Frame Format

The IEEE 802.5 RIF is structured as shown in Figure 10-4. Figure 10-4 The IEEE 802.5 RIF

802.5 MAC frame

Destination

R

Source

RIF

Data

FCS

address

I I

address

Type

Length

D

Largest

Not used

Routing

Route

Route

control

descriptor

descriptor

Ring

Bridge

number

number

The fields of the RIF are as follows:

• The routing control field, which consists of the following subfields:

— The type subfield in the RIF indicates whether the frame should be routed to a single node, a group of nodes that make up a spanning tree of the internetwork, or all nodes. The first type is called a specifically routed frame; the second type is called a spanning- tree explorer; and the third type is called an all-paths explorer. The spanning-tree explorer can be used as a transit mechanism for multicast frames. It can also be used as a replacement for the all-paths explorer in outbound route queries. In this case, the destination responds with an all-paths explorer.

— The length subfield indicates the total length (in bytes) of the RIF.

— The D bit indicates the direction of the frame (forward or reverse).

— The largest field indicates the largest frame that can be handled along this route.

• The route descriptor field, of which there can be more than one. Each route descriptor field carries a ring number/bridge number pair that specifies a portion of a route. Routes, then, are simply alternating sequences of LAN and bridge numbers that start and end with LAN numbers.

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