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:
- VC Multiplexing (VC Mux): used when only one protocol is to be carried on a single ATM VC. Separate VCs are used if multiple protocols are being transported.
- Figure: VC Mux IPv4 Routed:
- VC Mux IPv6 Routed
- VC Mux Bridged Ethernet/802.3 (FCS)
- VC Mux Bridged Ethernet/802.3 (no FCS)
- Logical Link Control (LLC): used for multiplexing multiple protocols over a single ATM virtual connection (VC).
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.
Routing Protocol | ATM Encapsulation Type |
---|---|
BGP |
`Bridged ATM':
`Routed ATM':
|
OSPF (v2 only) Note: Supported for both Point-to-Point and Point-to-MultiPoint links. |
`Bridged ATM':
`Routed ATM':
|
LDP Note: Discovery Mode must be set to Basic, and Advertising Mode must be set to Downstream on Demand (DoD). |
`Bridged ATM':
`Routed ATM':
|
RSVP-TE |
`Bridged ATM':
|
ISIS |
`Bridged ATM':
|
RIP |
`Bridged ATM':
|
RIPng |
`Bridged ATM':
|
IGMP |
`Bridged ATM':
|
MLD |
`Bridged ATM':
|
PIM-SM |
`Bridged ATM':
|
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:
- BRIDGED:
- 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)
- ROUTED:
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)