Ethan Banks On productivity.

NMC DOiT Vol.2 Scenario 12 – EIGRP Variance + Netflow Selective Sampling + PBR with CDP + IPv6 to IPv4 NAT-PT

  • The EIGRP variance command sets a multiplier to allow load-balancing across links with unequal metrics.  Yup, it’s true – with variance, you could, for instance, tell EIGRP to allow a path with twice the metric of the installed route to also be installed.  By default, that unequal cost route with twice the metric would only see half the traffic.  EIGRP load-balances in proportion to the metric of the installed routes.
  • You can configure IOS netflow to selectively sample certain kinds of traffic.  Probably most of us who are CCIE candidates have configured a router to do netflow exports to some kind of collector that makes pretty graphs.  It’s fun to know who’s eating all the WAN bandwidth.  :-)  Tweaking netflow to sample specific kinds or intervals of traffic may be beyond what a lot of us have done.  I know I hadn’t run into it before.
  • When a task asks you to forward to a certain next-hop if CDP indicates the next-hop is available, they are talking about the ability of policy based routing to do a “set ip next-hop verify-availability“.  CDP is a mechanism used for this.  See also this example.
  • You can do one-to-one NAT IPv4 to IPv6 and vice-versa.  This known as “NAT-PT”, network address translation – protocol translation.  What you need:
    • ipv6 nat” statements on interfaces – similar to “ip nat inside” or “ip nat outside” in IPv4 NAT.
    • ipv6 nat v4v6 source” or “ipv6 nat v6v4 source“, depending on which direction you need the NAT to go.  This goes on a dual-stack router, translating an IPv4 address into an IPv6 address, or vice-versa.
    • The “ipv6 nat prefix” statement defines an IPv6 prefix that will be used to draw NAT addresses from.

Okay – I think that mostly wraps it up for DOiTv2 scenario 12.  Right now, I’m scheduled for scenario 13 on 11/10, scenario 14 on 11/17, scenario 15 on 11/19, scenario 16 on 11/20, scenario 17 on 11/21, and scenario 18 on 11/24.  That should wrap up November.  We’ll see if I can do that 3 day stretch 11/19 – 11/21 or not.  I’m hopeful, but we’ll see.  That’s ambitious to say the least.


  • I have strange issue with NAT-PT. Almoust all times, when I configured NAT-PT I have pings of such type:
    !.!.!(one successful, one time exceeded) despite of the type of router (IPv4 or IPv6 router).

    It’s seems that I find how to resolve this issue. I disabled IPv6 CEF and IPv4 CEF on the translator and had straight successful pings.

  • I didn’t have that issue on the IOS I’m running, and CEF is enabled. Maybe a version difference between what you’re running and what I’m running? I did struggle getting the NAT to work, because the IOS configuration guide wasn’t as well written as it might have been. It took me a while to figure out that I needed the “ipv6 nat” and the “ipv6 nat prefix” statements. But once I got all the required statements, it worked without a hitch.

By Ethan Banks
Ethan Banks On productivity.

You probably know Ethan Banks because he writes & podcasts about IT. For example, he co-authored "Computer Networks Problems & Solutions" with Russ White.

This site is Ethan on productivity--not tech so much.

Find out more on his about page.