Cisco Multiservice Switching Networks

Joining two PNNI networks always entails some inherent risks, such as keeping routing information and the network topology protected. To minimize this particular risk, two interconnect specifications address this problem. The initial interconnect specification, Interim Inter-Switch Protocol (IISP), applied to UNI 3.0 and UNI 3.1 protocols. Originally, IISP was conceived to define how two ATM routing nodes were joined prior to the release of the PNNI 1.0 specification, but IISP is now commonly used to join PNNI 1.0 networks controlled by different entities. IISP is also called PNNI 0. The follow-up specification, ATM Inter-Network Interface (AINI), provides support for the UNI 4.0 protocol. In both IISP and AINI, no routing messages are exchanged between the two networks (or nodes), and static routes direct call signaling messages across interconnected networks.

On point-to-point ATM links implementing IISP, SVC, and SPVC, circuits are still built across these links using standard ITU-T Q.2931 signaling messages. IISP does not allow dynamic routing updates across this link; rather, all routing is done using static routes encoded into the switches on opposite sides of the IISP link. IISP performs this hop-by-hop routing using a per-switch routing database. Database routing entries contain three fields eachATM address, ATM address length, and interface index. The ATM address is the same 20-byte address discussed earlier. ATM address length specifies how many bits of the associated ATM address are significant during computation of the longer match routing algorithm. Functionally, the ATM address length field is equivalent to the network mask used in IP routing algorithms. Finally, the interface index denotes the physical or logical port that the routed ATM cell should exit.

AINI only serves as a signaling standard for ATM connections built across network boundaries. It does not cover the distribution of connection routing information across these boundaries. AINI does specify interworking standards used to connect a network using the B-ISUP signaling protocol, with another network implementing the PNNI signaling protocol. AINI also defines the connection signaling used between two PNNI networks. Although AINI allows ATM service providers and enterprise network operators to use AESA or E.164 addressing techniques, it does not require providers and operators to support all four ATM addressing formats.

IISP is an asymmetric protocol. IISP has a network side and a used side. The network side is responsible for assigning VPI and VCI identifiers. Conversely, AINI is a symmetric protocol. An AINI interface joins two networks. Therefore, in an AINI interface, administrators must define by mutual agreement which side is the assigning network (responsible for allocating the connection identifier VPI and VCI values) and which side is the receiving network. The preceding and succeeding side can vary call by call.