Token Ring Bridge Relay Function (TrBRF) and the Token Ring Concentrator Function (TrCRF)

Token Ring Bridge Relay Function ( TrBRF ) and the Token Ring Concentrator Function (TrCRF)

The Token Ring bridge relay function (TrBRF) functions like a multiport bridge. Its purpose is to bridge rings. The rings that it bridges are called Token Ring concentrator relay functions (TrCRF). (These probably would have been called virtual rings, but that name was already taken.) Multiple TrCRFs can attach to a single TrBRF, much like multiple rings can attach to a single bridge. TrCRFs can exchange data through source-route bridging (SRB) or source-route transparent bridging (SRT). If you are not familiar with these bridging technologies, you might want to read these sections of Chapter 13, "Configuring Bridging and Data Link Switching Plus."

The TrBRF spans Catalyst switches, much like a Ethernet trunk. This allows TrCRFs on different Catalysts to belong to the same bridge number. Remember that the TrCRF is "the ring." A TrBRF must be created before a TrCRF is defined. Each TrBRF is identified by a bridge number and VLAN ID. When you create a TrCRF, you identify it with a ring number and another unique VLAN ID. The TrCRF must be assigned to a parent TrBRF.

Figure 2-18 illustrates the TrBRF TrCRF relationship in the Catalyst 3920. The trbrf-default and the trcrf-default are the actual default TrBRF and TrCRF for all Catalyst switches. Think of them like an abstract VLAN 1 on Ethernet switches. Like Ethernet switches, Token Ring switches were designed to be "plug-and-play" on smaller networks. By default, all ports will be assigned to the trcrf-default, and the trcrf-default has TrBRF as its parent.

Figure 2-18. Default TrBRF TrCRF Relationship

The concept of default TrBRFs and TrCRFs made LAN migrations from shared hub/MSAU environments easy. Out of the box, a Token Ring switch could be deployed without any configuration in place of a hub or MSAU. Like Ethernet switches ports that reside on the same default VLAN, Token Ring ports reside on the same default ring and bridge. Table 2-16 lists the default VLAN settings for Cisco switches.

Table 2-16. Default VLAN Settings

Feature	Default Value
Native or Default VLAN	VLAN 1
Port VLAN assignments	All ports assigned to VLAN1; Token Ring ports assigned to VLAN 1003
VTP mode	Transparent
VLAN state	Active
Normal VLAN range	VLAN 2 to VLAN 1001
VLAN reserved range [*] VLAN extended range [*]	VLAN 1006 to VLAN 1009 VLAN 1025 to VLAN 2094
MTU size	1500 bytes for Ethernet 4472 bytes for Token Ring
SAID value	100,000 plus VLAN number VLAN2 = SAID 100002
Prune eligibility	VLANS 2-1000 are prune-eligible
MAC address reduction	Disabled
Spanning Tree mode	PVST
Default FDDI VLAN	VLAN 1002
Default Token Ring TrCRF VLAN	VLAN 1003
Default FDDI Net VLAN	VLAN 1004
Default Token Ring TrBRF VLAN	VLAN 1005 with bridge number 0F
Spanning Tree version for TrBRF VLANs	IBM
TrCRF bridge mode	SRB

^[*] The VLAN reserved range is used on the Catalyst 6500 series and is used to map nonreserved VLANs. The VLAN extended range is available on the Catalyst 6500 series switch. This range is an extension of the normal VLAN range. The extended and reserved VLAN range is not propagated by VTP. Token Ring and FDDI VLANs are listed on Ethernet-only switches because of global VTP information. Likewise, Token Ring switches' VLAN database lists Ethernet VLANs.

Figure 2-19 diagrams the logical layout of a Token Ring switch. Two TrBRFs were created on the switch. TrBRF brf100 will be bridge 10. This BRF is the parent to the TrCRF crf-ring10, which will be ring 10. All ports on the switch assigned to this CRF will be in ring 10. In this figure, ports 16 to 20 will all be on ring 10. The second TrBRF is used to link bridge 11 to ring 11; ports 21-24 will be on ring 11.

Figure 2-19. Logical TrBRF TrCRF Relationship

When this switch is configured, you will have two bridging domains. Figure 2-20 shows another conceptual view of the same configuration, represented in a more traditional fashion.

Figure 2-20. Conceptual View of TrBRFs and TrCRFs

For the bridging domains to communicate, they need to be connected with another source-route bridge, usually in the form of a router. Figure 2-21 illustrates how the two domains can be linked with a router. If the network were transporting a bridged protocol such as SNA, the router could be configured for source-route bridging, and the two domains could be source-route bridged. If the network were transporting a routed protocol such as IP, the router would be configured to "route" traffic between the two domains.

Figure 2-21. Linking Two Token Ring Bridging Domains