Table of Contents

Configuring Tag Switching

This chapter describes tag switching, a high-performance packet-forwarding technology that assigns tags to mulitprotocol frames for transport across packet- or cell-based networks.

Note   This chapter provides advanced configuration instructions for the Catalyst 8540 MSR, Catalyst 8510 MSR, and LightStream 1010 ATM switch routers. For an overview of tag switching, refer to the Guide to ATM Technology. For complete descriptions of the commands mentioned in this chapter, refer to the ATM Switch Router Command Reference publication.

This chapter includes the following sections:

Tag Switching Overview

In conventional Layer 3 forwarding, as a packet traverses the network, each router extracts forwarding information from the Layer 3 header. Header analysis is repeated at each router (hop) through which the packet passes.

In a tag switching network, the Layer 3 header is analyzed just once. It is then mapped into a short fixed-length tag. At each hop, the forwarding decision is made by looking only at the value of the tag. There is no need to reanalyze the Layer 3 header. Because the tag is a fixed-length, unstructured value, lookup is fast and simple.

For an overview of how tag switching works and its benefits, refer to the Guide to ATM Technology.

Hardware and Software Requirements and Restrictions (Catalyst 8540 MSR)

The Catalyst 8540 MSR hardware requirements for tag switching include the following:

Tag switching has the following software restrictions:

Hardware and Software Requirements and Restrictions (Catalyst 8510 MSR and LightStream 1010)

The Catalyst 8510 MSR and LightStream 1010 ATM switch router hardware requirements for tag switching include the following:

Tag switching has the following software restrictions:

Configuring Tag Switching

This section describes how to configure tag switching on ATM switch routers, and includes the following procedures:

Configuring a Loopback Interface

You should configure a loopback interface on every ATM switch router configured for tag switching. The loopback interface, a virtual interface, is always active. The IP address of the loopback interface is used as the Tag Distribution Protocol (TDP) identifier for the ATM switch router. If a loopback interface does not exist, the TDP identifier is the highest IP address configured on the ATM switch router. If that IP address is administratively shut down, all TDP sessions through the ATM switch router restart. Therefore, we recommend that you configure a loopback interface.

To configure the loopback interface, perform the following steps, beginning in global configuration mode:

Example

In the following example, loopback interface 0 is created with an IP address of 1.0.1.11 and a subnet mask of 255.255.255.255:

Displaying Loopback Interface Configuration

The following example shows the loopback 0 configuration using the show interfaces privileged EXEC command:

Enabling Tag Switching on the ATM Interface

Note   Configure all parallel interfaces between ATM switch routers for either IP unnumbered or with a specific IP address. Unnumbering some parallel interfaces and assigning specific IP addresses to others might cause TDP sessions to restart on some parallel interfaces when another parallel interface is shut down. Therefore, we highly recommend that you unnumber all parallel interfaces to loopback.

To enable tag switching on the ATM interface, perform the following steps, beginning in global configuration mode:

Examples

In the following example, ATM interface 1/0/1 is configured for IP unnumbered to loopback interface 0:

In the following example, ATM interface 0/0/3 is configured with a specific IP address and subnet mask (1.3.11.3 255.255.0.0):

Displaying the ATM Interface Configuration

To display the ATM interface configuration, use the following EXEC command:

The following example shows that tag switching is configured on ATM interfaces 0/0/3 and 1/0/1:

Configuring OSPF

Enable OSPF on the ATM switch router so that it can create routing tables, which identify routes through the network. Then add the addresses and associated routing areas to the OSPF process so that it can propagate the addresses to other ATM switch routers:

Note   Since the 12.0(1a)W5(5b) release of the system software, addressing the interface on the route processor (CPU) has changed. The ATM interface is now called atm0, and the Ethernet interface is now called ethernet0. Old formats (atm 2/0/0 and ethernet 2/0/0) are still supported.

Example

The following is an example of OSPF enabled and assigned process number 10000. All addresses are in area 0:

Note   An IP address of 1.1.1.1 with a subnet mask of 255.255.255.0 is entered as an IP network prefix of 1.1.1.0 with a subnet mask of 0.0.0.255. Likewise, an IP address of 1.2.1.1 with a subnet mask of 255.255.255.0 is entered as an IP network prefix of 1.2.1.0 with a subnet mask of 0.0.0.255.

Displaying the OSPF Configuration

To display the OSPF configuration, use the following privileged EXEC command:

The following example shows the OSPF configuration using the show ip ospf privileged EXEC command:

Configuring a VPI Range (Optional)

Although not necessary for most configurations, you might need to change the default tag virtual path identifier (VPI) range on the switch if:

To change the default tag VPI range, perform the following steps, beginning in global configuration mode:

Examples

The following example shows how to select a VPI range from 5 to 6 (a range of two), an acceptable range if the TDP neighbor is a router:

The following example shows how to select a VPI range from 5 to 7 (a range of three), an acceptable range if the TDP neighbor is a switch:

Note   Although the example shows a VPI range of three, you are not limited to a range of three if the TDP neighbor is a switch. The maximum VPI range is 0 to 255 if the TDP neighbor is a switch.

Displaying the Tag Switching VPI Range

To display the tag switching VPI range, use the following EXEC command:

Example

The following example shows the tag switching VPI range on ATM interface 1/0/1:

Configuring TDP Control Channels (Optional)

Although not necessary for most configurations, you can change the default Tag Distribution Protocol (TDP) control channel VPI and virtual channel identifier (VCI) if you want to use a nondefault value. The default TDP control channel is on VPI 0 and VCI 32. TDP control channels exchange TDP HELLOs and Protocol Information Elements (PIEs) to establish two-way TDP sessions. TVCs are created by the exchange of PIEs through TDP control channels.

To change the TDP control channel, perform the following steps, beginning in global configuration mode:

Figure 15-1 shows an example TDP control channel configuration between a source switch and destination switch on ATM interface 0/0/1. Note that the VPI and VCI values match on the source switch and destination switch.

Examples

In the following example, a TDP control channel is configured on the source switch:

In the following example, a TDP control channel is configured on the destination switch:

If you are having trouble establishing a TDP session, verify that the VPI and VCI values match on the TDP control channels of the source switch and destination switch.

Displaying the TDP Control Channels

To display the TDP control channel configuration, use the following EXEC command:

The following example shows the TDP control channel configuration on interface ATM 0/0/3:

Configuring Tag Switching on VP Tunnels

If you want to configure tag switching on virtual path (VP) tunnels, perform the following steps, beginning in global configuration mode:

Note   This procedure is optional.

Because a VP tunnel runs between switches, you must also configure a VP tunnel on the connecting ATM interface on the destination switch. The examples that follow show how to configure VP tunnels between switches.

Note   The intermediate switch configuration follows in the next section, "Connecting the VP Tunnels."

Figure 15-2 shows an example VP tunnel between a source switch and destination switch.

Examples

In the following example, ATM interface 0/1/1 on the source switch has no IP address and PVP 51 is configured for IP unnumbered to loopback interface 0:

In the following example, ATM interface 0/1/3 on the destination switch has no IP address and PVP 101 is configured for IP unnumbered to loopback interface 0:

To connect the source and destination switch VP tunnels, proceed to the next section, "Connecting the VP Tunnels."

Displaying the VP Tunnel Configuration

To display the VP tunnel configuration, use the following EXEC command:

The following example shows PVP 51 configured on ATM interface 0/1/1:

Connecting the VP Tunnels

To complete the VP tunnel, you must configure the ATM ports on the intermediate switch to designate where to send packets coming from the source switch and going to the destination switch.

To connect the permanent virtual path (PVP), perform the following steps, beginning in interface configuration mode:

Figure 15-3 shows an example configuration on an intermediate switch.

Example

In the following example, PVP 51 on ATM interface 0/1/1 is connected to PVP 101 on ATM interface 0/1/3:

Displaying the VP Tunnel Configuration

The following example shows PVP 51 on ATM interface 0/1/1 connected to PVP 101 on ATM interface 0/1/3:

Configuring VC Merge

VC merge allows the switch to aggregate multiple incoming flows with the same destination address into a single outgoing flow. Where VC merge occurs, several incoming tags are mapped to one single outgoing tag. Cells from different VCIs going to the same destination are transmitted to the same outgoing VC using multipoint-to-point connections. This sharing of tags reduces the total number of virtual circuits required for tag switching. Without VC merge, each source-destination prefix pair consumes one tag VC on each interface along the path. VC merge reduces the tag space shortage by sharing tags for different flows with the same destination.

Note   VC merge support requires FC-PFQ on the route processor. If you do not have FC-PFQ, and you try to enable VC merge, the TVCs remain point-to-point. (Catalyst 8510 MSR and LightStream 1010)

VC merge is enabled by default. To disable VC merge, enter the following command in global configuration mode:

Displaying the VC Merge Configuration

To display the VC merge configuration, use the following EXEC command:

The following example shows that VC merge configuration is enabled on ATM interface 0/1/0:

Configuring Tag Switching CoS

Quality of service (QoS) allows ATM to meet the transmission quality and service availability of many different types of data. The need for delay-sensitive data, such as voice, can be given a higher priority than data that is not delay-sensitive, such as e-mail. The following service categories were created for ATM Forum VCs to meet the transmission needs of various types of data: VBR-RT, VBR-NRT, ABR, and UBR. See "Configuring Resource Management," for more information about the standard ATM Forum implementation of QoS. This section describes tag switching class of service (CoS).

Up to eight QoS classes (0 to 7) can be allocated to each physical interface port. Each port has an independent logical rate scheduler (RS) and a weighted round-robin (WRR) scheduler. The RS guarantees minimum bandwidth and has first priority on supplying an eligible cell for transmission. Second priority is given to the service classes, which have been assigned relative weights that are based on the ratio of the total leftover bandwidth. The service class relative weights are configurable so you can change the priority of the default values. The VCs within a service class also have relative weights. The service classes and VCs within a service class are scheduled by their relative weights.

With tag switching CoS, tag switching can dynamically set up to four tag virtual channels (TVCs) with different service categories between a source and destination. TVCs do not share the same QoS classes reserved for ATM Forum VCs (VBR-RT, VBR-NRT, ABR, and UBR). The following four new service classes were created for TVCs: TBR_1 (WRR_1), TBR_2 (WRR_2), TBR_3 (WRR_3), and TBR_4 (WRR_4). These new service classes are called Tag Bit Rate (TBR) classes. TVCs and ATM Forum VCs can only coexist on the same physical interface, but they operate in ships in the night (SIN) mode and are unaware of each other.

TBR classes support only best-effort VCs (similar to the ATM Forum service category UBR); therefore, there is no bandwidth guarantee from the RS, which is not used for TVCs. All of the TVCs fall into one of the four TBR classes, each carrying a different default relative weight. The default values of the relative weights for the four TBR classes are configurable, so you can change the priority of the default values.

Table 15-1 lists the TBR classes and ATM Forum class mappings into the service classes for physical ports.

When tag switching is enabled on a hierarchical VP tunnel, the tunnel can only be used for tag switching. Because hierarchical VP tunnels support only four service classes, both TVCs and ATM Forum VCs map to the same service classes. Therefore, both ATM Forum VCs and TVCs cannot coexist in a hierarchical VP tunnel. The relative weights assigned to the service classes depend on which is active (either tag switching or ATM Forum). The class weights change whenever a hierarchical VP tunnel is toggled between ATM Forum and tag switching. By default, a hierarchical VP tunnel comes up as an ATM Forum port.

Table 15-2 lists the TBR classes and ATM Forum service category mappings for hierarchical VP tunnels.