IPv6 Question

[cisco]

Hi,

Sometimes whenever i try to ping in a IPv6 domain , it ask for a source interface, why is that ?

regards

Neo

[Peter Paluch]

Hello,

This can happen if you are trying to ping the link-local address (i.e. those starting with FE80). This address has only link-local scope and therefore might not be unique. Therefore the router is not certain on which interface you want to send the ping packets, therefore it asks.

Best regards,

Peter

[cisco]

Hi Peter,

If these address have link-local scope that means it is unique to that link, isnt ? Please correct me if i am wrong.

regards

Neo

[Peter Paluch]

Hi Neo,

Yes - the link-local addresses are unique on the link they appear on. But a single link-local address can be reused on various physically distinct links. For example, the serial interface do not have a MAC address so for purposes of link-local address creation, the serial interfaces borrow a MAC address from another interface in the router. I haven't done thorough tests but it seems that often the MAC address of the Fa0/0 interface is used (anyone having better experiences please correct me here!).

So this is the first issue about link-local addresses - while they are unique on a link, they might not be unique among multiple links.

Further on, the link-local addresses do not use any net/subnet semantics. They can't really be found in a routing table. Your router does not know when you ping a particular link-local address which interface should it use. The only way to determine it is to ask you. And so it does :)

Best regards,

Peter