Cisco Multiservice Switching Networks

If the planned PNNI network is destined to interconnect with other PNNI routing domains that are owned by other corporations or governments, it behooves the designer to secure globally unique ATM End System Address (AESA) prefixes from a recognized standards body. Globally unique prefixes prevent address duplication in the interconnected networks.

Table 9-2 lists the three standardized AESAs and the authorities that control the distribution of addresses within each.

Typically, telephony service providers that provide intercountry services use E.164 addressing. The ICD format's 2-byte code designator field is used to assign an organization or company such as Cisco Systems, Inc. a globally unique identifier. Many enterprises choose ICD AESA formatting for this reason.

Within an AESA, the network designer is free to define the format of the remaining bytes of the HO-DSP, ESI, and selector portions. Typically, the 6-byte ESI is filled with the IEEE formatted MAC address of the ATM switch's Ethernet management interface. The user-defined hierarchy levels are carved out of the unrestricted HO-DSP bits.

For example, the DCC AESA format allows the private entity purchasing the DCC-formatted address to specify the least-significant 6 bytes of the 13-byte DCC prefix. Following the guideline to allot 4 AESA bits per hierarchy level, potentially 12 additional user-defined hierarchy levels can be constructed. Because AESAs are mostly displayed in hexadecimal format, the 4-bit addressing principal ensures that the AESA portion associated with each hierarchy level is easily identifiable, because each hexadecimal number represents 4 bits. (This principle is discussed in the "Identifying the Hierarchy Levels" section in Chapter 8, "PNNI Explained.")

Continuing with this DCC example, a network designer devising the address plan might set the lowest hierarchy level at 88 bits. Using the 4-bit level selection process, the 88-bit level allows up to four levels of lower-layer expansion. Lower-layer expansion is useful when a rapidly expanding business unit within a company subdivides. On the opposite end of 88, eight higher-level layers are available.

A well designed address structure is of paramount importance to optimize routing, simplify planning, and allow multi peer group migration. The structure can be geographically based (for example, 8 bits for the continent, next 8 bits for the country, and then the region, city, and so on) or administratively assigned. PNNI addressing allows a structure that scales globally.