Kevin Dorrell, CCIE #20765

IOS “gotchas”

IOS “Gotchas” 

Here is a list of my favorite IOS “gotchas”.  Some of these will be obvious to many of the more experienced readers, but they are all things that have caught me out at one time or another.  Links will become live if (and only if) I write a page about a particular gotcha.

If you know of a similar list somewhere, or if you have any suggestions to add to the list, then please let me know.

Configuration fundamentals and miscellaneous

  1. no exec[-timeout]
  2. ipv6 address
  3. The “Section” filter
  4. frame-relay interface-dlci nnn ppp Virtual-Template n
  5. Copying configs to flash
  6. con f
  7. debug all (The quickest way to crash a router)

LAN Switching

  1. Under certain circumstances, a vtp client can destructively update a VTP domain.  Clients are not necessarily safe.


  1.   timers basic 0 n n n (Worse than a crash … a lockout)


  1. When you configure EIGRP hello timer, you may also need to modify the hold timer.  Do this on the local router, not the remote one!  It is the local router that tells the remote router how long to hold the adjacency for.
  2. If you add metric to a route by adding delay on the interface, the advertised distance of the neighbor remains unchanged.  If you add metric to a route with an offset-list, it adds it to the apparent advertised metric of the neighbor.  If you are load-sharing with variance, this can disqualify the route altogether because the feasible successor criterion fails.

Route redistribution

  1. If you redistribute OSPF through a route-map, match route-type internal does not include the connected prefixes.
  2. no redistribute rip route-map RIP–>OSPF does not remove the redistribution; it just opens it up.
  3. If you redistribute routes that are tagged, usually the tags are preserved (provided the destination protocol supports them).  Except if you are redistributing into RIP v2.  In that case the tag is stripped.  If you need the tag, use a route-map to regenerate it.


  1. Don’t the timers have to be agreed on as a requirement of EIGRP neighborship?

    Comment by Marko — 08 May 2008 @ 23:54

  2. No, it’s OSPF where they have to agree. In EIGRP, they are independent in each direction.

    Comment by dorreke — 09 May 2008 @ 20:44

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