Which AS path matches the AS path regular expression shown in the exhibit?
A.
65100 65200 21870 21870
B.
65100 65101 65200 21780 21780
C.
65100 52870 65200 65300
D.
65100 65101 65200 65300 21870
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I think this is A.
65100. Match 65500 if it appears anywhere in the AS path
(65200|65300) 65200 or 65300
+21870 One or more occurrences of “21870”
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Sorry, it is D 1000% OOPS
The way to read it:
First ASN is 65100
Second ASN is any (65101 in this case)
Third ASN could be one or more times either 65200 or 65300
Last ASN is AS 21870.
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I maked a test with the operador “+” joined to last one term (term after operador) , the operator is applied to penultimate term, not the last one.
This is in the same way that the follow: https://www.juniper.net/documentation/en_US/junos/topics/usage-guidelines/policy-configuring-as-path-regular-expressions-to-use-as-routing-policy-match-conditions.html
“operator—(Optional) An operator specifying how the term must match. Most operators describe how many times the term must be found to be considered a match (for example, any number of occurrences, or zero, or one occurrence). Table 1 lists the regular expression operators supported for AS paths. You place operators immediately after term with no intervening space, except for the pipe ( | ) and dash (–) operators, which you place between two terms, and parentheses, with which you enclose terms.”
Then: the correct answer is the “D” 65100 65101 65200 65300 21870
The way to read it:
First ASN is 65100
Second ASN is any (65101 in this case)
Third ASN could be one or more times either 65200 or 65300, this because the + operador is applied to (65200 |65300)
Last ASN is AS 21870.
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Answer is A
1 – 1st AS is 65100
2 – 2nd AS number must be 65200 or 65300
3 – 3RD AS one or more occurrences of 21870
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Ignore my last comment – Answer is D – agree with basur1 and dix
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