ATM Internetworking Design

Asynchronous Transfer Mode (ATM) is the first networking architecture developed specifically for supporting multiple services. ATM networks are capable of supporting audio (voice), video and data simultaneously. ATM is currently architected to support up to 2.5 Gbps bandwidth. Data networks immediately get a performance enhancement when moving to ATM due to the increased bandwidth over a WAN. Voice networks realize a cost savings due in part to sharing the same network with data and through voice compression, silence compression, repetitive pattern suppression, and dynamic bandwidth allocation. The ATM fixed-size 53-byte cell enables ATM to support the isochronicitiy of a time-division multiplexed (TDM) private network with the efficiencies of public switched data networks (PDSN).

Most network designers are first challenged by the integration of ATM with the data network. Data network integration requires legacy network protocols to traverse a cell-based switched network. ATM can accomplish this in several ways. The first of these is LAN emulation.

1. LAN emulation (LANE)
   1. LAN Emulation Client (LEC)

ATM employs a standards based specification for enabling the installed base of legacy LANs and the legacy network protocols used on these LANs to communicate over an ATM network. This standard is known as LAN emulation (LANE). LANE uses the Media Access Control (MAC) sublayer of the OSI data link control Layer 2. Using MAC encapsulation techniques enables ATM to address the majority of Layer 2 and Layer 3 networking protocols. ATM LANE logically extends the appearance of a LAN thereby providing legacy protocols with equivalent performance characteristics as are found in traditional LAN environments. illustrates a typical ATM topology with LANE support.

• Data forwarding
• Address resolution
• Registering MAC addresses with the LANE server
• Communication with other LECs using ATM virtual channel connections (VCCs).

End systems that support the LEC functions are:

• ATM-attached workstations
• ATM-attached servers
• ATM LAN switches (Cisco Catalyst family)
• ATM attached routers (Cisco 12000, 7500, 7000, 4700, 4500 and 4000 series)
• LAN Emulation Configuration Server (LECS)

The ELAN database is maintained by the LAN emulation configuration server (LECS). In addition, the LECS builds and maintains an ATM address database of LAN Emulation Servers (LES). The LECS maps an ELAN name to a LES ATM address. The LECS performs the following LANE functions:

• Accepts queries from a LEC
• Responds to LEC query with an ATM address of the LES for the ELAN/VLAN
• Serves multiple emulated LANs
• Manually defined and maintained

The LECS assigns individual clients to a ELAN by directing them to the LES that corresponds to the ELAN.

1. 1. 1. LAN Emulation Server (LES)

      LECs are controlled from a central control point called a LAN Emulation Server (LES). LECs communicate with the LES using a Control Direct Virtual Channel Connection (VCC). The Control Direct VCC is used for forwarding registration and control information. The LES uses a Control Distribute VCC, a point-to-multipoint VCC, enabling the LES to forward control information to all the LECs. The LES services the LAN Emulation Address Resolution Protocol (LE_ARP) request which it uses to build an maintain a list of LAN destination MAC addresses.

      2. Broadcast Unknown Server (BUS)

      ATM is based on the notion that the network is point-to-point. Therefore, there is no inherent support for broadcast or any-to-any services. LANE provides this type of support over ATM by centralizing broadcast and multicast functions on a Broadcast And Unknown Server (BUS). Each LEC communicates with the BUS using a Multicast Send VCC. The BUS communicates with all LECs using point-multipoint VCC known as the Multicast Forward VCC. A BUS reassembles received cells on each Multicast Send VCC in sequence to create the complete frame. Once a frame is complete is then sent to all the LECs on a Multicast Forward VCC. This ensures the proper sequence of data between LECs.

      3. LANE Design Considerations

The following are guidelines for designing LANE services on Cisco routers:

• The AIP has a bi-directional limit of 60 thousand packets per second (pps).
• The ATM interface on a Cisco router has the capability of supporting up to 255 subinterfaces.
• Only one active LECS can support all the ELANs. Other LECS operate in backup mode.
• Each ELAN has one LES/BUS pair and one or more LECs.
• LES and BUS must be defined on the same subinterface of the router AIP.
• Only one LES/BUS pair per ELAN is permitted.
• Only one active LES/BUS pair per subinterface is allowed.
• LANE Phase 1 standard does not provide for LES/BUS redundancy.
• The LECS can reside on a different router than the LES/BUS pair.
• VCCs are supported over switched virtual circuits (SVCs) or permanent virtual circuits (PVCs).
• A subinterface supports only one LEC.
• Protocols such as , AppleTalk, IP and IPX are routable over a LEC if they are defined on the AIP subinterface.
• AN ELAN should be in only one subnet for IP.

1. 1. 1. Network Support

The LANE support in Cisco IOS enables legacy LAN protocols to utilize ATM as the transport mechanism for inter-LAN communications. The following features highlight the Cisco IOS support for LANE:

• Support for Ethernet-emulated LANs only. There is currently no token-ring LAN emulation support.
• Support for routing between ELANs using IP, IPX or AppleTalk.
• Support for bridging between ELANs
• Support for bridging between ELANs and LANs
• LANE server redundancy support through simple server redundancy protocol (SSRP)
• IP gateway redundancy support using hot standby routing protocol (HSRP)
• DECnet, Banyan VINES, and XNS routed protocols
• 
  
• Addressing

LANE requires MAC addressing for every client. LANE clients defined on the same interface or subinterface automatically have the same MAC address. This MAC address is used as the end system identifier (ESI) value of the ATM address. Though the MAC address is duplicated the resulting ATM address representing each LANE client is unique. All ATM addresses must be unique for proper ATM operations. Each LANE services component has an ATM address unique form all other ATM addresses.

• LANE ATM Addresses

1. 1. 
      

LANE uses the NSAP ATM address syntax however it is not a Layer 3 network address. The address format used by LANE is :

• A 13-byte prefix that includes the following fields defined by the ATM Forum:

• AFI (Authority and Format Identifier) field (1 byte)
• DCC (Data Country Code) or ICD (International Code Designator) field (2 bytes)
• DFI field (Domain Specific Part Format Identifier) (1 byte)
• Administrative Authority field (3 bytes)
• Reserved field (2 bytes)
• Routing Domain field (2 bytes)
• Area field (2 bytes)

• A 6-byte end-system identifier (ESI)

• A 1-byte selector field

1. 1. 1. Cisco's Method of Automatically Assigning ATM Addresses

The Cisco IOS supports an automated function of defining ATM and MAC addresses. Theses addresses are used in the LECS database. The automation process uses a pool of eight MAC address that are assigned to each router ATM interface. The Cisco IOS applies the addresses to the LANE components using the following methodology:

• All LANE components on the router use the same prefix value. The prefix value identifies a switch and must be defined within the switch.
• The first address in the MAC address pool becomes the ESI field value for every LANE client on the interface.
• The second address in the MAC address pool becomes the ESI field value for every LANE server on the interface.
• The third address in the MAC address pool becomes the ESI field value for the LANE broadcast-and-unknown server on the interface.
• The fourth address in the MAC address pool becomes the ESI field value for the LANE configuration server on the interface.
• The selector field for the LANE configuration server is set to a 0 value. All other components use the subinterface number of interface to which they are defined as the selector field.