ServerNet Cluster Manual

ServerNet Cluster Description
ServerNet Cluster Manual—520575-003
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Node-Numbering Agent (NNA) FPGA
Figure 1-11 shows an NNA PIC and an ECL PIC.
External Routing and the NNA
To understand the role of the NNA FPGA in a ServerNet cluster, you must understand
ServerNet packets and ServerNet IDs.
ServerNet packets are the unit of transmission in a ServerNet network. A ServerNet
packet consists of a header, a variable-size data field, and a 32-bit, cyclic redundancy
check (CRC) checksum covering the entire packet. Figure 1-12 shows a detail of the
ServerNet ID portion of the header. The header contains fields for control information,
the destination ServerNet ID, and the source ServerNet ID to identify the processor or
I/O device transmitting and receiving the packet.
Each packet has a destination ServerNet ID (DID) and a source ServerNet ID (SID).
(Recall that the node number is derived from the first six bits of the 20-bit ServerNet
ID.)
The SID for ServerNet packets within a server is the same for all NonStop S-series
servers. Therefore, packets that must be sent to another node must be modified. For
outbound packets, the SID must be replaced with an SID for external routing. For
inbound packets, the DID must be replaced with a DID for internal routing.
Figure 1-11. NNA and ECL PICs
NNA PIC
ECL PIC
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