Category: PPP

More real-life DHCPv6 Prefix Delegation gotchas

The murky details of IPv6 implementations never crop up till you start deploying it (or, as Randy Bush recently wrote: “it is cheering to see that the ipv6 ivory tower still stands despite years of attack by reality”).

Here’s another one: in theory the prefixes delegated through DHCPv6 should be static and permanently assigned to the customers for long periods of time.

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DHCPv6 Prefix Delegation with Radius Works in IOS Release 15.1

A while ago I described the pre-standard way Cisco IOS used to get delegated IPv6 prefixes from a RADIUS server. Cisco’s documentation always claimed that Cisco IOS implements RFC 4818, but you simply couldn’t get it to work in IOS releases 12.4T or 15.0M. In December I wrote about the progress Cisco is making on the DHCPv6 front and [email protected] commented that IOS 15.1S does support RFC 4818. You know I absolutely had to test that claim ... and it’s true!

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Framed-IPv6-Prefix used as delegated DHCPv6 prefix

Chris Pollock from io Networks was kind enough to share yet another method of implementing DHCPv6 prefix delegation on PPP interfaces in his comment to my DHCPv6-RADIUS integration: the Cisco way blog post: if you tell the router not to use the Framed-IPv6-Prefix passed from RADIUS in the list of prefixes advertised in RA messages with the no ipv6 nd prefix framed-ipv6-prefix interface configuration command, the router uses the prefix sent from the RADIUS server as delegated prefix.

This setup works reliably in IOS release 15.0M. 12.2SRE3 (running on a 7206) includes the framed-IPv6-prefix in RA advertisements and DHCPv6 IA_PD reply, totally confusing the CPE.

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Delegated IPv6 prefixes – RADIUS configuration

In the Building Large IPv6 Service Provider Networks webinar I described how Cisco IOS uses two RADIUS requests to authenticate an IPv6 user (request#1) and get the delegated prefix (request#2). The second request is sent with a modified username (-dhcpv6 is appended to the original username) and an empty password (the fact that is conveniently glossed over in all Cisco documentation I found).

FreeRADIUS server is smart enough to bark at an empty password, to force the RADIUS server to accept a username with no password you have to use Auth-Type := Accept:

Site-A-dhcpv6   Auth-Type := Accept
cisco-avpair = "ipv6:prefix#1=fec0:1:2400:1100::/56"
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DHCPv6-RADIUS integration: the Cisco way

Yesterday I described how the IPv6 architects split the functionality of IPCP into three different protocols (IPCPv6, RA and DHCPv6). While the split undoubtedly makes sense from the academic perspective, the service providers offering PPP-based services (including DSL and retrograde uses of PPP-over-FTTH) went berserk. They were already using RADIUS to authenticate PPP users ... and were not thrilled by the idea that they should deploy DHCPv6 servers just to make the protocol stack look nicer.

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Last week I got an interesting tweet: “Hey @ioshints can you tell me what is the radius parameter to send ipv6 dns servers at pppoe negotiation?” It turned out that the writer wanted to propagate IPv6 DNS server address with IPv6CP, which doesn’t work. Contrary to IPCP, IPv6CP provides just the bare acknowledgement that the two nodes are willing to use IPv6. All other parameters have to be negotiated with DHCPv6 or ICMPv6 (RA/SLAAC).

The following table compares the capabilities of IPCP with those offered by a combination of DHCPv6, SLAAC and RA (IPv6CP is totally useless as a host parameter negotiation tool):

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DHCPv6 over PPPoE: Total disaster

Every time someone throws me an IPv6 curveball, I’m surprised when I discover another huge can of worms (I guess I should have learned by now). This time it started pretty innocently with a seemingly simple PPPoE question:

What happens if an ISP decides to assign dynamic IPv6 subnets? With static assignment, the whole stuff is pretty straight-forward due to ND, RA & DHCPv6, but if dynamic addresses are used, what happens if the subnet changes - how will the change be propagated to the end-user devices? The whole thing is no problem today due to the usage of NAT / PAT...

LAN address allocation with changing DHCPv6 prefix is definitely a major problem, but didn’t seem insurmountable. After all, you can tweak RA timers on the LAN interface, so even though the prefix delegated through DHCPv6 would change, the LAN clients would pick up the change pretty quickly. WRONG ... at least if you use Cisco IOS.

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L2TP: The revenge of hardware switching

Do you like the solutions to the L2TP default routing problem? If you do, the ASR 1000 definitely doesn’t share your opinion; so far it’s impossible to configure a working combination of L2TP, IPSec (described in the original post) and PBR or VRFs:

PBR on virtual templates: doesn’t work.

Virtual template interface in a VRF: IPSec termination in a VRF doesn’t work.

L2TP interface in a VRF: This one was closest to working. In some software releases IPSec started, but the L2TP code was not (fully?) VRF-aware, so the LNS-to-LAC packets used global routing table. In other software releases IPSec would not start.

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L2TP default routing: solutions

There are three tools that can (according to a CCIE friend of mine) solve any networking-related problems: GRE tunnels, PBR and VRFs. The solutions to the L2TP default routing challenge nicely prove this hypothesis; most of them use at least one of those tools.

Policy-based routing on virtual template interface. Use the default route toward the Internet and configure PBR with set default next-hop on the virtual template interface. The PBR is inherited by all virtual access interfaces, ensuring that the traffic from remote sites always passes the network core (and the firewall, if needed).

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