Instead of drinking beer and lab-testing vodka during the PLNOG party I enjoyed DHCPv6 discussions with Tomasz Mrugalski, the “master-of-last-resort” for the ISC’s DHCPv6 server. I mentioned my favorite DHCPv6 relay problem (relay redundancy) and while we immediately agreed I’m right (from the academic perspective), he brought up an interesting question – is this really an operational problem?

Some intro first

I wrote about DHCPv6 relaying before, but let’s revisit how it works:

1. CPE sends DHCPv6 solicit message to figure out who the DHCPv6 server is;
2. PE relays the message toward the DHCPv6 server (DHCPv6 relaying is not a simple packet forwarding, DHCPv6 relay wraps the original message into a new envelope);
3. DHCPv6 server replies with the advertise message which is relayed to the CPE by the PE-router;
4. CPE might get multiple advertise messages and selects one of the potential DHCPv6 servers;
5. CPE sends DHCPv6 request message with IA-PD option to the PE-router.
6. PE-router relays the message to the DHCPv6 server.
7. DHCPv6 server allocates a prefix to the CPE router and returns the prefix in the DHCPv6 reply message.
8. PE-router receives the reply message;
9. PE-router installs a static route for the delegated prefix pointing toward the CPE-router;
10. PE-router forwards the DHCPv6 reply message to the CPE;
11. CPE starts using the delegated prefix to address its interfaces.

A brief overview of the process (actually, just the second half of it) is shown in the following diagram:

Relaying in Redundantly Designed Networks

Now imagine you have a switched layer-2 access network and two PE-routers connected to it for redundancy purposes.

This is not a good idea – the first-hop switch in the service provider network should be a layer-3 device for security reasons.

The CPE router still sends a single solicit message, but each PE-router relays that to the DHCPv6 server. DHCPv6 server will send back two advertise messages, both will get relayed to the CPE router (each PE-router will relay the response to its relayed solicit message), and the CPE router will pick one or the other.

The CPE router subsequently sends a DHCPv6 request message to the selected PE-router (acting as a relay), eventually gets back the delegated prefix, and the PE-router has a static route … but the other PE-router is not aware of what’s going on.

The Problem

One of the problems is obvious – you might get suboptimal routing, because one of the PE-routers lacks routes toward destination(s) with next hop in directly connected access network … but we can solve that with routing protocols that support third-party next hops.

However, matters get much worse if one of the PE-routers crashes. The static routes are lost (they can be regained with BULK LEASE mechanism once the PE-router completes the reload), but in the meantime the CPE devices served by that PE-router have no connectivity, even though you have a totally redundant access network.

The Question

Is this scenario relevant? Do you use something like that in your access network? Would you be impacted? Write a comment – Tomasz needs real-life use cases.

More information

• Enterprise IPv6 – the first steps webinar gives you an overview of IPv6 technologies, 4-to-6 migration alternatives and design recommendations.
• Service Provider IPv6 Introduction is an introductory-level webinar targeting ISP environments.
• Building Large IPv6 Service Provider Networks webinar describes access- and core-layer technologies and designs.

All three webinars (and numerous others) are part of the yearly subscription.