350-001 415Q Q201 EtherChannel 避免 SPT loop?

supisces

QUESTION 201
The network administrator wants to enable an EtherChannel between two switches in "on" mode. The administrator connects the cables and enables the
; ~5 ~' j0 E, Y3 j; I7 Cinterfaces, but while configuring the EtherChannel in the first switch, a spanning-tree loop was detected. Which two of these procedures can avoid this problem?
# A8 ?# V5 I2 e8 O* X1 a(Choose two.)

2 A0 a4 u7 h/ Q4 F0 K+ g( RA. Configure the EtherChannel as "desirable" first.
B. Assign all interfaces to the same VLAN.
C. Disable PortFast on the interfaces in the EtherChannels.
D. Disable all interfaces first.
' p* {$ @8 l8 i* p5 ?E. Fast Ethernet and Gigabit Ethernet ports cannot be assigned to the same EtherChannel.
F. Fix cabling problems.
# ?) B6 @- e0 `6 E( c- s
8 b6 b  s7 z! hAnswer: AD

選項 D 沒有問題.
$ ~  d. f" n. p( O6 ]: V3 M選項 A 使 EtherChannel 建立 trunk 之後, 不曉得為何能防止 SPT loop?

新出的 448Q 的答案捨去 A 而選擇 C,
大意是說 switch 之間不能用 portfast, 否則環路很嚴重. 看起來還頗有道理的...
4 ?3 g: C" L' o# }& w論壇上好幾位前輩說 448Q 答案錯很多, 不知道是否包含這題?
4 y9 S  g5 S' d% A4 N0 h
* n. b. l3 h2 ~Explanation
If a workstation or a server is connected with a single Network Interface Card (NIC) to a switch port, this connection cannot
create a physical loop. These connections are considered leaf nodes. There is no reason to make the workstation wait 30
- f; L& T5 }3 S' pseconds while the switch checks for loops when the workstation cannot cause a loop. With the addition of the PortFast or
8 y6 \, r2 H* @0 }3 ~" Nfast-start feature, the STP for this port assumes that the port is not part of a loop. In this case, the port immediately moves
5 V; E4 P7 @3 K" E& I0 m6 ^8 V- g) zto the forwarding state, and skips the blocking, listening, or learning states. This command does not turn STP off. This
( G3 J- D7 u" [" A/ r/ bcommand makes STP skip a few steps in the beginning on the selected port, although unnecessary in this circumstance.

6 o/ ~/ Z+ g) y% sNote: The PortFast feature must never be used on switch ports that connect to other switches, hubs, or routers. These
( w6 T7 a) _) {' n4 D" E0 bconnections can cause physical loops, and it is very important that Spanning Tree go through the full initialization
$ U; P2 Q- B. p. l5 c& Kprocedure in these situations. A Spanning Tree loop can bring the network down. If the PortFast feature is turned on for a
3 m$ V1 X! ~: A' Jport that is part of a physical loop, it can cause packets to be continuously forwarded and even multiply in such a way that
the network cannot recover.
Reference
- [7 t( N. d4 jhttps://supportforums.cisco.com/docs/DOC-4897
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srqlyh

davidliang

AD是正确的。portfast不是enabled by default.

tomqq1982

tomqq1982

TOBABYLI

感谢分享