Encapsulated remote SPAN (ERSPAN): encapsulated Remote SPAN (ERSPAN),
as the name says, brings generic routing encapsulation (GRE) for all
captured traffic and allows it to be extended across Layer 3 domains.
ERSPAN is a Cisco proprietary feature and is available only to
Catalyst 6500, 7600, Nexus, and ASR 1000 platforms to date. The ASR
1000 supports ERSPAN source (monitoring) only on Fast Ethernet,
Gigabit Ethernet, and port-channel interfaces.
To control traffic between the VLANs, you use something like an ACL on the router that routes traffic between the VLANs.
