From: ccie2be (ccie2be@nyc.rr.com)
Date: Wed Apr 20 2005 - 12:38:38 GMT-3
Sam,
Thanks for bearing with me on this.
I think I now understand the first part of your explanation but I'm not sure
about the other part.
In the original question, it was given that a site local address of FEC0/125
should be used. And, in a Site local address, the 16 bits that follow the
first 48 represent the subnet-id which is why you used this:
FEC0:0:0:D
for the 1st half of the address. OK, this part makes sense.
Now, for the 2nd half of the address. You used the 5th hex grouping for the
SLA. If I understand you're reasoning, the 5th hex didn't have to used. The
6th or 7th hex field could have been used but using the 5th hex field makes
summarization easier. Do I have that right?
What confused me most about this problem was that I originally thought D
should be used the way you said. But, the "official" solution swapped the
position of the subnet id and SLA values. It has this:
FEC0:0:0:B::D:x/125 where x is the router number.
I tend to expect the "official" answers to be correct. But, sometimes, like
in this case, they aren't.
Thanks again. Tim
-----Original Message-----
From: Sam Joseph [mailto:samjoseph747@hotmail.com]
Sent: Wednesday, April 20, 2005 10:51 AM
To: ccie2be@nyc.rr.com
Cc: ccielab@groupstudy.com
Subject: RE: ipv6: SLA and subnet ID
Tim,
You are getting it on your Last Paragraph. On a Site Local Address (ipv6),
the first 48 bits (3hex's) are always fixed. That would be FEC0:0:0. The
Next Portion is 16 Bit Subnet-ID ( 1hex). The Last 64 bits (4hex) is the
interface ID. This interface ID can be an extension or may also represent of
the Subnet. Cisco calls it as concatenation of Subnet ID.
Since in this question, you are specifically told to use Subnet_Id of D, we
are left with the choice of
using the 5th hex as site level identifier.
If you ask me, do i have to use only the fifth hex as site level aggregator
?. The answer is it depends. When you are aksed to summarize it somewhere
upstream, following an hierachical addressing will be hell a lot easier.
Hope I made sense in this explanation. If any one can improve upon, pls come
forward.
Thx.
>From: "ccie2be" <ccie2be@nyc.rr.com>
>To: "'Sam Joseph'" <samjoseph747@hotmail.com>
>CC: <ccielab@groupstudy.com>
>Subject: RE: ipv6: SLA and subnet ID
>Date: Wed, 20 Apr 2005 08:56:09 -0400
>
>Sam,
>
>Thanks for getting back to me. I appreciate it.
>
>I think your answer is correct but the issue I still have is this:
>
>Why did you decide to use
>
>FEC0::D:B:0:0:2/125 on R2 and FEC0::D:B:0:0:1/125 on R1
>
>instead of, for example,
>
>FEC0::B:D:0:0:2/125 on R2 and FEC0::B:D:0:0:1/125 on R1
>
>or this:
>
>FEC0::D:0:0:B:2/125 on R2 and FEC0::D:0:0:B:1/125 on R1
>
>In this task is there one and ONLY one correct answer or many possible
>correct answers?
>
>The trouble I'm having with this problem is that in the Cisco doc's they
>show examples of ipv6 addresses using a SLA and a subnet ID but never both
>of these fields in the same type of address. And, in the examples, both the
>SLA and subnet ID occupy the same position - bits 49 to 64 from the left -
>in the address.
>
>Clearly, its not possible for both the subnet id and the SLA to occupy the
>same bits at the same time in the same type of address. So, why is it you
>choose to make the subnet, B, part of the interface id and place it where
>you did?
>
>Thanks, Tim
>
>-----Original Message-----
>From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of Sam
>Joseph
>Sent: Tuesday, April 19, 2005 9:28 PM
>To: ccie2be@nyc.rr.com
>Cc: ccielab@groupstudy.com
>Subject: RE: ipv6: SLA and subnet ID
>
>Hi Tim,
>
>How about the following Address:
>
>FEC0::D:B:0:0:2/125 on R2 and FEC0::D:B:0:0:1/125 on R1
>
>My Explanation:
>
>First 48 bits are FEC0:0:0 ( 3hex)
>Subnet ID is 000D (1 hex)
>Interface ID is B:0:0:1 (SLA in this case since this is specific to site or
>local to the organization)
>
>If any better thoughts, please come forward.
>
>Thx,
>
>Sam
>
> >From: "ccie2be" <ccie2be@nyc.rr.com>
> >Reply-To: "ccie2be" <ccie2be@nyc.rr.com>
> >To: "Group Study" <ccielab@groupstudy.com>
> >Subject: ipv6: SLA and subnet ID
> >Date: Tue, 19 Apr 2005 18:43:16 -0400
> >
> >Hi guys,
> >
> >If I'm given this info in the lab, how do I figure out where to put what:
> >
> >1. All ipv6 addresses will use FEC0/125 unless otherwise specified.
> >
> >2. Configure a site-local subnet D between R1 and R2 and use SLA number
>B.
> >
> >
> >From the Cisco documentation, a site local address has a subnet ID but
>not
> >a
> >SLA. While a global unicast address has a SLA but not a subnet ID.
> >
> >Now, I realize that with a netmask of /125, there's lots of room to put
> >things where I want but if given only the above info, how do I figure out
> >what's OK?
> >
> >TIA, Tim
> >
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