This document answers frequently asked questions about bridging over
ATM router interfaces.
Does the Gigabit Switch Router (GSR) support bridging?
A. No. The GSR runs Cisco IOS® Software
Releases 12.0S and 11.2GS, which are designed to provide robust IP routing and
enhanced IP services for the Internet Service Provider (ISP) community. They do
not provide support for full bridging protocols such as transparent bridging or
source route bridging, nor do they support integrated routing and bridging
(IRB). However, you can use bridged-style permanent virtual circuits (BPVCs) to
allow ATM line cards to connect to a Catalyst switch or to another remote
device that supports bridged-format request for comments
Does the Catalyst 6000 FlexWAN support bridged-format RFC 1483 protocol
data units (PDUs)?
A. Yes. The performance of this feature varies with your Cisco IOS
Software Release, which determines whether such PVCs are supported in the
software or hardware forwarding path.
Cisco IOS 12.1(13)E introduces hardware support RFC 1483 Bridged PDUs
on the Cisco 7600 Series with a PA-A3 ATM port adapter and FlexWAN. Cisco
recommends upgrading to this release to ensure maximum forwarding performance.
In previous releases, RFC 1483 Bridged PDUs significantly reduces the
forwarding performance of the Catalyst 6000 when using "IRB" and is not
If you cannot upgrade to Cisco IOS 12.1(13)E, an alternate solution is
to use the OC-12 ATM module (WS-X6101) for the Catalyst 6000 Series to support
strictly layer-2 applications such as bridged-format PDUs or LAN emulation
(LANE). If you cannot use an OC-12 interface and your application is digital
subscriber line (DSL) aggregation, use a Cisco 7200 or 7400 Series router
running routed bridge encapsulation (RBE).
Cisco IOS 12.1(5a)E1 introduced support for RFC 1483 Bridged PDUs on
the FlexWAN with the PA-A3. In this release, the cwan atm bridge
hidden command is required. We recommend running Cisco IOS
12.1(11a)E1 or higher to avoid CSCdw22284 and CSCdw44684. The cwan
atm bridge command is not needed when using Cisco IOS 12.1(13)E
RFC 1483 Bridged PDUs are supported in both Cisco IOS (Native IOS) and
Hybrid Mode (MSFC) starting with Cisco IOS 12.1(13)E or newer software. The
following table illustrates a sample configuration for RFC 1483 Bridged PDUs on
the FlexWAN and PA-A3. Ensure that you map the correct VLAN to the correct PVC.
This configuration is done in the distributed path of the FlexWAN.
Sample Configuration for RFC 1483 Bridged PDUs 12.1(13)E1 or
no ip address
switchport access vlan 30
no ip address
The ATM OC-12 Optical Services Module (OSM) for the Cisco 7600 Series
also supports RFC 1483 bridging. For more information and a sample
the OC-12 ATM Optical Services Modules.
Why don't the Catalyst 5000/6000 ATM modules bridge between two
subinterfaces in the same virtual LAN (VLAN)?
A. The configuration guides for the ATM modules note the following
restriction: If two PVCs are configured on the same VLAN and ATM module,
packets received from one PVC are not forwarded to the other PVC. The reason
has to do with the architecture of the modules. The WS-X515x series and the
WS-X5166 use a unidirectional backplane interface chip that prevents the module
from sending packets that it receives. The WS-X516x series uses two
unidirectional backplane interface chips. However, it cannot send packets that
it receives because of automatic echo suppression, which is enabled by default
to avoid loops since the Spanning Tree logic in the Catalyst 5000 works
per-port, per-VLAN and not on individual PVCs.
The workarounds are:
Use a full-mesh rather than a hub-and-spoke ATM PVC
Use a router configured with the standard Cisco IOS bridging. You
must configure two different subinterfaces in the same bridge group to allow
broadcasts and other traffic to pass between two remote
Use a route switch module (RSM), route switch feature card (RSFC), or
multilayer switch feature card (MSFC). Terminate the PVCs on the router and
then route or bridge between them.
Do Cisco routers support RFC 1483 aal5mux bridged encapsulation
A. No. Virtual Circuit (VC)-based multiplexing requires that the
encapsulated or carried protocol be manually configured on the ATM interface.
Cisco supports the following protocols with aal5mux encapsulation:
7500-1(config-subif)#atm pvc 1 0 200 aal5mux ?
apollo Apollo Domain
ipx Novell IPX
vines Banyan VINES
xns Xerox Network Services
With VC-based multiplexing, the network protocol carried across the ATM
network is identified implicitly by the VC connecting the two ATM stations.
That is, each protocol must be carried over a separate VC. This is not
available on the Cisco 7600 with the ATM OSM.
Will a router pass an 802.1Q header through an ATM interface?
A. No. In the following topology, the router will not preserve the 802.1Q
label when bridging over the ATM link. The 802.1Q label is removed at the
However, there is an alternate way to build end-to-end VLANs. Configure
both routers with the same set of bridge groups and 802.1Q subinterfaces. In
other words, the two router ends have a one-to-one mapping between a
bridge-group and an 802.1Q subinterface. This is not applicable to the Cisco
Router Configuration for end-to-end VLANs
no ip address
encapsulation dot1Q 100
interface atm 5/0.100
In addition, Cisco IOS now supports layer-2 virtual private networks
(VPNs) to transparently carry a Layer 2 architecture across an ATM cloud. Any
Transport over Multiprotocol Label Switching (AToM) is the Cisco solution for
transporting Layer 2 packets over an Internet Protocol/Multiprotocol Label
Switching (IP/MPLS) backbone. Atom extends the usability of IP networks by
enabling the transport of Layer 2 frames over an MPLS backbone. AToM is
required for supporting legacy services over MPLS infrastructures and for
supporting several new connectivity options, including Layer 2 VPNs and Layer 2
virtual leased lines. For more information on AToM, see
- Cisco Any Transport over MPLS.
Cisco IOS Software Releases 12.0(10)ST/12.0(21)ST and 12.0(22)S
introduce support for ATM Atom (AAL5 over MPLS) on Cisco 12000 Series ATM line
AToM-ATM AAL5 over MPLS for more information. Cisco IOS Software Release
12.0(22)SY introduces ATM cell relay over MPLS on the 7200 Series and 7500