A Note on LC-ATM Interfaces

We can differentiate between two types of LC-ATM interfaces:

• Directly connected LC-ATM interfaces These have one logical port per physical line.

• Indirectly connected LC-ATM interfaces These are connected by means of a virtual path. They are also called VP-Tunnel interfaces. There can be multiple logical ports per physical line.

As we mentioned, the difference resides in the label space, as well as in the VPI used for the control-vc. Normally, both VPI/VCI fields are used as label space. In a VP-Tunnel interface, only the VCI field of the ATM cell header is used as label space.

We can use the command show mpls interface detail in the LSC or in an eLSR to differentiate between the two types of LC-ATM interfaces. See Example 6-82.

Example 6-82. Directly and Indirectly Connected LC-ATM Interfaces

 P1_LSC_c7204#! Indirectly connected LC-ATM interface (VP-Tunnel) P1_LSC_c7204#show mpls interfaces xTagATM 331 detail Interface XTagATM331:         IP labeling enabled (ldp)         LSP Tunnel labeling not enabled         Tag Frame Relay Transport tagging not enabled         BGP tagging not enabled         Tagging operational         Optimum Switching Vectors:           IP to MPLS Turbo Vector           MPLS Punt Vector         Fast Switching Vectors:           IP to MPLS Fast Switching Vector           MPLS Turbo Vector         MTU = 4470         ATM labels: Label VPI = 9 (VP Tunnel)                 Label VCI range = 33 - 65535                 Control VC = 9/32 P1_LSC_c7204#! Directly connected LC-ATM interface P1_LSC_c7204#show mpls interfaces xTagATM 133 detail Interface XTagATM133:         IP labeling enabled (ldp)         LSP Tunnel labeling not enabled         Tag Frame Relay Transport tagging not enabled         BGP tagging not enabled         Tagging operational         Optimum Switching Vectors:           IP to MPLS Turbo Vector           MPLS Punt Vector         Fast Switching Vectors:           IP to MPLS Fast Switching Vector           MPLS Turbo Vector         MTU = 4470         ATM labels: Label VPI range = 2 - 10                 Label VCI range = 33 - 65535                 Control VC = 0/32 P1_LSC_c7204# 

In the LSC only, we can also use the command show controller xtagatm to see the two LC-ATM interface types. See Example 6-83.

Example 6-83. Directly and Indirectly Connected LC-ATM Interfaces

 P1_LSC_c7204#! Indirectly connected LC-ATM interface (VP_Tunnel) P1_LSC_c7204#show controllers xTagATM 331 Interface XTagATM331 is up   Hardware is Tag-Controlled ATM Port (on BPX switch BPX-VSI2)   Control interface ATM3/0 is up   Physical descriptor is 0.3.3.1   Logical interface 0x00030301 (0.3.3.1)   Oper state ACTIVE, admin state UP   VPI range 9-9, VCI range 32-65535   VPI may be translated at end of link   Tag control VC must be strictly in VPI/VCI range   Available channels: ingress 241, egress 241   Maximum cell rate: ingress 2867, egress 2867   Available cell rate: ingress 2867, egress 2867   Endpoints in use: ingress 7, egress 7, ingress/egress 1   Rx cells 182513     rx cells discarded 0, rx header errors 0     rx invalid addresses (per card): 364124     last invalid address 0/0   Tx cells 281966     tx cells discarded: 0 P1_LSC_c7204#! Directly connected LC-ATM interface P1_LSC_c7204#show controllers xTagATM 133 Interface XTagATM133 is up   Hardware is Tag-Controlled ATM Port (on BPX switch BPX-VSI2)   Control interface ATM3/0 is up   Physical descriptor is 0.13.3.0   Logical interface 0x000D0300 (0.13.3.0)   Oper state ACTIVE, admin state UP   VPI range 2-10, VCI range 32-65535   VPI is not translated at end of link   Tag control VC need not be strictly in VPI/VCI range   Available channels: ingress 475, egress 475   Maximum cell rate: ingress 50000, egress 50000   Available cell rate: ingress 50000, egress 50000   Endpoints in use: ingress 18, egress 18, ingress/egress 1   Rx cells 223934     rx cells discarded 0, rx header errors 0     rx invalid addresses (per card): 44     last invalid address 0/0   Tx cells 211581     tx cells discarded: 0 P1_LSC_c7204# 

NOTE

Every time we use the command show controller xtagatm in the LSC, we are pulling the counters from the VSI slave (in this case, the BXM card). Those counters reside on the VSI slave and are retrieved for display using VSI commands. If the controlled switch is a BPX-8600 or IGX-8400, the counters can also be seen from the switch CLI using dsptrkstats or dspportstats.

If we use only one VPI in a non-VP-Tunnel LC-ATM interface, the VPI cannot be different at both ends of the link. This is because the VPI will be used as label space, and there could be a disjoint VPI range (null intersection). As a result, LDP will not come up. Consequently, if we use a PVP and the VPI is different at both ends, we need to define the XTagATM interfaces as VP-Tunnel.

In the LSC, a VP-Tunnel interface is auto-discovered with VSI messages as soon as we configure a virtual trunk or VNNI interface. On the eLSR, we need to specify that the interface is VP-Tunnel and also specify the VPI it will be using.

In RFC 3036, "LDP Specification," the field V-bits in the ATM Label TLV specifies whether VPI or VCI or both are significant.

The following is a related excerpt from RFC 3035, "MPLS Using LDP and ATM VC Switching":

7.2. Connections via an ATM VP

Sometimes it can be useful to treat two LSRs as adjacent (in some LSP) across an LC-ATM interface, even though the connection between them is made through an ATM "cloud" via an ATM Virtual Path. In this case, the VPI field is not available to MPLS, and the label MUST be encoded entirely within the VCI field.

In this case, the default VCI value of the non-MPLS connection between the LSRs is 32. Other values can be configured, as long as both parties are aware of the configured value. The VPI is set to whatever is required to make use of the Virtual Path.