CCIE Pursuit Blog

April 2, 2008

Question Of The Day: 02 April, 2008

Topic: OSPF

Which of the following OSPF network types do not use Designated Routers:

1) point-to-point
2) non-broadcast
3) point-to-multipoint non-broadcast
4) point-to-multipoint
5) broadcast

Click Here For Answer


Yesterday’s Question

Question Of The Day: 01 April, 2008 

Topic: IPv6

FastEthernet0/0 is up, line protocol is up
  Hardware is AmdFE, address is cc01.17b4.0000 (bia cc01.17b4.0000)
  Internet address is 100.100.100.100/8

r2(config)#ipv6 unicast-routing
r2(config)#int fa0/0
r2(config-if)#ipv6 enable

What will be the IPv6 address of interface fa0/0?

Answer: FE80::CE00:17FF:FEB4:0

When you use the ‘ipv6 enable’ command under the interface, it will assign a link local address based on the MAC address.  The host portion of this address is determined using the same algorithm as modified eui-64:

Step 1 – Invert the seventh most significant bit of the MAC address.

This requires us to do some quick and dirty hexadecimal to binary conversion.  Luckily each hex character represents 4 bits, so we only need to convert the first two hex characters of the MAC address to include the seventh most significant bit:

cc = 1100 1100

Now let’s flip that seventh bit

1100 1110

Finally let’s convert this back to hex:

1100 1110 = ce

Step 2 – Split the MAC address in half and insert ‘fffe’

ce00.17 | b4.0000
ce0017fffeb40000
ce00:17ff:feb4:0000

Voila!  Now our 48-bit MAC address has been converted to 64-bits.  We just need to combine it with the IPv6 network portion of our link-local address (always FE80::) and we will have our dynamically assigned link-local address:

FE80:CE00:17FF:FEB4:0000
FE80:CE00:17FF:FEB4:0 <-after zero compression

r1#sh ipv6 int br | sec net0/0
FastEthernet0/0            [up/up]
    FE80::CE00:17FF:FEB4:0

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4 Comments »

  1. […] Click Here For Answer […]

    Pingback by Question Of The Day: 01 April, 2008 « CCIE Pursuit — April 2, 2008 @ 10:23 am | Reply

  2. The work leading to the solution changes the 1st 8bits from FF to FE. Is there a typo somewhere?

    Comment by Chris Dudley — April 2, 2008 @ 11:15 pm | Reply

  3. @Chris. You’re right. My fat fingers are to blame. 🙂 The link-local address should begin with FE80 and not FF80.

    Thank you for catching that error.

    Comment by cciepursuit — April 3, 2008 @ 6:05 am | Reply

  4. The mac address is cc01.17b4.0000. The final link-local address should be fe80::ce01……., is it a typo you got fe80::ce00..?

    Comment by Igor Urosevic — August 18, 2009 @ 2:42 am | Reply


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