Configuring Token Ring Inter-Switch Link
Technology Overview
BC-147
Cisco IOS Bridging and IBM Networking Configuration Guide
78-11737-02
Token Ring VLANs
A VLAN is essentially a broadcast domain. In transparent bridging, there is only one type of broadcast
frame and, therefore, only one level of broadcast domain and one level of VLAN. In source routing,
however, there are two types of broadcast frames:
• Those that are confined to a single ring
• Those that traverse the bridged domain
Therefore, there are two levels of VLANs in a Token Ring switched network.
The first level is the Token Ring Concentrator Relay Function (TrCRF). At this level, the VLAN is a
logical ring and, as such, is assigned a ring number. On a Token Ring switch, the logical ring (TrCRF)
contains one or more physical ports. On a router, the logical ring (TrCRF) does not contain any physical
ports, but rather is used only in processing source-routed traffic to terminate the routing information field
(RIF).
The second level is the Token Ring Bridge Relay Function (TrBRF). This is the parent VLAN to which
TrCRF VLANs are assigned. At this level, the VLAN is a logical bridge and, as such, is assigned a bridge
number. The logical bridge (TrBRF) contains the virtual ports that establish a connection between the
TrBRF and its TrCRFs. The TrBRF can be extended across a network of switches and routers via ISL,
as shown in Figure 52.
Figure 52 Physical View of Switches Interconnected via ISL
When you extend the TrBRF across an ISL link, you are essentially extending the bridge across devices,
as shown in Figure 53.
Figure 53 Logical View of Switches Interconnected via ISL
Therefore, if you use source-route bridging between the TrCRFs that belong to the TrBRF, only one hop
appears in the RIF.
Traffic is switched between the ports in a TrCRF and bridged via SRB or SRT between the TrCRFs in a
TrBRF.
TrBRF 3
ISL
Switch A
TrCRF
200
TrCRF
350
TrCRF
400
1859
TrCRF
400
TrCRF
350
TrCRF
200
