Exam 300-101 topic 1 question 22 discussion

Actual exam question from Cisco's 300-101

Question #: 22
Topic #: 1

An engineer has configured a router to use EUI-64, and was asked to document the IPv6 address of the router. The router has the following interface parameters: mac address 2201.420A.0004 subnet 2001:DB8:0:1::/64
Which IPv6 addresses should the engineer add to the documentation?

A. 2001:DB8:0:1:01:42AF:FE0F:4
B. 2001:DB8:0:1:FFFF:2201:420F:4
C. 2001:DB8:0:1:FE80:2201:420F:4
D. 2001:DB8:0:1:C601:42AE:800F:4

Show Suggested Answer

Suggested Answer: A 🗳️
Extended Unique Identifier (EUI), as per RFC2373, allows a host to assign iteslf a unique 64-Bit IP Version 6 interface identifier (EUI-64). This feature is a key benefit over IPv4 as it eliminates the need of manual configuration or DHCP as in the world of IPv4. The IPv6 EUI-64 format address is obtained through the 48-bit
MAC address. The Mac address is first separated into two 24-bits, with one being OUI (Organizationally Unique Identifier) and the other being NIC specific. The
16-bit 0xFFFE is then inserted between these two 24-bits to for the 64-bit EUI address. IEEE has chosen FFFE as a reserved value which can only appear in EUI-
64 generated from the EUI-48 MAC address.
Here is an example showing how the Mac Address is used to generate EUI.

Next, the seventh bit from the left, or the universal/local (U/L) bit, needs to be inverted. This bit identifies whether this interface identifier is universally or locally administered. If 0, the address is locally administered and if 1, the address is globally unique. It is worth noticing that in the OUI portion, the globally unique addresses assigned by the IEEE has always been set to 0 whereas the locally created addresses has 1 configured. Therefore, when the bit is inverted, it maintains its original scope (global unique address is still global unique and vice versa). The reason for inverting can be found in RFC4291 section 2.5.1.
Reference:
https://supportforums.cisco.com/document/100566/understanding-ipv6-eui-64-bit-address

by Bastex at Sept. 16, 2019, 8:07 a.m.

Comments

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Bastex

Highly Voted 5 years, 9 months ago

The options list is wrong. Right answer is 2001:DB8:0:1:2001:42FF:FE0A:4.

upvoted 9 times

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pudu123

Most Recent 5 years, 7 months ago

I think you meant this one 2001:DB8:0:1:2201:42FF:FE0A:4 the mac address is 2201.420A.0004

upvoted 1 times

d719273

5 years, 6 months ago

As written before "MAC should be splitet in firsthalf*ff.fe*secondhalf and changed to opposit the seventh bit from the begining." Right answer is 2001:DB8:0:1:2001:42FF:FE0A:4

upvoted 1 times

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Gngogh

5 years, 5 months ago

The 7th bit from the left must be inverted. Hence 2 becomes 0. the correct EUI-64 is 2001:42FF:FE0A:4

upvoted 1 times

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d719273

5 years, 7 months ago

agree with Bastex, right address is 2001:DB8:0:1:2001:42FF:FE0A:4 As MAC should be splitet in firsthalf*ff.fe*secondhalf and changed to opposit the seventh bit from the begining.

upvoted 2 times

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