Cisco 7609 Configuration Manual page 134

Understanding How VLANs Work
Figure 9-1
Token
Ring
001
For source routing, the Cisco 7600 series router appears as a single bridge between the logical rings. The
TrBRF can function as a source-route bridge (SRB) or a source-route transparent (SRT) bridge running
either the IBM or IEEE STP. If an SRB is used, you can define duplicate MAC addresses on different
logical rings.
The Token Ring software runs an instance of STP for each TrBRF VLAN and each TrCRF VLAN. For
TrCRF VLANs, STP removes loops in the logical ring. For TrBRF VLANs, STP interacts with external
bridges to remove loops from the bridge topology, similar to STP operation on Ethernet VLANs.
Caution Certain parent TrBRF STP and TrCRF bridge mode configurations can place the logical ports (the
connection between the TrBRF and the TrCRF) of the TrBRF in a blocked state. For more information,
see the
To accommodate IBM System Network Architecture (SNA) traffic, you can use a combination of SRT
and SRB modes. In a mixed mode, the TrBRF determines that some ports (logical ports connected to
TrCRFs) operate in SRB mode while other ports operate in SRT mode
Token Ring TrCRF VLANs
Token Ring Concentrator Relay Function (TrCRF) VLANs define port groups with the same logical ring
number. You can configure two types of TrCRFs in your network: undistributed and backup.
TrCRFs typically are undistributed, which means each TrCRF is limited to the ports on a single network
device. Multiple undistributed TrCRFs on the same or separate network devices can be associated with
a single parent TrBRF (see
between the undistributed TrCRFs.
To pass data between rings located on separate network devices, you can associate the rings to the same
Note
TrBRF and configure the TrBRF for an SRB.
Cisco 7600 Series Router Cisco IOS Software Configuration Guide—12.1E
9-4
Interconnected Token Ring TrBRF and TrCRF VLANs
SRB or SRT
SRS SRS
Token Token
Ring Ring
001 011
"VLAN Configuration Guidelines and Restrictions" section on page
Figure
BRF
CRF
SRS
Token Token Token
Ring Ring Ring
002 002 002
9-2). The parent TrBRF acts as a multiport bridge, forwarding traffic
Chapter 9 Configuring VLANs
9-8.
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