IPv6 Static Routes to an Interface without next hop subnet

vinbuck · September 2011

I'm working on a lab from gns3vault and I think i've come across something that can't be done.

IPv6 Static Route | IPv6

In this lab (item #5) It asks you to configure a static route to a loopback on a directly connected router using only the outbound interface and not a next hop address.

If you specify a command like ipv6 route 2001::4/128 FastEthernet 0/0 and 2001::4/128 is the Loopback 0 address of a dricetly connected router, it' doesn't seem like you should be able to reach that address without another static route or an IGP.

stuh84 · September 2011

I take it you aren't familiar with using interfaces in static routes rather than IPs as the destination?

vinbuck · September 2011

I am in the v4 world but it's been a while since I touched v6 so I'm doing some labbing to refresh. The way I understood the v6 method of routing to an interface, because it uses link-local addresses, the subnet you are trying to reach has to be on the physical interface of the directly connected router on the other end or you would need another route.

Yea or Nay?

stuh84 · September 2011

Think about it in the IPv4 world. Would the loopback you are directing towards be directly connected to the interface? Nope. Yet it can still reach it right? Yep.

So what is different now? You are pointing towards the other side of a link (same as before), the interface on the other side is not in the same subnet as the route (same as before), and now you are directing traffic via that interface (same as before).

Just to confirm, have you actually tried to configure it?

vinbuck · September 2011

I've labbed it both ways and when I put the global unicast subnets on the FastEthernet interface on each router, I can ping all day long. However when I move each unicast subnet to a loop, neither is reacheable. I don't beleive that IPv6 functions the same as IPV4 in this regard. I think you would have to deploy an IGP or use the Global Unicast subnet as the next hop instead of the interface.

Here is a doc from cisco that leads me to believe this is the case

Cisco IOS IPv6 Configuration Guide, Release 12.4 - Implementing Static Routes for IPv6 [Cisco IOS Software Releases 12.4 Mainline] - Cisco Systems

Directly Attached Static Routes

In directly attached static routes, only the output interface is specified. The destination is assumed to be directly attached to this interface, so the packet destination is used as the next-hop address. This example shows such a definition:

ipv6 route 2001:DB8::/32 ethernet1/0

QHalo · September 2011

Static Routing – differences between specifying an exit interface, an IP address, and both « Cisconinja’s Blog

This has configurations that show all the ways, including the way you're attempting when they configure R1 to R3's loopback via IPv6. It's just below the configs of each device where they talk about it. Check it over. HTH

stuh84 · September 2011

Today I learned then! It's been a while since I did anything with IPv6 if I'm honest, I had thought I had done it with just an interface at some point but I guess not. Sorry if I came across harsh or anything like that, it just appeared from your posts that you hadn't tried it like that.

As an idea, maybe try something like a /127 as the interface addressing, so that the other side only has one host they can get to? I don't know if it would get treated as a point-to-point interface then, worth a go though.

Edit: just re-read the initial sites description....yeah I'm not sure how this can be done. Labbing time.

IPv6 Static Routes to an Interface without next hop subnet

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