Token Ring VLANs

This section discusses VLANs as they are applied to Token Ring networks. Only the Catalyst 5000 and the Catalyst 3900 switches support Token Ring—both using CLI-based commands.

Recall from the discussion in Chapter 3, "Basic Switch and Port Configuration," the basic topology of Token Ring networks. End stations are connected to multistation access units (MSAUs), which interconnect with other MSAUs to form a ring. Multiple rings can be interconnected by bridges for segmentation and frame forwarding using source-route bridging and the RIF information. Figure 4-11 shows a typical Token Ring network with two rings and a source-route bridge in the left half. The right half of the figure shows a similar network topology with three rings and a source-route bridge, using the Token Ring switching features of Catalyst switches. The functionality of rings and bridges are performed within the switches, using Token Ring switching functions.

Figure 4-11 Token Ring Networks: Traditional and Switched

Source route bridge

Figure 4-11 Token Ring Networks: Traditional and Switched

Source route bridge

Traditional Token Ring bridging

TrBRF

TrCRF

Ring A Ring B

ports ports

Ring C ports

TrBRF

TrCRF

Traditional Token Ring bridging

Ring A Ring B

ports ports

Token Ring switching with Catalyst switches

Ring C ports

Token Ring switching follows the same topology, but performs the various functions within the switch. Where groups of end stations are connected by MAUs in a ring, the IEEE has defined the Concentrator Relay Function (CRF). The function of a multiport bridge to connect individual rings is defined as the Bridge Relay Function (BRF). These functions as performed by Catalyst switches are further described in the sections that follow.

TrBRF

A Catalyst switch connects logical Token Ring Concentrator Relay Functions (TrCRFs) with a logical multiport bridge, or Token Ring Bridge Relay Function (TrBRF). In the hierarchy of bridged Token Rings, each TrCRF must be connected to a parent TrBRF. The hierarchical structure of Token Ring VLANs is shown in the right half of Figure 4-11.

By default, the TrBRF interconnects only TrCRFs located on the local switch. However, if trunking is used with ISL encapsulation, the TrBRF can extend to TrCRFs located on other Catalyst switches.

Each TrBRF exists as a special VLAN within a Catalyst switch. A switch can support many TrBRFs, but only one VLAN can be assigned to each TrBRF. By default, one TrBRF is defined as "trbrf-default" on VLAN 1005. Each TrBRF can operate as either a source-route bridge

(SRB), a source-route transparent (SRT) bridge, or both as a mixed mode. Additionally, each TrBRF runs a separate instance of either the IBM or IEEE Spanning-Tree Protocol to prevent bridging loops. The Spanning-Tree Protocol is covered in Chapter 5.

NOTE To create and use Token Ring VLANs, VTP version 2 must be enabled on all Catalyst switches in the Token Ring domain. Enabling VTP version 2 was discussed in the previous "VTP Configuration" section.

To define a TrBRF on a Catalyst switch, use the following command:

Switch(enable) set vlan vlan-num [name name] type trbrf bridge bridge-num [stp {ieee | ibm}]

The only two required fields for a TrBRF are the VLAN number and the bridge number. Bridge numbers are defined by a single byte value, as a hexadecimal number from 0x1 to 0xf. The default Spanning-Tree Protocol is ibm. Notice that the type of bridging is not defined with the TrBRF, although the TrBRF performs the actual bridging function. Instead, the type of bridging is defined at the TrCRF. This way, multiple TrCRFs can be connected by a single parent TrBRF, each bridged with the desired method.

TrCRF

In a Catalyst switch, individual Token Ring ports can be connected to a logical ring, or Token Ring VLAN, by assigning them with identical ring numbers. Internally, the Catalyst performs the TrCRF to maintain the ring connectivity. Frame forwarding between ports on a common ring is performed with source-route switching, using either MAC addresses or route descriptors.

The TrCRF can be confined within a single switch or can be spread across mutliple switches, depending on the topology and switch configuration. When a TrCRF is contained completely within a switch, it is referred to as an undistributed TrCRF. However, a TrCRF can be distributed across multiple switches if ISL trunking is enabled between switches and TrCRFs with identical VLAN numbers are defined.

By default, one TrCRF is defined on every Catalyst switch as "trcrf-default" on VLAN 1003. The default TrCRF is also assigned to the default TrBRF on VLAN 1005. If ISL trunking is in use, every Token Ring port on every switch will be defined to the same distributed TrCRF. Because only one TrBRF is defined by default, no bridging will occur. Instead, source-route switching will be performed to forward frames between switch ports within the TrCRF.

To define a TrCRF on a Catalyst switch, use the following command:

Switch(enable) set vlan vlan-num [name name] type trcrf

{ring hex-ring-num | decring decimal-ring-num} parent vlan-num

Both the ring number and the parent VLAN number must be specified to define a TrCRF. The ring number can be defined as a hexadecimal value of 0x1 to Oxfff with the ring option, or as a decimal value of 1 to 4095 with the decring option. The parent VLAN number must match the VLAN number assigned to the parent TrBRF.

On the Catalyst 5000, a single TrCRF can be distributed across multiple switches. To enable this feature, use the set tokenring distrib-crf enable command.

After a TrCRF VLAN has been created, switch ports can be assigned to it. As with Ethernet switching, use the following command to assign ports to a VLAN:

Switch(enable) set vlan vlan-num mod-num/port-num

To view the current Token Ring VLAN configuration, use the show vlan command. The output of which is demonstrated in Example 4-6.

Example 4-6 show vlan Command Output Displays the Current Token Ring VLAN Configuration

To view the current Token Ring VLAN configuration, use the show vlan command. The output of which is demonstrated in Example 4-6.

Example 4-6 show vlan Command Output Displays the Current Token Ring VLAN Configuration

Switch(enable) show

vlan

VLAN

Name

Status

Mod/Ports, Vlans

1

default

active

1/1-2

800

brf800

active

801,802

801Floor_1

active

802Floor_2

active

1002

fddi-default

active

1003

trcrf

-default

active

2/1-16

1004

fddinet-default

active

1005

trbrf

-default

active

1003

VLAN

Type

SAID

MTU

Parent

RingNo BrdgNc

Stp BrdgMode

Trans1

Trans2

1

enet

100001

1500

0

0

800

trbrf

100800

4472

- 0x2

ibm -

0

0

801

trcrf

100201

4472

800

0X01 -

- srb

0

0

802

trcrf

100202

4472

800

0X02 -

- srb

0

0

1002

fddi

101002

1500

0X0 -

0

0

1003

trcrf

101003

4472

1005

0xccc -

- srb

0

0

1004

fdnet

101004

1500

- 0X0

ieee -

0

0

1005

trbrf

101005

4472

- 0xf

ibm -

0

0

Notice in this example that one TrBRF has been assigned to VLAN 800, and two TrCRFs to VLANs 801 and 802. TrBRF on VLAN 800 shows the two active TrCRFs. In the lower group of output lines, the two TrCRFs on VLANs 801 and 802 show the parent TrBRF as 800 with source-route bridging enabled. Even though the defaults are not being used in this example, the default TrBRF is listed as VLAN 1005, and the default TrCRF is VLAN 1003, assigned to the default TrBRF.

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    What is a token ring vlan?
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    What are the components of Token Ring VLAN?
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