The source-route
bridging (SRB) algorithm was developed by IBM and proposed to the IEEE
802.5 committee as the means to bridge between all LANs. Since its
initial proposal, IBM has offered a new bridging standard to the IEEE
802 committee: the source-route transparent (SRT)
bridging solution. SRT bridging eliminates pure SRBs entirely, proposing
that the two types of LAN bridges be transparent bridges and SRT
bridges. Although SRT bridging has achieved support, SRBs are still
widely deployed. SRT is covered in "Mixed-Media Bridging."
This chapter summarizes the basic SRB frame-forwarding algorithm and
describes SRB frame fields.
SRBs are so named because they assume
that the complete source-to-destination route is placed in all inter-LAN
frames sent by the source. SRBs store and forward the frames as
indicated by the route appearing in the appropriate frame field. Figure
25-1 illustrates a sample SRB network.
In Figure 25-1 , assume that Host X wants
to send a frame to Host Y. Initially, Host X does not know whether Host
Y resides on the same or a different LAN. To determine this, Host X
sends out a test frame. If that frame returns to Host X without a
positive indication that Host Y has seen it, Host X must assume that
Host Y is on a remote segment.
Figure 25-1: An SRB network contains LANs and
bridges.
To determine the exact remote location of
Host Y, Host X sends an explorer frame. Each bridge receiving
the explorer frame
(Bridges 1 and 2, in this example) copies the frame onto all outbound
ports. Route information is added to the explorer
frames as they travel through the internetwork. When Host X's explorer
frames reach Host Y, Host Y replies to each individually, using the
accumulated route information. Upon receipt of all response frames, Host
X chooses a path based on some predetermined criteria.
In the example in Figure
25-1 , this process will yield two routes:
- LAN 1 to Bridge 1 to LAN 3 to Bridge 3
to LAN 2
- LAN 1 to Bridge 2 to LAN 4 to Bridge 4
to LAN 2
Host X must select one of these two
routes. The IEEE 802.5
specification does not mandate the criteria Host X should use in
choosing a route, but it does make several suggestions, including the
following:
- First frame received
- Response with the minimum number of
hops
- Response with the largest allowed
frame size
- Various combinations of the preceding
criteria
In most cases, the path contained in the
first frame received is 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 setting
the most significant bit within the Source Address field, called the
routing information indicator (RII)
bit.
The IEEE 802.5 RIF is
structured as shown in Figure 25-2.
The RIF illustrated in Figure 25-2
consists of two main fields: Routing Control and Routing
Descriptor. These fields are described in the summaries that
follow.
Figure 25-2: A IEEE 802.5 RIF
is present in frames destined for other LANs.
The Routing
Control field consists of four subfields: Type, Length, D bit, and
Largest frame. The fields are summarized in the following list:
Each -routing
descriptor-routing descriptor field consists of two subfields:
Bridges add to the frame their bridge
number and the ring number onto which the frame is forwarded. (The first
bridge also adds the ring number of the first ring.)
Routes are alternating sequences of ring
and bridge numbers that start and end with ring numbers. A single RIF
can contain more than one -routing descriptor field. The IEEE specifies
a maximum of 14 -routing descriptor fields (a maximum of 13 bridges or
hops, because the last bridge number always equals zero).
Until recently, IBM specified a maximum
of eight -routing Descriptor fields (a maximum of seven bridges or
hops), and most bridge manufacturers followed IBM's implementation.
Newer IBM bridge software programs combined with new LAN adapters support
13 hops.
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