4 Protocol Examples

The following figure shows a scenario where Device 1 is connected to Bridge 1 via IPo1394. Bridge 1 is connected to Bridge 2 via IPo1394, and Bridge 2 is connected to Device 2 via Ethernet.

Scenario of four devices connected in a network

Figure 3: Scenario of four devices connected in a network

Consider the scenario outlined in the previous figure. Assume that all devices have already acquired their IP addresses, and Bridge 1 and Bridge 2 implement the spanning tree algorithm (as specified in [IEEE802.1D] section 8 and 9) for loop detection and termination. Also, assume that Bridge 2 is not yet connected to Device 2.

  1. At initialization time, the IPo1394 components of Bridge 1 and Bridge 2 report link-layer addresses of their 1394 interfaces up to higher-layer protocols. In this example, both Bridge 1 and Bridge 2 report an EUI-48 that has the Locally Administered bit set and the remaining bits generated randomly. The Spanning Tree Algorithm Protocol (STP), being one of the higher-layer protocols, uses the addresses of each interface to generate a Bridge Identifier, as specified in [IEEE802.1D] section 8.5.3.7. Bridge 1 has only 1394 interfaces, so the Bridge Identifier is constructed from the EUI-48 on the lowest numbered interface (as specified in [IEEE802.1D] section 7.12.5). Similarly, Bridge 2 also constructs a Bridge Identifier from the address of its lowest numbered interface, which in the example is its Ethernet interface.

  2. The Ethernet cable is first plugged into Bridge 2. This media change triggers the STP implementation to send an STP packet on both interfaces in Bridge 2, as specified in [IEEE802.1D] section 9. Therefore, the packet that appears on the 1394 cable looks like the following: Bridge 2 sends an STP packet on both interfaces—one using Ethernet to Device 2 and the other using IPo1394 to Bridge 1. The format of the packet sent from Bridge 2 to Bridge 1 via IPo1394 follows:


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    lf = 0x0

    Reserved = 0x0

    ether_type = 0x0777

    Standard STP message (variable)

    ...

    The preceding packet is sent on the 1394 broadcast channel, and the Bridge Identifier used by STP is the one generated from its Ethernet media access control (MAC) address.

  3. Bridge 1 receives the packet and delivers it up to the STP component.

  4. Based on STP details, the STP component on Bridge 1 then decides to send an STP packet on the 1394 cable toward Device 1. The packet that appears on the 1394 cable looks the same as in the preceding diagram, but the STP message is a new STP message, and the Bridge Identifier used by STP is based on the random EUI-48 generated for the 1394 cable toward Device 1. This packet is sent on the 1394 broadcast channel.

  5. Device 1 receives the STP packet via IPo1394. Because Device 1 is not a bridge, and is not running STP, there is no STP component to deliver the packet up to, and the packet is ignored.

The following figure shows the sequence of packets exchanged in this example.

Sequence of packets exchanged

Figure 4: Sequence of packets exchanged

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