Re: ipv6 eui-64 and link-local

From: ccie2be (ccie2be@nyc.rr.com)
Date: Mon Feb 07 2005 - 19:34:04 GMT-3

Andy,

Thanks again for bearing with me on this.

Please see in-line comments.

----- Original Message -----
From: "Edwards, Andrew M" <andrew.m.edwards@boeing.com>
To: "ccie2be" <ccie2be@nyc.rr.com>; "Group Study" <ccielab@groupstudy.com>
Sent: Monday, February 07, 2005 4:48 PM
Subject: RE: ipv6 eui-64 and link-local

I'm refering to your question about overriding the link-local address.

The router has a fixed number of mac addresses available to it in a pool
that it selects from to determine the IPv6 link-local address. What
happens if two routers select the same mac address on a serial link and
use that as the basis for their IPv6 address on two adjacent routers?

********************
Do different Cisco routers ( pick 1 of the following choices: a- always b-
sometimes c- never )

have the same pool of mac addresses to pick from?

(Your comments imply either choice a or b but I thought it was choice c.)

But, in any case, the critical point to understand is that by using the
link-local option
on WAN interfaces, I avoid the potential problem of each interface on a wan
link
having the same ipv6 address, correct?

*****************************

Now, there is a mechanism to mitigate this problem, however, I would
think it best to just remove the potential for this problem. Especially
in instances where the router reloads. 8)

*****************************

I'm still very unclear about the eui-64 option -

    when do I HAVE to use this option vs
    when is it a good idea to use this option vs
    when don't I need this at all.

Any chance you could elaborate on this?

Thanks, Tim

Andy

-----Original Message-----
From: ccie2be [mailto:ccie2be@nyc.rr.com]
Sent: Monday, February 07, 2005 10:13 AM
To: Edwards, Andrew M; Group Study
Subject: Re: ipv6 eui-64 and link-local

Hi Andy,

Thanks for getting back to me.

I do understand the overall process by which ipv6 replaces arp but I
don't understand what role the eui-64 has in that process.

For example, I know that when node A wants to to talk with node B, it
sends out an icmp v6 neighbor solicitation mcast using as the
destination address a multicast address that corresponds to node B's
ipv6 address. And, when node B gets that message, it responds with an
icmp v6 neighbor advertisement which includes node B's link layer
address.

Maybe, if you understand this, you could explain the role of the eui-64
in that process so I can understand when I need to use that option.

Regarding link layer option, are you saying that it's a good idea to use
this option on serial interfaces because unlike ethernet interfaces,
they don't have a mac address burned in?

Thanks again for taking the time to respond to this inquiry. I
appreciate it.

Tim

----- Original Message -----
From: "Edwards, Andrew M" <andrew.m.edwards@boeing.com>
To: "ccie2be" <ccie2be@nyc.rr.com>; "Group Study"
<ccielab@groupstudy.com>
Sent: Monday, February 07, 2005 12:47 PM
Subject: RE: ipv6 eui-64 and link-local

Tim,

How does IPv6 work for L3 to L2 resolution?

And, if routers only have a fixed number of predefined L2 addresses
available to them to pick from, what happens if two adjacent router pick
the same L2 address from the pool especially possible when it comes to
things like serial interfaces.

Once you look at this, then you will know why you might want to hard
code the link-local address. 8)

Something else interesting to note, that because of the way IPv6 works
for L2 to L3 resolution, neat things start to become possible, like
pinging from one IPv6 address to another IPv6 address on the same
physical segment but on different subnets without requiring a router in
between like IPv4.

Look at the IPv6 ICMP usage.

HTH,

andys

-----Original Message-----
From: ccie2be [mailto:ccie2be@nyc.rr.com]
Sent: Monday, February 07, 2005 5:21 AM
To: Group Study
Subject: ipv6 eui-64 and link-local

HI guys,

I'm very unclear about when I need to use the eui-64 or link-local
options when configuring an ipv6 address.

For example, what's the difference between the following 2 options?

Rack1R1(config-if)#ipv6 address fec0:157:1:14::/64 ?
  eui-64 Use eui-64 interface identifier
  <cr>

If I just hit enter <cr>, what will be the value of the low 64 bit's?

Also, regarding the link local option, I understand that ipv6 will
automatically generate a link-local address from the site local or the
aggregatable global unicast address. So, why would I ever want to
override the auto generated link-local address?

Thanks for your help. I'm really struggling with this ipv6 stuff.

Tim

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