Elisa Jasinska covered several IPAMs in her overview of open-source network automation tools, and we had Jeremy Stretch talking about NetBox in the Building Network Automation Solutions online course, but if you’re looking for a really robust easy-to-implement solution, check out this document from 1998 (deployment experience, including a large-scale one).

If you’ve been a networking engineer (or a sysadmin) for a few years, you must be pretty familiar with DHCP and might think you know everything there is to know about this venerable protocol. So did I… until I read the article by Chris Marget in which he answers two interesting questions:

• How does the DHCP server (or relay) send DHCP offer to the client that doesn’t have an IP address (and doesn’t respond to ARP)?
• How does the DHCP client receive the DHCP responses if it doesn’t have an IP address?

Martin Bernier has decided to open another can of IPv6 worms: how do you address multiple subnets in a very typical setup where you use a firewall (example: ASA) to connect a SMB network to the outside world?

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.

Jernej Horvat sent me the following question:

I know DHCPv6-based prefix delegation should be as stable as possible, so I plan to include the delegated prefix in my RADIUS database. However, for legacy reasons each username can have up to four concurrent PPPoE sessions. How will that work with DHCPv6 IA_PD?

Short answer: worst case, DHCPv6 prefix delegation will be royally broken.

Years ago our IT assigned a /28 to my home office. It seemed enough; after all, who would ever have more than ~10 IP hosts at home (or more than four computers at a site).

When the number of Linux hosts and iGadgets started to grow, I occasionally ran out of IPv4 addresses, but managed to kludge my way around the problem by reducing DHCP lease time. However, when the start of school holidays coincided with the first snow storm of the season (so all the kids used their gadgets simultaneously) it was time to act.

Beatrice Ghorra (@beebux) was kind enough to share the results of her IPv6-over-PPPoE tests with me.

Short summary: everything works as expected on ASR 1K running IOS XE 3.7.

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?

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!

DHCPv6 server on Cisco IOS got several highly useful enhancements since the first time I started looking into its behavior. Seems like most of them are implemented only in 15.xS trains (where they are most badly needed one would assume), but there’s hope those changes will eventually trickle down into mainstream IOS.

Last week 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"

Last week I ran numerous lab tests while preparing router configurations for the Building IPv6 Service Provider Core webinar (register here or buy a recording). One of the fantastic test results: DHCPv6 relaying works correctly on a 7200 running 12.2(33)SRE2, even when the client requests IA_PD option.

Due to extreme student interest, I’ve added a whole new remote access section to my Building IPv6 Service Provider Core webinar (register here or buy a recording). It covers PPPoE and Carrier Ethernet access methods (PPPoE configuration can be used in any dial-up environment; Carrier Ethernet configuration is probably applicable to cable as well) and describes the following topics:

• SLAAC on access networks for hosts connecting to the IPv6 Internet;
• DHCPv6 prefix delegation required by IPv6-enabled CPE routers;
• Prefix allocation (SLAAC and DHCPv6) from local pools;
• DHCPv6 relays and SLAAC/DHCPv6-RADIUS integration.

As always, attendees of past webinars can download the updated materials immediately and will get access to the new recording after the next week’s session.

Srinivas sent me the following printout a few days ago and asked me whether I could explain the weird DHCP bindings (I removed the lease expiration column from the printout):

Switch#sh ip dhcp binding
Bindings from all pools not associated with VRF:
IP address          Client-ID/              Type
                    Hardware address/
                    User name
192.168.101.140     0152.4153.2000.188b.    Automatic
                    cfb7.f800.0000.0000.
                    00
192.168.101.141     0152.4153.2000.188b.    Automatic
                    cfb7.f800.0001.0000.
                    00

My DHCPv6+PPPoE post received a very comprehensive comment from Ole Troan (thank you!) in which he explains the context in which DHCPv6 was developed (a mechanism to give a static IPv6 prefix to a customer) and its intended usage (as the prefix is static, it should have a very long lifetime).

However, when you deploy DHCPv6 in some modern access networks (it’s not just PPPoE, Carrier Ethernet fares no better), you might experience subtle problems. Let’s start with a step-by-step description of how DHCPv6 works: