Ethan Banks Not writing about IT.

NMC DOiT Vol.2 Scenario 21 – OSPF Database Filter + OSPF ISPF (Incremental SPF) + Advertising Routes to HSRP Virtual IP + neighbor local-as + neighbor disable-connected-check


I was originally scheduled to do DOiT scenario 22 on Monday, but I opted to finish off scenario 21 instead.  I spent time with the family for the rest of the day, plus yesterday (of course).  I hope you all had a Merry Christmas!  I did, and I’m ready to be back at the rack.  Before I do, I need to blog about scenario 21.

  • To disable Dynamic Trunking Protocol (DTP) on a switchport, use the command “switchport nonegotiate“.
  • The OSPF scenario had a task where 3 routers (R1, R2, and R4) were supposed to form a full-mesh of adjacencies with one another.  However, R2 and R4 were not supposed to advertise routes to each other.  You could not use OSPF priority to solve the problem.  So, how do you deal with it?

    OSPF has a feature where you can filter LSA floods on a per-interface basis – “ip ospf database-filter all out“.  This works for broadcast, non-broadcast, and point-to-point OSPF network types on an interface basis, but it’s not neighbor-specific.  For neighbor-specific granularity, change your OSPF network type to one that requires neighbor statements, and then do a “neighbor xx.xx.xx.xx database-filter all out” command.

  • Incremental SPF is an OSPF feature that allows for partial tree calculations in reaction to a change in a type-1 or type-2 LSA.  This reduces convergence time and saves on router CPU.  ISPF and non-ISPF router can co-exist in the same area.  To enable ISPF, use the command “ISPF” in the “router ospf” paragraph.
  • NMC put together an interesting RIP scenario, where a manually configured RIP router neighbor was actually a virtual IP assigned to an HSRP group.  That allowed RIP advertisements to float from one router to another, depending on which HSRP router was active.  This was a creative way to solve the problem NMC presented.  At first, I was thinking there was yet some other RIP feature I hadn’t run into yet.  In fact, the issue was tied into the first-hop redundancy task later in the scenario.
  • When migrating BGP AS numbers, you might need a BGP router to be in one AS, but act as if he’s in another.  This can be done by manipulating the AS path using the “neighbor local-as” command.  Read more about BGP Support for Dual AS Configuration for Network AS Migrations.
  • Using BGP “neighbor disable-connected-check“, you can peer 2 directly connected routers using loopback IPs, but without using “ebgp-multihop“.

More in the next post.

By Ethan Banks
Ethan Banks Not writing about IT.

You probably know Ethan Banks because he writes & podcasts about IT. This site is his, but covers other stuff.

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