What is Integrated Routing and Bridging


This document answers frequently asked questions about bridging over ATM router interfaces.

Q. Does the Gigabit Switch Router (GSR) support bridging?

Answer: No. The GSR runs the Cisco IOS® Software Releases 12.0S and 11.2GS designed for robust IP routing and advanced IP services for the Internet Service Provider (ISP) community. They neither support complete bridging protocols such as transparent bridging or source routing bridging, nor integrated routing and bridging (IRB). However, you can use bridge-style persistent virtual circuits (BPVCs) to allow ATM line cards to connect to a Catalyst switch or other remote device that only requests bridge format requests for comments on RFC 1483 -PDUs supported.

Q. Does the Catalyst 6000 FlexWAN support RFC 1483 Protocol Data Units (PDUs) in bridge format?

Answer: Yes. The performance of this feature depends on the Cisco IOS software version, which determines whether these PVCs are supported in the software or hardware routing path.

Cisco IOS 12.1 (13) E introduces RFC 1483 bridged PDUs with PA-A3 ATM port adapters and FlexWAN on the Cisco 7600 series. Cisco recommends upgrading to this version for maximum forwarding performance. In previous releases, the forwarding performance of the Catalyst 6000 when using "IRB" was significantly reduced by RFC 1483 bridged PDUs.

Alternatively, if you are unable to upgrade to Cisco IOS 12.1 (13) E, you can use the OC-12 ATM module (WS-X6101) for the Catalyst 6000 series to only support Layer 2 applications such as PDUs in bridge format or to support LAN emulation (LANE). If you cannot use an OC-12 interface and your application is digital subscriber line aggregation (DSL), use a Cisco 7200 or 7400 series router with routed bridge encapsulation (RBE).

Cisco IOS 12.1 (5a) E1 provides support for RFC 1483 bridged PDUs on the FlexWAN with the PA-A3. In this version the command is cwan atm bridge secret required. We recommend running Cisco IOS 12.1 (11a) E1 or higher to avoid CSCdw2284 and CSCdw44684. The command cwan atm bridge is not required when using Cisco IOS 12.1 (13) E or newer.

RFC 1483 Bridged PDUs are supported in Cisco IOS (Native IOS) and Hybrid Mode (MSFC) from Cisco IOS 12.1 (13) E-Software. The following table shows an example configuration for RFC 1483 bridged PDUs on FlexWAN and PA-A3. Make sure you assign the correct VLAN to the correct PVC. This configuration takes place via the distributed path of the FlexWAN.

Example configuration for RFC 1483 Bridged PDUs 12.1 (13) E1 or higher
vlan 30! interface FastEthernet7 / 1 no ip address duplex full speed 100 switchport switchport access vlan 30! interface ATM9 / 1/0 no ip address mtu 4096 atm bridge-enable bandwidth 2000 pvc 0/39 bridge-vlan 30 encapsulation aal5snap! router rip network network!

The ATM OC-12 Optical Services Module (OSM) for the Cisco 7600 Series also supports RFC 1483 bridging. For more information and a sample configuration, see Configuring the OC-12 ATM Optical Services Module.

Q. Why don't the Catalyst 5000/6000 ATM modules bridge between two subinterfaces on the same virtual LAN (VLAN)?

Answer: In the configuration guides for the ATM modules, the following restriction is noted: If two PVCs are configured on the same VLAN and ATM module, packets received by one PVC will not be forwarded to the other PVC. The reason for this is the architecture of the modules. The WS-X515x series and WS-X5166 use a unidirectional interface chip for the backplane 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 it receives due to the automatic echo cancellation, which is enabled by default to avoid loops, as the spanning tree logic in the Catalyst 500 works per port, per VLAN, and not on individual PVCs.

The workarounds are:

  • Instead of a hub-and-spoke ATM PVC design, use a full mesh design.

  • Use a router configured with Cisco IOS standard bridging. You must configure two different subinterfaces in the same bridge group so that broadcasts and other traffic can be carried between two remote hosts.

  • Use a Route Switch Module (RSM), a Route Switch Feature Card (RSFC), or a Multilayer Switch Feature Card (MSFC). End the PVCs on the router, then forward or bypass them.

Q. Do Cisco routers support the bridged RFC 1483 aal5mux encapsulation format?

Answer: No. For VC-based multiplexing, the encapsulated or the transmitted protocol must be configured manually 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 appletalk AppleTalk decnet DECnet ip IP ipx Novell IPX vines Banyan VINES xns Xerox Network Services

With VC-based multiplexing, the network protocol transmitted in the ATM network is implicitly identified by the VC that connects the two ATM stations. This means that each protocol must be transmitted via a separate VC. This feature is not available on the Cisco 7600 with the ATM OSM.

Q. Does a router pass an 802.1Q header over an ATM interface?

Answer: No. In the following topology, the router does not retain the 802.1Q label when bridging over the ATM connection. The 802.1Q label is removed from the Ethernet interface.

However, there is another way to create end-to-end VLANs. Configure both routers with the same bridge groups and the same 802.1Q subinterfaces. In other words, the two router ends have a 1: 1 assignment between a bridge group and an 802.1Q subinterface. This does not apply to the Cisco 7600.

Router configuration for end-to-end VLANs
interface FastEthernet4 / 0 no ip address! interface FastEthernet4 / 0.100 encapsulation dot1Q 100 bridge-group 1! interface atm 5 / 0.100 bridge-group 1

In addition, Cisco IOS now supports Virtual Private Networks (VPNs) on Layer 2 to transparently transmit a Layer 2 architecture over an ATM cloud. Any Transport over Multiprotocol Label Switching (AToM) is the Cisco solution for the transmission of Layer 2 packets over an IP / MPLS backbone (Internet Protocol / Multiprotocol Label Switching). Atom extends the usability of IP networks by allowing Layer 2 frames to be transported over an MPLS backbone. AToM is required to support legacy services over the MPLS infrastructure and to support several new connection options such as Layer 2 VPNs and virtual Layer 2 leased lines. For more information on AToM, see Overview - Cisco Any Transport over MPLS.

Cisco IOS software versions 12.0 (10) ST / 12.0 (21) ST and 12.0 (22) S provide support for ATM Atom (AAL5 over MPLS) on Cisco ATM Line Cards 12000 series. For more information, see MPLS AToM- ATM AAL5 over MPLS. Cisco IOS Software Version 12.0 (22) SY introduces an ATM Cell Relay over MPLS on the 7200 and 7500 Series.

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