Ethan Banks On productivity. CCIE-LAB List – Best of 1/15/2008 – Prefix Lists + PAgP “non-silent” + OSPF Flood Wars + Duplicate EIGRP RIDs


Gleanings from the CCIE-LAB mailing list over the last few days…

  • An interesting practice page for prefix-lists.
  • Although nothing definitive was unearthed, there was some talk about what the PAgP keyword “non-silent” accomplishes. Related links on this topic follow.
    • Finally, I read this in the Doc CD about 5 times before it made sense. It’s worded a little awkwardly. It almost makes it sound like you should use “non-silent” for interswitch links – which I’ve never done. It’s a rabbit trail to go down at some point, although I don’t think it’s a critical feature to understand when it comes to passing the lab. Still interesting.

      If your switch is connected to a partner that is PAgP-capable, you can configure the switch port for nonsilent operation by using the non-silent keyword. If you do not specify non-silent with the auto or desirable mode, silent mode is assumed.

      Use the silent mode when the switch is connected to a device that is not PAgP-capable and seldom, if ever, sends packets. An example of a silent partner is a file server or a packet analyzer that is not generating traffic. In this case, running PAgP on a physical port connected to a silent partner prevents that switch port from ever becoming operational. However, the silent setting allows PAgP to operate, to attach the port to a channel group, and to use the port for transmission.

  • I’d been meaning to play with prefix-lists to find the best way to replicate a “permit any” ACE. That appears to be “ip prefix-list name seq 10 permit le 32“, and that makes sense. I had been using “ip prefix-list name seq 10 permit ge 1” which I knew wasn’t right, but for whatever reason I hadn’t yet taken the cycles needed to figure it out.
  • An OSPF “flood war” (about which I can find nothing on, and almost nothing via Google) probably indicates that you have a duplicate router ID in the OSPF domain. You may see errors starting with “%OSPF-4-FLOOD_WAR” describing an LSA being flushed and then reoriginated.
  • Duplicate router IDs in the EIGRP domain can cause external routes to be rejected by the router with the duplicated RID. Read more about this here.


  • Another reason for the OSPF flood-war that I just ran into (IE Vol2, lab 16) is simple Cisco funkyness. R4 and R5 have an ethernet connection to each other. They are also hubs to R3 in a FR hub and spoke triangle. OSPF is everywhere, same area.

    R5 kept getting this flood-war message. After having wasted hours troubleshooting weird crap like this, only to have it resolved via a reboot, that’s exactly what I did right up front. It fixed it.

  • I should add that there was never a duplicate router-id, or even a duplicate IP, anywhere. I verified this, and changed nothing before rebooting.

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.