Archive for the Layer 2 Technologies Category

Inverse ARP for Dummies

Posted in Layer 2 Technologies with tags , on August 4, 2009 by Gerren

As much as I like to think I know what I’m talking about, inverse ARP and FR in general were never one of my strong points. I didn’t study it much during my CCNA studies or during my CCNP studies. With that said, here’s my general explanation (simplified for my simple brain) on how inverse ARP works.

R1 has inverse ARP enabled, but R2 doesn’t.

1.  R1 –> InverseARP –> R2

2. R2: “Even thought I have inverse ARP disabled, I am still required to reply to R1′s request. AND while I’m at it, add a dynamic entry for R1.

And that folks…is my brief lesson on inverse ARP.

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DTP Negotiation for Dummies

Posted in Layer 2 Technologies with tags , on July 31, 2009 by Gerren

Warning: there is nothing formal about this post, just a way to help me (and maybe you, who knows) remember DTP port states:

Dynamic Desirable: “Hey, I want to trunk and I’m letting everyone know” (3550′s are DD by default)

Dynamic Auto: “I want to trunk, but I’m not announcing it. BUT! I’ll answer DTP trunking requests” (3560′s are DA by default)

DD + DA = Trunk (And use ISL unless configured otherwise — assuming Cisco switches)

DD + DD = Trunk

DA + DA = No Trunk for You!

You can disabled DTP by either:

  1. Setting the interface to static access using the switchport mode access command
  2. Turn off DTP all together with switchport nonegotiate


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