IPv6 interface: Global address + Link local address

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Sep 21st, 2010
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Hi,


Is there anybody who would know the reason why a router always have two IPv6 addresses Link local address (FE80) and Global address. Is there any special reason or case why we would like to used a link local address.


I also would like to know if these link local addresses can be ping and if it can be redistribuedby RIPng protocol is enabled.


Thanks for your help

Stephane

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Peter Paluch Tue, 09/21/2010 - 13:15
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Stephane,


The link-local addresses are important for IPv6 because they allow you to be independent on the (non)existence of global addressing, and they provide you with a sort of "bootstrap" for IPv6 start.


For IGP routing protocols (OSPFv3, IS-IS, RIPng, EIGRPv6), the link-local addresses serve as the next-hop IP addresses for all dynamically discovered networks. Thus, the routing in IGP is based on link-local addresses, and on a purely transit link, you do not need to assign any global IPv6 prefix - just use the ipv6 enable command.


The link-local IPv6 address is also used for communication on a single segment - say, when stations talk to its default gateway. Also, you have to bear in mind that all IPv6 communication including the supervisory communication like MAC address resolution is carried inside IPv6 packets. So it is necessary to actually some sort of address always present on an interface even if no global IPv6 address has been assigned yet. This is precisely the link-local address.


Link-local addresses can be pinged only from a device directly connected to the network where the pinged station is located, and when pinging the link-local address, the ping command on a Cisco router will ask for the name of the outgoing interface, as the egress interface cannot be deduced from a link-local address (it has no subnet prefix).


The link-local addresses cannot be advertised in the RIPng. That would actually break their definition of being valid only on a single link.


Best regards,

Peter

Steph1963 Mon, 09/27/2010 - 07:28
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Hi Peter,


Thanks for this very interesting reply. Did not realize that local addresses were used as next hop for IGP, I would imagine that the fact that link local has no prefix means that each router would only have a knowledge of the local link connected to itself and routing of any global addresses can be done via default/static route using link local addresses.


Should we used a name like fa0/0 to determine the name of the egress interface when we ping a remote link local address.


Thanks for the answer

Stephane

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