i found this, this morning…
Feb 18, 2013 4:56 AM
1,162 posts since
Sep 11, 2011
i use anki… one of the dangers of anki is programming yourself with long term misinformation…
CCIE Routing and Switching v4.0 Quick Reference, 2nd Edition
Alternative port: This port role is new to 802.1w and is a quickly converging backup port to the current DP on a segment.
Backup port: This port role is new to 802.1w and is a quickly converging backup to the root port for a system.
these are backwards… i looked for errata at ciscopress but found none…
from wendell in ccie r&s pg 86:
The Alternate Port concept is like the UplinkFast concept—it offers protection against the loss of a switch’s RP by keeping track of the Alternate Ports with a path to the root. The concept and general operation is identical to UplinkFast, although RSTP might converge more quickly via its active messaging between switches.
The Backup Port role has no equivalent with Cisco-proprietary features; it simply provides protection against losing the DP attached to a shared link when the switch has another physical port attached to the same shared LAN.
and scott morris from here:
Backup port = Backup to same segment (non-designated)
Alternate port = Alternate port to different segment towards root (non-root)
So by that logic, the backup port cannot guarantee it will have a better path towards the root. While it may become the path of choice, the first choice is actually the alternate port. (2nd place to root port)
and from here:
This distinction is already made internally within 802.1D. This is essentially how Cisco UplinkFast functions. The rationale is that an alternate port provides an alternate path to the root bridge and therefore can replace the root port if it fails. Of course, a backup port provides redundant connectivity to the same segment and cannot guarantee an alternate connectivity to the root bridge. Therefore, it is excluded from the uplink group.