In IS-IS the router knows the interfaces addresses of the other routers. So would it be possible to connect two IPv6 areas over an IPv4 area?

A router in area A knows that the destination ipv6 address is at the router in area B. So the route would be set, isn't it?

But what would be the destination of this packet when it is on the IPv4 area?

best regards


IS-IS is a routing protocol. It can forward on information about what direction to send traffic for a number of protocols, including IPv4 and IPv6.

The key point there is that it's information about what direction traffic should be forwarded but it's not the means by which the traffic is actually forwarded.

The simplest analogy is that of roads, road signs and addresses: the road sign tells you which road to take to get to a given address but isn't the road itself. The road exists whether or not there are signs and the simple existence of signs isn't an assurance that any possible type of address is reachable via a given set of roads.

In concrete terms the implicit item in your question is the distinction between the control plane and the forwarding plane. The control plane is concerned with exchanging reachability information between independent nodes. The forwarding plane is purely concerned with the act of receiving, analyzing and forwarding packets (or frames) on the wire. The control plane both drives the forwarding plane and utilizes it to actually exchange information but the two are ultimately distinct. A control plane protocol (...like IS-IS) can carry information about any number of subjects without the forwarding plane necessarily being able to handle corresponding traffic types.

So - back to your question - the IS-IS process might be carrying information about how to reach IPv4, IPv6, IPX, X.25 and CLNS addresses (and, yes, all of these actually have/had specs defined at some point) but this doesn't mean that the routers actually running the protocol are able to forward datagrams for all of them. IS-IS might tell you about your IPv6 routes but if some of the routers along the way can't actually forward v6 then it's not going to do you much good..

  • Thank you so much :) Wouldn’t it be useful to have a protocol to route packet in an AS completely on layer 2? Something like every router has its NET address and knows like in IS-IS the information about all other router (interface addresses, hostname, metrics…). Then the router could send a layer 2 packet within everything we want. On routing the other router would know where this NET address is and would it send forward this address. So if there are any problem on layer 3 of our system (mismatch of subnets in one link, other types of IP protocol…) it would pass anyway… – denis bourqui May 29 '17 at 5:14
  • 1
    You've more-or-less described MPLS - common L2 transport, carries anything and a control plane that's a lot more flexible and extensible (MP-BGP). – rnxrx May 30 '17 at 2:13

IPv4 and IPv6 are two incompatible protocols. In order to forward your IPv6 data across an IPv4 network, you have to use some sort of tunneling mechanism, where the IPv6 packet is encapsulated in an IPv4 packet. This is because IPv4 devices do not understand IPv6.

IS-IS or any other routing protocol, has nothing to do with this process. The function of routing protocols is to populate the routing table so that the router knows which interface to forward the packet out of.


IS-IS doesn't help you with crossing IPv4 territory for IPv6, you'll need to use a tunnel.

  • 2
    could you explain why? – denis bourqui May 26 '17 at 11:09
  • rnxrx explains this quite well below. – Zac67 May 27 '17 at 10:17

IS-IS can carry routes for different protocols, but it does not tunnel the packets for the protocols themselves. IPv4 and IPv6 are two completely separate protocols. There are solutions for tunneling IPv6 in IPv4, and you would still need to do something like that.

It really is no different than using BGP-MP or OSPFv3, both of which can carry routes for both IPv4 and IPv6, but you will need some method to encapsulate IPv6 packets inside IPv4 packets to cross an IPv4-only area. The routers inside the IPv4-only area will have no idea what to do with IPv6 packets entering the area, so they would be dropped.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.