Archive for forward delay

The (Necessary) Evils of Spanning Tree, Part II

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on June 13, 2011 by jjrinehart

Timing Is Everything

One of the more important aspects of 802.1d Spanning Tree Protocol operation has to do with the use of specific timers, three of them specifically.  These critical timers are as follows:

  1. Hello Timer: As the name suggests, this timer is concerned with the frequency at which hello messages are sent from the root switch and subsequently propagated throughout the switched network.  By default, the interval is set at two (2) seconds, and the root switch sends out the Hello BPDU out all of its actively functioning interfaces.
  2. Max Age Timer: The Max Age timer is not the point at which a worker should retire, but rather the length of time switches should wait before triggering changes in the spanning-tree topology.  This happens in response to events in which the hello messages are failing to appear.  Rather than being dependent on a “Max Age” setting, this timer is derived by multiplying the hello interval by 10, yielding a default setting of twenty (20).
  3. Forward Delay: Ever paranoid about loops, spanning-tree sets yet another timer as a blocked port moves to a full forwarding state (more about port states later).  There are two steps in this process, each of which are allocated fifteen (15) seconds each.

For spanning-tree to converge, the Max Age (20 seconds and Forward Delay (15+15=30 seconds) timers have to expire, which ensures that no accidental path loops will be introduced during a topology change (50 seconds total).  I would imagine you cannot think of anyone that would be willing to wait as long as a full minute for network resources to become available again, right?  To get around this issue, Cisco created some new feature to trim or eliminate the ridiculously long convergence time, as follows:

  1. Etherchannel: Since spanning-tree blocks multiple links, why not bond ports to create a single, larger logical connection?  This is exactly what Etherchannel does, it creates a bundle of similar interfaces into a larger entity “Port Channel” with the benefits of greater bandwidth and nullifying the blocking issue of multiple ports.  If one link drops, spanning-tree never has to reconverge.
  2. Portfast: Many devices connected to the network (workstations, servers, etc.) pose no loop threat and do not even participate in spanning tree, so the portfast feature puts the port into forwarding mode immediately.
  3. Uplinkfast: Miss America pageants have first runner-up contestants that can immediately be promoted to first place without having to stage an entirely new pageant.  Uplinkfast tracks secondary root (alternate) and designated (backup) ports for this very purpose.
  4. Backbonefast: If indirect links upstream/downstream of the switch fail, the switch can query its neighbors for new path information without waiting for the Max Age timer to expire.

Next time, we will look at 802.1d port states and improvements created in a newer version of spanning-tree, Rapid Spanning Tree (802.1w).

– Joe