300-115 4/22精简题库第86题疑问

popeye2008

题库给的答案是B，总觉得有些疑问。
D是不是更好些？

这些是在网上找到的：

When unidirectional link occurs, UDLD can put that port into errdisable state (same as shutdown). The administrator must manually shut/no shut to bring that interface up. If we want the interface to automatically recover then configure the “errdisable autorecovery”. For example:

errdisable recovery cause udld errdisable recovery interval 30

By doing so, the port will be place back in up state (no err-disabled state) after 30 seconds, if the port still has violation it will be placed again in “err-disabled” state, otherwise it will remain in up state.

With UDLD aggressive mode enabled, when a port on a bidirectional link that has a UDLD neighbor relationship established stops receiving UDLD packets, UDLD tries to reestablish the connection with the neighbor. After eight failed retries, the port is disabled.

To prevent spanning tree loops, nonaggressive UDLD with the default interval of 15 seconds is fast enough to shut down a unidirectional link before a blocking port transitions to the forwarding state (with default spanning tree parameters).

When you enable UDLD aggressive mode, you receive additional benefits in the following situations:

• One side of a link has a port stuck (both Tx and Rx)
• One side of a link remains up while the other side of the link has gone down
  

In these cases, UDLD aggressive mode disables one of the ports on the link, which prevents traffic from being discarding.

Reference: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst6500/ios/15-0SY/configuration/guide/15_0_sy_swcg/udld.html#wp1027627

软件工人

这题答案没错，应该选B，UDLD只能检测出问题的光纤，并把相应的port置为errdisable状态以避免Spanning-tree loop，并没有fix the damaged fiber link的功能，光纤坏了必须人工换……