ATM Interfaces

On Asynchronous Transport Mode (ATM) is a Layer 2, connection-oriented, switching protocol, based on L2 Virtual Circuits (VCs). For operation in a connectionless IP routing or bridging environment, the IP PDUs must be encapsulated within the payload field of an ATM AAL5 CPCS-PDU (ATM Adaptation Layer 5 Common Part Convergence Sublayer Protocol Data Unit). The ATM CPCS-PDUs are divided into 48-byte segments which receive 5-byte headers to form 53-byte ATM cells.

The ATM cells are then switched across the ATM network, based on the Virtual Port Identifiers (VPIs) and the Virtual Connection Identifiers (VCIs). The relationship between VPIs (identifying one hop between adjacent nodes) and VCIs (identifying the end-to-end virtual connection) is illustrated in the following figure.

Figure: ATM VPI/VCI Pairs (PVCs)

Bridged ATM' Versus Routed ATM

The ATM AAL5 frames allow for the overlay of the connectionless IP bridging or routing environment over the network of ATM nodes (that have frame handling capability). Each ATM node examines the payload of the AAL5 frame, and forwards the frame to the next node, based on the payload's MAC destination address (for IP bridging) or IP destination address (for IP routing). In effect, the ATM environment functions as a simulated Ethernet or IP network, respectively.

In the case of Label Distribution Protocol (LDP) routing over ATM, the process becomes more complex since MPLS tunnels are created over ATM core networks. For more information on the signaling, session setup, and label distribution for LDP routing over ATM, see the IxNetwork Users Guide: NetworkProtocols - LDP chapter.

ATM Encapsulation Types

There are two main types of ATM Multiplexing encapsulations defined by RFC 2684, `Multiprotocol Encapsulation over ATM Adaptation Layer 5.' The ATM AAL5 Frame is described in ATM Frame Formats. The various encapsulation types and references to diagrams of the encapsulated frame payloads are listed as follows:

Note: The Protocol Configuration Wizards for BGP, OSPFv2, and ISIS allow configuration on ATM ports, but ONLY for the VC Mux Bridged Ethernet/802.3 (FCS) encapsulation type.

Encapsulation Types by Protocol

The types of RFC 2684 ATM encapsulations available for each Ixia routing protocol emulation are listed in the following table.

ATM Encapsulations for Protocols
Routing Protocol ATM Encapsulation Type

BGP

`Bridged ATM':

  • VC Mux Bridged Ethernet/802.3 (FCS) - (the default)
  • VC Mux Bridged Ethernet/802.3 (no FCS)
  • LLC Bridged Ethernet (FCS)
  • LLC Bridged Ethernet (no FCS)

`Routed ATM':

  • VC Mux IPv4 Routed
  • VC Mux IPv6 Routed
  • LLC Routed AAL5 Snap

OSPF (v2 only)

Note: Supported for both Point-to-Point and Point-to-MultiPoint links.

`Bridged ATM':

  • VC Mux Bridged Ethernet/802.3 (FCS) - (the default)
  • VC Mux Bridged Ethernet/802.3 (no FCS)
  • LLC Bridged Ethernet (FCS)
  • LLC Bridged Ethernet (no FCS)

`Routed ATM':

  • VC Mux IPv4 Routed
  • LLC Routed AAL5 Snap

LDP

Note: Discovery Mode must be set to Basic, and Advertising Mode must be set to Downstream on Demand (DoD).

`Bridged ATM':

  • VC Mux Bridged Ethernet/802.3 (FCS) - (the default)
  • VC Mux Bridged Ethernet/802.3 (no FCS)
  • LLC Bridged Ethernet (FCS)
  • LLC Bridged Ethernet (no FCS)

`Routed ATM':

  • VC Mux IPv4 Routed
  • LLC Routed AAL5 Snap

RSVP-TE

`Bridged ATM':

  • VC Mux Bridged Ethernet/802.3 (FCS) - (the default)
  • VC Mux Bridged Ethernet/802.3 (no FCS)
  • LLC Bridged Ethernet (FCS)
  • LLC Bridged Ethernet (no FCS)

ISIS

`Bridged ATM':

  • VC Mux Bridged Ethernet/802.3 (FCS) - (the default)
  • VC Mux Bridged Ethernet/802.3 (no FCS)
  • LLC Bridged Ethernet (FCS)
  • LLC Bridged Ethernet (no FCS)

RIP

`Bridged ATM':

  • VC Mux Bridged Ethernet/802.3 (FCS) - (the default)
  • VC Mux Bridged Ethernet/802.3 (no FCS)
  • LLC Bridged Ethernet (FCS)
  • LLC Bridged Ethernet (no FCS)

RIPng

`Bridged ATM':

  • VC Mux Bridged Ethernet/802.3 (FCS) - (the default)
  • VC Mux Bridged Ethernet/802.3 (no FCS)
  • LLC Bridged Ethernet (FCS)
  • LLC Bridged Ethernet (no FCS)

IGMP

`Bridged ATM':

  • VC Mux Bridged Ethernet/802.3 (FCS) - (the default)
  • VC Mux Bridged Ethernet/802.3 (no FCS)
  • LLC Bridged Ethernet (FCS)
  • LLC Bridged Ethernet (no FCS)

MLD

`Bridged ATM':

  • VC Mux Bridged Ethernet/802.3 (FCS)-(the default)
  • VC Mux Bridged Ethernet/802.3 (no FCS)
  • LLC Bridged Ethernet (FCS)
  • LLC Bridged Ethernet (no FCS)

PIM-SM

`Bridged ATM':

  • VC Mux Bridged Ethernet/802.3 (FCS)-(the default)
  • VC Mux Bridged Ethernet/802.3 (no FCS)
  • LLC Bridged Ethernet (FCS)
  • LLC Bridged Ethernet (no FCS)

ATM Frame Formats

The image below shows the format of the Figure: ATM AAL5 CPCS-PDU (ATM AAL5 Frame). The formats of the various types of AAL5 CPCS-PDU payloads for these frames are shown in the following diagrams:

Figure: ATM AAL5 CPCS-PDU (ATM AAL5 Frame)

Figure: VC Mux Bridged Ethernet/802.3 (FCS)

Figure: VC Mux Bridged Ethernet/802.3 (no FCS)

Figure: LLC Bridged Ethernet (FCS)

Figure: LLC Bridged Ethernet (no FCS)

Figure: VC Mux IPv4 Routed

Figure: VC Mux IPv6 Routed

Figure: LLC Routed AAL5 Snap