PCI Express Switch

A Switch is defined as a logical assembly of multiple virtual PCI-to-PCI Bridge devices as illustrated in Figure 1-3. All Switches are governed by the following base rules.

Logical Block Diagram of a Switch

• Switches appear to configuration software as two or more logical PCI-to-PCI Bridges.
• A Switch forwards transactions using PCI Bridge mechanisms; e.g., address based routing except when engaged in a Multicast, as defined in Section 6.13.
• Except as noted in this document, a Switch must forward all types of Transaction Layer Packets between any set of Ports.
• Locked Requests must be supported as specified in Section 6.5. Switches are not required to support Downstream Ports as initiating Ports for Locked requests.
• Locked Requests must be supported as specified in Section 6.5. Switches are not required to support Downstream Ports as initiating Ports for Locked requests.
• Each enabled Switch Port must comply with the flow control specification within this document.
• A Switch is not allowed to split a packet into smaller packets, e.g., a single packet with a 256-byte payload must not be divided into two packets of 128 bytes payload each.
• Arbitration between Ingress Ports (inbound Link) of a Switch may be implemented using round robin or weighted round robin when contention occurs on the same Virtual Channel. This is described in more detail later within the specification.
• Endpoints (represented by Type 00h Configuration Space headers) must not appear to configuration software on the Switch’s internal bus as peers of the virtual PCI-to-PCI Bridges representing the Switch Downstream Ports.

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