This document illustrates a sample configuration between three routers and an ATM switch, using Logical Link Control (LLC) encapsulation. Router A routes on the Ethernet and performs bridging between Router B and Router C. Router B and Router C bridge between the ATM and Ethernet. No mapping is done on the PVC for bridging, because all VCs on a bridged subinterface are automatically used for bridging.
In the sample configuration, Router B and Router C are only used as Layer 2 devices, with end stations attached to their Ethernets. Therefore, you need to turn off ip routing on Router B and C.
Note: This document focuses on permanent virtual circuit (PVC) configurations on Cisco routers that run Cisco IOS® software. For PVC configuration examples on Cisco WAN switches, click here.
There are no specific requirements for this document.
The information in this document is based on these software and hardware versions:
The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.
For more information on document conventions, refer to Cisco Technical Tips Conventions.
When PVCs are used, a user has two ways to carry multiple protocols over Asynchronous Transfer Mode (ATM).
virtual circuit (VC) multiplexing—The user defines one PVC per protocol. This method uses more VCs than LLC encapsulation, but reduces overhead. This is because a header is not necessary.
LLC/SNAP Encapsulation—The user multiplexes multiple protocols over a single ATM VC. The protocol of a carried protocol data unit (PDU) is identified by prefixing the PDU with a Logical Link Control (LLC)/Subnetwork Access Protocol (SNAP) header.
LLC/SNAP headers use a routed format or a bridged format. The format of the ATM Adaptation Layer 5 (AAL5) common part convergence sublayer (CPCS)-PDU Payload field for bridged Ethernet/802.3 PDUs is seen here:
A bridged format does not necessarily mean that the encapsulated protocol is not routable. Rather, it typically is used when one side of the link supports only the bridged-format PDUs. For example, in a connection between a router and a Catalyst switch in a corporate campus ATM network. In this application, the router interface typically serves as the default gateway for the remote users. Then, integrated routing and bridging (IRB), routed bridge encapsulation (RBE) or bridged-style PVCs (BPVCs) provide the mechanism to route traffic off-network.
These protocols allow the ATM interface to receive bridged-format PDUs. However, they have important differences in performance. Cisco recommends that you consider RBE when the configuration supports it.
In this section, you are presented with the information to configure the features described in this document.
Note: To find additional information on the commands used in this document, use the Command Lookup Tool (registered customers only) .
This document uses this network setup:
Network Diagram Notes:
In the example, 1/116 is switched to 1/116 by the ATM switch and 1/118 is switched to 1/118.
The topology is a hub-and-spoke topology where Router A is the hub. Each PVC uses a different subinterface to ensure that PDUs received from Router B can be forwarded back out to Router C. Otherwise, flooded traffic that comes on one PVC on a subinterface is not flooded back on another PVC on the same subinterface.
All ATM subinterfaces are configured as multipoint. A multipoint subinterface supports multiple VCs. A point-to-point subinterface supports only one VC.
This example uses IRB for routing off-network. Refer to Configuring Integrated Routing and Bridging in the Cisco IOS Bridging and IBM Networking Configuration Guide for guidance on the use of IRB commands. See the Related Information