300-101 第202题疑问

陳罐西

QUESTION 202
" m  v; X$ N/ H9 z7 O% ]" EWhen unicast reverse path forwarding is configured on an interface, which action does
the interface take first when it receives a packet?
A. It check the ingress access list
B. It check the egress access list
C. It verifies that the source has a valid CEF adjacency
4 w" w! [0 _) P7 |' FD. It verifies a reverse path via the FIB to the source
Correct Answer: D
Section: part 5
  g8 R1 Y* T1 ~  E: PExplanation
Explanation/Reference:
When a packet is received at the interface where Unicast RPF and ACLs have been configured, the
' h: k* b- y' O, z8 dfollowing actions occur:
Step 1: Input ACLs configured on the inbound interface are checked.
Step 2: Unicast RPF checks to see if the packet has arrived on the best return path to the source, which it
does by doing a reverse lookup in the FIB table
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3 V' p4 j6 p" Z下面的解析中的第一步 说的是先检查  进口ACL  那么为啥不会选A呢。。。。。
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willyou

我觉得题库是错的，不过有一点需要考虑，就是题干并没有指明ACL有被配置。
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: V* d/ @5 f; l& o思科官网链接：https://www.cisco.com/c/en/us/td ... 7-9679-0390DD2F6FC1
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思科给出的带ACL的uRPF配置案例：

int eth0/1/1
ip address 192.168.200.1 255.255.255.0
ip verify unicast reverse-path 197
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; m: ?$ s6 i3 u9 Y4 p$ T8 Yint eth0/1/2
ip address 192.168.201.1 255.255.255.0
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! K, P2 {% q' o- u( Jaccess-list 197 deny   ip 192.168.201.0 0.0.0.63 any log-input
1 H2 D: c) s" K. ]3 Z& G. X2 Waccess-list 197 permit ip 192.168.201.64 0.0.0.63 any log-input
access-list 197 deny   ip 192.168.201.128 0.0.0.63 any log-input
access-list 197 permit ip 192.168.201.192 0.0.0.63 any log-input
access-list 197 deny ip host 0.0.0.0 any log