System information

Troubleshooting IBM 10-215
Frame Format
• First frame received
• Response with the minimum number of hops
• Response with the largest allowed frame size
• Various combinations of the above criteria
In most cases, the path contained in the first frame received will be used.
After a route is selected, it is inserted into frames destined for Host Y in the form of a routing
information field (RIF). A RIF is included only in those frames destined for other LANs. The
presence of routing information within the frame is indicated by the setting of the most significant
bit within the source address field, called the routing information indicator (RII) bit.
Frame Format
The IEEE 802.5 RIF is structured as shown in Figure 10-4.
Figure 10-4 The IEEE 802.5 RIF
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.
Destination
address
Source
address
R
I
I
DataRIF FCS
802.5 MAC
frame
Routing
control
Route
descriptor
Bridge
number
Ring
number
Not usedLargestLengthType D
Route
descriptor