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. This chapter includes the following sections:

Note For a complete description of the commands mentioned in this chapter, refer to the LightStream 1010 ATM Switch Command Reference publication.

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.

A tag switching network consists of tag edge routers and tag switches, as shown in Figure 16-1. Tag edge routers are located at the edge of a tag switching network. They use standard routing protocols (such as Open Shortest Path First [OSPF]) to create routing tables, which identify routes through the network. Based on the routing tables, tag edge routers use the Tag Distribution Protocol (TDP) to apply and distribute tags to other tag edge routers or tag switches. Tag switches are located at the core of a tag switching network. They receive TDP information from the tag edge routers and build their own forwarding database. Tag switches then switch the packets based on the tags only (without looking at the Layer 3 header).

Figure 16-1: Tag Switching Network

How it Works

When a tag edge router at the entry point of a tag switching network receives a packet for forwarding:

1. The router analyzes the network layer header and performs any applicable network layer services (such as security, accounting, or quality of service [QoS] classification).

2. The router chooses a route for the packet based on the information in its routing table, applies a tag, and forwards the packet to the next-hop tag switch.

3. The tag switch receives the tagged packet and switches the packet from switch to switch based on the tag only. The switches do not reanalyze the network layer header, but only look at the short fixed-length tag.

4. The packet reaches the tag edge router at the exit point of the tag switched network, where the tag is removed and the packet is delivered.

Benefits

Tag switching offers the following benefits:

Combines the performance and traffic management capabilities of Layer 2 (data link layer) switching with the scalability and flexibility of Layer 3 (network layer) routing. You can direct packet flows across a router-based network along predetermined paths, such virtual circuits, rather than hop-by-hop as in a typical router-based network. This allows routers to perform advanced traffic management tasks, such as load balancing, in ATM switches.
Uses standard routing protocols and TDP to distribute tag values with other tag switches and with tag edge routers. Unlike standard ATM, there is no call setup procedure.

Hardware and Software Requirements and Restrictions

The hardware requirements for tag switching include:

LightStream 1010 ATM switch (used as a tag switch).
Feature card per-flow queueing (FC-PFQ) daughter card on the ATM switch processor (ASP), if you want to enable VC merge (multipoint-to-point connection). Note that FC-PFQ requires 64 MB of dynamic random-access memory (DRAM).
Cisco 7000 Route Switch Processor (RSP), 7505, 7507, or 7513 router (used as a tag edge router) with an Optical Carrier 3 (OC3) ATM interface processor (AIP) installed.

Note The DS3 and E3 port adapter modules (PAMs) are not currently supported for the LightStream 1010 ATM switch.

Tag switching has the following software restrictions:

OSPF is the only routing protocol currently supported.
IP is the only network layer protocol supported.

Configuring Tag Switching

This section describes how to configure tag switching on LightStream 1010 ATM switches, and includes the following procedures:

Configuring a Loopback Interface
Enabling Tag Switching on the ATM Interface
Configuring OSPF
Configuring Tag Switching on VP Tunnels
Connecting the VP Tunnels
Configuring a VPI Range (optional)
Configuring TDP Control Channels (optional)
Configuring VC Merge (optional)

Configuring a Loopback Interface

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

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

Step Command Task

1
interface loopback number

Enter interface configuration mode and assign a number to the loopback interface.

2
ip address ipaddress mask

Assign an IP address and subnet mask to the loopback interface.
Note We recommend a 32-bit subnet mask (255.255.255.255) for the loopback interface. If you do not use a 32-bit subnet mask, two TVCs¹ terminate for the same address---one for a 32-bit subnet mask and the other for the mask you entered. Entering a 32-bit subnet mask reduces the number of TVCs to one.

¹TVCs = tag virtual channels.

Step	Command	Task
1	interface loopback number	Enter interface configuration mode and assign a number to the loopback interface.
2	ip address ipaddress mask	Assign an IP address and subnet mask to the loopback interface. Note We recommend a 32-bit subnet mask (255.255.255.255) for the loopback interface. If you do not use a 32-bit subnet mask, two TVCs¹ terminate for the same address---one for a 32-bit subnet mask and the other for the mask you entered. Entering a 32-bit subnet mask reduces the number of TVCs to one.

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:

Switch(config)# interface loopback 0
Switch(config-if)# ip address 1.0.1.11 255.255.255.255
Switch(config-if)# exit

Displaying the Loopback Interface Configuration

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

Switch# show interface loopback 0
Loopback0 is up, line protocol is up
  Hardware is Loopback
  Internet address is 1.0.1.11/24
  MTU 1500 bytes, BW 8000000 Kbit, DLY 5000 usec, rely 255/255, load 1/255
  Encapsulation LOOPBACK, loopback not set, keepalive set (10 sec)
  Last input 00:00:03, output never, output hang never
  Last clearing of "show interface" counters never
  Queueing strategy: fifo
  Output queue 0/0, 0 drops; input queue 0/75, 0 drops
  5 minute input rate 0 bits/sec, 0 packets/sec
  5 minute output rate 0 bits/sec, 0 packets/sec
     0 packets input, 0 bytes, 0 no buffer
     Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
     73 packets output, 0 bytes, 0 underruns
     0 output errors, 0 collisions, 0 interface resets
     0 output buffer failures, 0 output buffers swapped out

Enabling Tag Switching on the ATM Interface

Note Configure all parallel interfaces between LightStream 1010 ATM switches 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:

Step Command Task

1
interface atm card/subcard/port

Enter interface configuration mode on the specified ATM interface.

2
ip unnumbered type number

Enable IP unnumbered on the ATM interface and assign the unnumbered interface to an interface that has an IP address. We recommend enabling IP unnumbered because it allows you to conserve IP addresses and it reduces the number of TVCs terminating on the switch.

or

or

ip address ip-address mask

Assign an IP address and subnet mask to the ATM interface.

3
tag-switching ip

Enable tag switching of IPv4 packets.

Step	Command	Task
1	interface atm card/subcard/port	Enter interface configuration mode on the specified ATM interface.
2	ip unnumbered type number	Enable IP unnumbered on the ATM interface and assign the unnumbered interface to an interface that has an IP address. We recommend enabling IP unnumbered because it allows you to conserve IP addresses and it reduces the number of TVCs terminating on the switch.
	or	or
	ip address ip-address mask	Assign an IP address and subnet mask to the ATM interface.
3	tag-switching ip	Enable tag switching of IPv4 packets.

Examples

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

Switch(config-if)# interface atm 1/0/1
Switch(config-if)# ip unnumbered loopback 0
Switch(config-if)# tag-switching ip
Switch(config-if)# exit

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):

Switch(config)# interface atm 0/0/3
Switch(config-if)# ip address 1.3.11.3 255.255.0.0
Switch(config-if)# tag-switching ip
Switch(config-if)# exit

Displaying the ATM Interface Configuration

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

Command Task

show tag-switching interfaces

Display the tag switching configuration on the ATM interface.

Command	Task
show tag-switching interfaces	Display the tag switching configuration on the ATM interface.

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

Switch# show tag-switching interfaces
Interface              IP    Tunnel   Operational
ATM0/0/3               Yes   No       No          (ATM tagging)
ATM1/0/1               Yes   No       No          (ATM tagging)

Configuring OSPF

To configure OSPF, you must first enable OSPF on the ATM switch 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 switches and routers.

To configure OSPF, follow these steps in global configuration mode:

Step Command Task

1
router ospf process_number

Enable OSPF and assign it a process number. The process number can be any positive integer.

2
network address wildcard-mask area area-id

Define the network prefix, a wildcard subnet mask, and the associated area number on which to run OSPF. An area number is an identification number for an OSPF address range.

Repeat this command for each additional area you want to add to the OSPF process.

Caution Ethernet interface 2/0/0 is used for system management only (for example, downloading system images or configuration files from a TFTP server). Do not add this interface to the OSPF process.

Step	Command	Task
1	router ospf process_number	Enable OSPF and assign it a process number. The process number can be any positive integer.
2	network address wildcard-mask area area-id	Define the network prefix, a wildcard subnet mask, and the associated area number on which to run OSPF. An area number is an identification number for an OSPF address range. Repeat this command for each additional area you want to add to the OSPF process. Caution Ethernet interface 2/0/0 is used for system management only (for example, downloading system images or configuration files from a TFTP server). Do not add this interface to the OSPF process.

Example

In the following example, 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.

Switch(config)# router ospf 10000
Switch(config-router)# network 1.1.1.0 0.0.0.255 area 0
Switch(config-router)# network 1.2.1.0 0.0.0.255 area 0
Switch(config-router)# network 1.3.0.0 0.0.255.255 area 0
Switch(config-router)# network 200.2.2.0 0.0.0.255 area 0
Switch(config-router)# network 1.0.1.0 0.0.0.255 area 0
Switch(config-router)# network 1.18.0.0 0.0.255.255 area 0
Switch(config-router)# exit

Displaying the OSPF Configuration

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

Command Task

show ip ospf

Display the OSPF configuration.

Command	Task
show ip ospf	Display the OSPF configuration.

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

Switch# show ip ospf
 Routing Process "ospf 10000" with ID 1.0.1.11
 Supports only single TOS(TOS0) routes
 SPF schedule delay 5 secs, Hold time between two SPFs 10 secs
 Number of DCbitless external LSA 0
 Number of DoNotAge external LSA 0
 Number of areas in this router is 1. 1 normal 0 stub 0 nssa
    Area BACKBONE(0) (Inactive)
        Number of interfaces in this area is 4
        Area has no authentication
        SPF algorithm executed 2 times
        Area ranges are
        Link State Update Interval is 00:30:00 and due in 00:14:42
        Link State Age Interval is 00:20:00 and due in 00:14:10
        Number of DCbitless LSA 0
        Number of indication LSA 0
        Number of DoNotAge LSA 0

Configuring a VPI Range

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

It is an administrative policy to use a VPI value other than 1, the default VPI.
There is a large number of TVCs on an interface.

Note You cannot enter a VPI range on a VP tunnel. On VP tunnels, the VPI is the permanent virtual path (PVP) number of the tunnel.

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

Step Command Task

1
interface atm card/subcard/port

Enter interface configuration mode on the specified ATM interface.

2
tag-switching atm vpi vpi - vpi

Enter the VPI range.

Note If the TDP neighbor is a router, the VPI range can be no larger than two. For example, from 5 to 6 (a range of two), not 5 to 7 (a range of three). If the TDP neighbor is a switch, the maximum VPI range is 0 to 255.

Step	Command	Task
1	interface atm card/subcard/port	Enter interface configuration mode on the specified ATM interface.
2	tag-switching atm vpi vpi - vpi	Enter the VPI range. Note If the TDP neighbor is a router, the VPI range can be no larger than two. For example, from 5 to 6 (a range of two), not 5 to 7 (a range of three). If the TDP neighbor is a switch, the maximum VPI range is 0 to 255.

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:

Switch(config)# interface atm 4/0/1 
Switch(config-if)# tag-switching ip
Switch(config-if)# tag-switching atm vpi 5 - 6

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:

Switch(config)# interface atm 4/0/1 
Switch(config-if)# tag-switching ip
Switch(config-if)# tag-switching atm vpi 5 - 7

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:

Command Task

show tag-switching interfaces detail

Display the tag switching VPI range on an interface.

Command	Task
show tag-switching interfaces detail	Display the tag switching VPI range on an interface.

Example

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

Switch# show tag-switching interfaces detail
Interface ATM0/0/3:
        IP tagging enabled
        TSP Tunnel tagging not enabled
        Tagging not operational
        MTU = 4470
        ATM tagging: Tag VPI = 1, Control VC = 0/32
Interface ATM1/0/1:
        IP tagging enabled
        TSP Tunnel tagging not enabled
        Tagging not operational
        MTU = 4470
                
 ATM tagging: Tag VPI range = 5 - 6, Control VC = 6/32
<information deleted>

Configuring TDP Control Channels

Although not necessary for most configurations, you can change the default 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. Tag virtual channels (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:

Step Command Task

1
interface atm card/subcard/port

Enter interface configuration mode on the specified ATM interface.

2
ip address ip-address mask

Assign an IP address and subnet mask to the ATM interface.

3
tag-switching ip

Enable tag switching of IPv4 packets.

4
tag-switching atm control-vc vpi vci

Change the TDP control channel.

Step	Command	Task
1	interface atm card/subcard/port	Enter interface configuration mode on the specified ATM interface.
2	ip address ip-address mask	Assign an IP address and subnet mask to the ATM interface.
3	tag-switching ip	Enable tag switching of IPv4 packets.
4	tag-switching atm control-vc vpi vci	Change the TDP control channel.

Figure 16-2 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.

Figure 16-2: Configuring TDP Control Channels

Examples

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

Switch(config)# interface atm 0/0/1
Switch(config-if)# ip address 1.2.0.11 255.255.255.0
Switch(config-if)# tag-switching ip
Switch(config-if)# tag-switching atm control-vc 6 32
Switch(config-if)# exit

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

Switch(config)# interface atm 0/0/1
Switch(config-if)# ip address 1.2.0.12 255.255.255.0
Switch(config-if)# tag-switching ip
Switch(config-if)# tag-switching atm control-vc 6 32
Switch(config-if)# exit

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:

Command Task

show tag-switching interfaces detail

Display the TDP control channel configuration on an interface.

Command	Task
show tag-switching interfaces detail	Display the TDP control channel configuration on an interface.

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

Switch# show tag-switching interfaces detail
Interface ATM0/0/3:
        IP tagging enabled
        TSP Tunnel tagging not enabled
        Tagging not operational
        MTU = 4470
        
 ATM tagging: Tag VPI = 1, Control VC = 0/32
<information deleted>

Configuring Tag Switching on VP Tunnels

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

Note This procedure is optional.


Step	Command	Task
1	interface atm card/subcard/port	Enter interface configuration mode on the specified ATM interface.
2	atm pvp vpi	Create a PVP. When configuring PVP connections, configure the lowest VPI numbers first.
3	exit	Return to global configuration mode.
4	interface atm card/subcard/port.sub-interface	Enter interface configuration mode.
5	ip unnumbered type number	Enable IP unnumbered on the ATM interface and assign the unnumbered interface to an interface that has an IP address. We recommend enabling IP unnumbered because it allows you to conserve IP addresses and reduces the number of TVCs terminating on the switch.
	or	or
	ip address ip-address mask	Assign an IP address and subnet mask to the ATM interface.
6	tag-switching ip	Enable tag switching of IPv4 packets.

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 16-3 shows an example VP tunnel between a source switch and destination switch.

Figure 16-3: Configuring VP Tunnels

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:

Switch(config-if)# interface atm 0/1/1
Switch(config-if)# atm pvp 51
Switch(config-if)# exit
Switch(config-if)# interface atm 0/1/1.51
Switch(config-subif)# ip unnumbered loopback 0
Switch(config-subif)# tag-switching ip
Switch(config-subif)# exit

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:

Switch(config)# interface atm 0/1/3
Switch(config-if)# atm pvp 101
Switch(config-if)# exit
Switch(config)# interface atm 0/1/3.101
Switch(config-subif)# ip unnumbered loopback 0
Switch(config-subif)# tag-switching ip
Switch(config-subif)# exit

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:

Command Task

show atm vp

Display the VP tunnel configuration on an interface.

Command	Task
show atm vp	Display the VP tunnel configuration on an interface.

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

Switch# show atm vp
Interface    VPI    Type  X-Interface     X-VPI     Status
ATM0/1/1     51      PVP     TUNNEL

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 PVP, perform the following steps, beginning in interface configuration mode:

Step Command Task

1
interface atm card/subcard/port

Enter interface configuration mode on the specified ATM interface.

2
atm pvp vpi interface atm card/subcard/port vpi-B

Connect the PVP from the source switch to the destination switch.

Step	Command	Task
1	interface atm card/subcard/port	Enter interface configuration mode on the specified ATM interface.
2	atm pvp vpi interface atm card/subcard/port vpi-B	Connect the PVP from the source switch to the destination switch.

Figure 16-4 shows an example configuration on an intermediate switch.

Figure 16-4: Connecting the VP Tunnels

Example

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

Switch(config)# interface atm 0/1/1
Switch(config-if)# atm pvp 51 interface atm 0/1/3 101
Switch(config-if)# exit

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:

Switch# show atm vp
Interface    VPI    Type  X-Interface     X-VPI     Status
ATM0/1/1     51      PVP     ATM0/1/3     101       DOWN
ATM0/1/3     101     PVP     ATM0/1/1     51        DOWN

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.

Figure 16-5 shows an example of VC merge. In Figure 16-5, routers A and B are sending traffic to prefix 171.69.0.0/16 on router C. The LightStream 1010 ATM switch in the middle is configured with a single outbound VCI 50 bound to prefix 171.69.0.0/16. Data flows from routers A and B congregate in the LightStream 1010 ATM switch and share the same outgoing VC. Cells coming from VCIs 40 and 90 are buffered in the input queues of the LightStream 1010 ATM switch until complete AAL5 frames are received. The complete frame is then forwarded to router C on VCI 50.

Figure 16-5: VC Merge

VC merge support requires a feature card per-flow queueing (FC-PFQ) daughter card on the ATM switch processor (ASP). VC merge is not possible with a feature card per-class queueing (FC-PCQ). To determine which feature card you have, enter the show hardware privileged EXEC command. Either FeatureCard1 (FC-PCQ) or FC-PFQ displays in the Ctrlr-Type column.

Note If you do not have a FC-PFQ, and you try to enable VC merge, the TVCs remain point-to-point.

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

Command Task

no tag-switching atm vc-merge

Enable VC merge.

Command	Task
no tag-switching atm vc-merge	Enable VC merge.

Displaying the VC Merge Configuration

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

Command Task

show tag-switching atm-tdp capability

Display the TDP control channel configuration on an interface.

Command	Task
show tag-switching atm-tdp capability	Display the TDP control channel configuration on an interface.

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

Switch# show tag-switching atm-tdp capability
 
                Control    VPI           VCI           Alloc   VC Merge
ATM0/1/0       VP    VC    Range         Range         Scheme  IN   OUT
  Negotiated   0     32    [7 - 8]       [33 - 1023]   UNIDIR  -    -    
  Local        -     -     [7 - 8]       [33 - 16383]  UNIDIR  Yes  Yes
  Peer         -     -     [7 - 8]       [33 - 1023]   UNIDIR  -    -

Table of Contents

Configuring Tag Switching

Example

Examples

Example

Examples

Example

Examples

Examples

Example