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New Member

IPv6 Question

Hi,

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

regards

Neo

3 REPLIES
Cisco Employee

Re: IPv6 Question

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

New Member

Re: IPv6 Question

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

Cisco Employee

Re: IPv6 Question

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

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