In the presence of a Class-of-Service (CoS) configuration for many logical interfaces of the same physical interface, the following log messages were observed on the MX-Series router:

Feb  7 11:54:36  router-re0 : %PFE-4: fpc2 ifd(ge-2/0/0) guaranteed_bw_remain (48000000) is less than ifl-350's configured guaranteed rate (50000000)
Feb  7 11:54:39  router-re0 : %PFE-4: fpc2 ifd(ge-2/0/0) guaranteed_bw_remain (-2000000) is less than ifl-351's configured guaranteed rate (50000000)
Feb  7 11:54:39  router-re0 : %PFE-4: fpc2 ifd(ge-2/0/0) guaranteed_bw_remain (-52000000) is less than ifl-352's configured guaranteed rate (50000000)

This article addresses the following questions:

• Are these logs caused by software or hardware issues on the MX router?
• Is this an expected behavior?
• What is the impact of these logs?

The example below illustrates a situation when these logs will occur on the box.

On the test router, 21 static pp0 interfaces are configured using the same template as shown bellow (the configuration is replicated for the logical interfaces pp0.5000-pp0.5021):

interfaces {
    pp0 {
        unit 5000 {
            pppoe-options {
                underlying-interface ge-2/0/0.100;
                server;
            }
            family inet {
                unnumbered-address lo0.0;
            }
        }
    }
}

Each of these pp0 logical interfaces has the same underlying physical interfaces ge-2/0/0 configured as:

interfaces {
    ge-2/0/0 {
        hierarchical-scheduler maximum-hierarchy-levels 2;
        vlan-tagging;
        unit 100 {   
            encapsulation ppp-over-ether;
            vlan-id 100;     
            pppoe-underlying-options {
                max-sessions 32000;
                service-name-table KB-TABLE;
            }                
        }
    }
}

For each of these logical interfaces, CoS is configured as below:

class-of-service {  
    traffic-control-profiles {
        KB-OUTPUT-TCP-50M {
            guaranteed-rate 50m;     
        }
    }
    interfaces {
        pp0 {        
            unit 5000 {
                output-traffic-control-profile KB-OUTPUT-TCP-50M;
            }
    }
}

In total, this configuration tries to reserve 50 * 21 = 1050 Mbps on the physical interface ge-2/0/0 where 1000 Mbps is available.

In the test described in this article the subscribers were logged in sequentially (i.e. initially subscriber logged in on the interface pp0.5000, then pp0.5001 etc). The logs started to appear after login of the 20th subscriber:

admin@router-re0> show log messages | match guaranteed_bw_remain
Feb  7 11:54:36  router-re0 : %PFE-4: fpc2 ifd(ge-2/0/0) guaranteed_bw_remain (48000000) is less than ifl-350's configured guaranteed rate (50000000)

The log shows that only 48 Mbps of bandwidth is available on the interface. This is because the interface must allocate 2 Mbps for a special internal unit 32768 (used for processing Layer 2 control traffic and created automatically after configuring 'vlan-tagging' knob on the physical interface), so the system was not able to satisfy the 20th subscriber's request and printed the logs.

These log messages are expected in such a configuration and does not indicate any hardware or software problems. They do not have any impact in a situation when the physical interface is not oversubscribed as shown from the output below:

admin@router-re0> show class-of-service scheduler-hierarchy interface pp0.5020
Interface/                    Shaping Guarnteed  Guaranteed/   Queue   Excess
Resource name                    rate      rate       Excess  weight   weight
                                kbits     kbits     priority          high/low
ge-2/0/0                      1000000
  ge-2/0/0 RTP                1000000         0                          1    1
    BE                        1000000         0     Low  Low     950
    NC                        1000000         0     Low  Low      50
  pp0.5020                      50000     50000                        500  500
    BE                          50000     47500     Low  Low     950
    NC                          50000      2500     Low  Low      50