[MX] Difference in auto-bandwidth LSP behavior when circuit cross-connects are in use

Summary:

This article describes how the auto-bandwidth signaling behavior of a label-switched path (LSP) differs when cross-connects are in use and how to avoid any issues with auto-bandwidth signaling due to this behavior on MX routers that run Junos OS release prior to release 17.4R1.

Symptoms:

An LSP in the circuit cross-connect (CCC) family does not signal auto-bandwidth properly when cross-connects are in use, as shown in the following output:

jtac@router1-re1> show mpls lsp name CC:router1:router2:123 extensive
Ingress LSP: 628 sessions

203.210.07.03
From: 203.210.07.03, State: Up, ActiveRoute: 0, LSPname: CC:router1:router2:123
Description: this is a test LSP
ActivePath: via-router3 (primary)
Node/Link protection desired
LSPtype: Static Configured, Penultimate hop popping
LoadBalance: Random
Autobandwidth
MinBW: 1000kbps, MaxBW: 7Gbps  
AdjustTimer: 3600 secs AdjustThreshold: 4%
Max AvgBW util: 6.26712Gbps, Bandwidth Adjustment in 2775 second(s).     
Overflow limit: 3, Overflow sample count: 18
Underflow limit: 3, Underflow sample count: 0, Underflow Max AvgBW: 0bps
Encoding type: Packet, Switching type: Packet, GPID: IPv4
*Primary   via-router3     State: Up, Preference: 200
Priorities: 2 2
Bandwidth: 0bps (ActualBandwidth: 1000kbps)        <<<< Auto-bandwidth not signaled properly. 
OptimizeTimer: 7200

Cause:

Auto-bandwidth allows an MPLS tunnel to automatically adjust its bandwidth based on the volume of traffic flowing through the tunnel.

However, it is observed that LSP auto-bandwidth is not signaled properly if the LSP is of type circuit cross-connect (CCC). This is because of a feature called "self-ping" that was introduced in Junos OS release 16.1.

An LSP should not carry traffic unless it is known to have been programmed in the forwarding plane. Data exchange in the LSP data plane, such as ping requests, happens in the ingress router before being switched to a new LSP or before the auto-bandwidth of an LSP is adjusted. After the self-ping completes successfully, the make-before-break process considers it to be completed and an LSP comes up.

This behavior, however, is not common in the case of Layer 2 cross-connects. In the case of circuit cross-connect LSPs, the self-ping mechanism is not supported. Therefore, the self-ping test would never be completed successfully.

Solution:

This issue has been resolved in Junos OS release 17.4R1 wherein self-ping will be disabled by default for LSPs that do not support self-ping.

In the case of circuit cross-connect LSPs prior to Junos OS release 17.4R1, the self-ping mechanism is not supported. Therefore, the self-ping test would never be completed successfully.

If the LSP auto-bandwidth must be signaled correctly, we need to stop the self-ping mechanism from being operational on these devices.

Steps

Check the encapsulation type. When you configure encapsulation type with the CCC family, it shows as follows:

fe-fpc/pic/port {
encapsulation ethernet-ccc;
unit 0;
}

To verify that the LSP belongs to the CCC family at the hierarchy levels, use the following command:

[edit protocols connections] <<<<Layer 2 switching cross-connects are configured under protocol connections.

To disable the self-ping mechanism on the LSP that belongs to the CCC family, use the following command:

jtac@router1-re1> set protocols mpls label-switched-path CC:router1:router2:123 no-self-ping

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