CCIE Pursuit Blog

October 6, 2007

How Uplinkfast Affects Spanning-Tree Port Cost

Read a book….learn something new.  🙂

I don’t muck with “spanning-tree uplinkfast” much and I’ve never considered how it does its magic.  After applying this command the switch’s priority will shoot up to 49,152 + sys-id-ext and will add 3000 to the existing STP port costs:

Before [note: this switch is connected to the root switch]:
sw2(config)#do sh sp v 1

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    24577
             Address     0012.018f.d580
             Cost        19
             Port        15 (FastEthernet0/13)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     0012.009c.ca00
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 15

Interface        Role Sts Cost      Prio.Nbr Type
—————- —- — ——— ——– ——————————–
Fa0/13           Root FWD 19        128.15   P2p
Fa0/14           Altn BLK 19        128.16   P2p
Fa0/15           Altn BLK 19        128.17   P2p

After:
sw2(config)#span uplinkfast
sw2(config)#do sh sp vlan 1

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    24577
             Address     0012.018f.d580
             Cost        3019
             Port        15 (FastEthernet0/13)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    49153  (priority 49152 sys-id-ext 1)
             Address     0012.009c.ca00
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300
  Uplinkfast enabled

Interface        Role Sts Cost      Prio.Nbr Type
—————- —- — ——— ——– ——————————–
Fa0/13           Root FWD 3019      128.15   P2p
Fa0/14           Altn BLK 3019      128.16   P2p
Fa0/15           Altn BLK 3019      128.17   P2p

The rational behind this is that the high priority value will make it unlikely that the switch will become a root.  The high STP port costs will make it unlikely that the switch will become a transit switch (a switch that forwards frames between other switches).  That way if one of the uplinks fails the switch can quickly switch to another uplink with little chance of causing a loop.

Advertisements

Leave a Comment »

No comments yet.

RSS feed for comments on this post. TrackBack URI

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Create a free website or blog at WordPress.com.

%d bloggers like this: