Description

This note aims to precise the usage of the overload bit in ISIS and when the overload bit is being cleared. That statement could be applicable for JUNOS OSPF overload bit.

Symptoms

The ISIS overload bit was first intended for signaling resource shortage, but however in today implementations it is used for a different purpose which is to avoid prefix blackholing upon router startup as stated in RFC 3277.  You may encounter JUNOS setting the overload bit due to resource shortage in few circumstances.

How does this overload bit being set or clear into the LSP?

Solution

Trigger for setting the overload bit, as described in [RFC3277 3. Deployment considerations], are left to the vendors. Some potential triggers could perhaps include N seconds after booting or N number of BGP prefixes in the BGP loc-RIB .

In JUNOS, the overload timer starts running after the initial RPD startup and after you have configured an ISIS instance with the overload bit option.  It will only take RPD startup time and not the instance activation time into account.  When specifying the overload timeout period in JUNOS the ISIS overload bit is N period of times after the RPD startup time.

From JUNOS 9.0 onward, the show isis overview CLI command displays the running overload timer as follows: 

user@router> show isis overview 
Instance: master
  Router ID: 1.1.2.1
  Adjacency holddown: enabled
  Maximum Areas: 3
  LSP life time: 1200
  Attached bit evaluation: enabled
  SPF delay: 200 msec, SPF holddown: 5000 msec, SPF rapid runs: 3
  Overload bit at startup is set
    Overload high metrics: disabled
    Overload timeout: 120 sec
  IPv4 is enabled, IPv6 is enabled
  Traffic engineering: enabled
  Restart: Enabled
    Restart duration: 210 sec
    Helper mode: Enabled
  Level 1
    Internal route preference: 15
    External route preference: 160
    Wide metrics are enabled, Narrow metrics are enabled
  Level 2
    Internal route preference: 18
    External route preference: 165
    Wide metrics are enabled


References: 
   [RFC3277] McPherson, D., "Intermediate System to Intermediate System (IS-IS)
             Transient Blackhole Avoidance", RFC 4291, April 2002.

Related Information