IOS features such as ARP and IP Address secondary

Hi There,

I was not going to create any more posts this week but earlier on today I got stuck with this question which sounded really simple until it became almost a nightmare. This is one of those perfect examples of a huge difference between knowing how to configure something and knowing what “it” actually does for a living.

I must admit that in order to resolve this one I had to spent some time on the internet researching which again boils down to a point the when you know how to configure something does not necessarily mean you know how it works.

See below our very simple diagram , can it get any simpler ?? Well you can always use 1 or 2 devices but …. 🙂


This time I am not going to include a GNS3 net file for this assuming all of you know exactly how to build it. The say that CCIE lab exam is over 50% psychology and myself having gone through various workbooks I totally agree, ok let’s look at the question (read this carefully )

Main Question :
You only have OSPF process 1 running between R1 and R5 in area 0.Configure SVI Vlan13 on SW1 and assign an IP address
Allow R5 to ping the IP address (SVI of SW1)
DO NOT assign any static , dynamic , or policy routes to achieve this.
DO NOT configure any routing protocols between R1 and SW1 and then redistribute to achieve this.
DO NOT configure any routing other than a default route pointing to R1 on SW1
IP address is available for your use. Configure only R5 and SW1

Easy bonus question :
On all devices prevent the hostname from being displayed in the prompt when in configuration mode.


One last thing , this is not a difficult question as long as you know your technologies and the reason why I decided to make this post was because when I first became involved with Cisco networking years back I was not as much keen on reading as I was on labbing which we all know is great fun but knowing theory about the technology is crucial !

Because it took me a while to figure it out the main question please go ahead and post your suggestions in the comments field

Enjoy !


About ccie4all
Hello, and welcome to the first post of my CCIE blog This blog has got one simple goal and that is to improve our skills in Cisco Networking field so we can become best engineers on a job market. Wordpress Blog information about the changes made to Gns3 BGP , MPLS and R&S CCIE labs. In order to access and download all provided materials and receive important updates from Gns3 BGP , MPLS and R&S CCIE labs under GNS3 tab in the main header please go ahead and subscribe to ! All other posts have not been affected and can be accessed at any given time. Enjoy ! Tom

4 Responses to IOS features such as ARP and IP Address secondary

  1. ashwani says:

    Can`t it be done using NAT, by configuring static entries in R1?

    • ccie4all says:

      Hello Ashwani,

      Correct it can be accomplished the way you’re suggesting however we are not suppose to configure anything on R1. We are only allowed to make configuration changes on SW1 and R5.
      Having access to Cisco gold labs I found this question yesterday and it was used in one of the CCIE lab exams under IOS services section. It did take me a while to figure it out.
      TIP: use IOS features such as , IP secondary address and ARP


  2. John H says:

    In theory:
    Apply as a secondary IP address to the f0/1 interface on R5 with an 8bit mask.
    (This results in R5 ‘thinking’ that the entire 150.x.x.x network is reachable via interface f0/1, thus when attempting to ping the address from R5… R5 will transmit an ARP request from its f0/1 interface requesting the MAC address that corresponds to IP address

    R1 f0/1 interface will have Proxy ARP turned on by default and this feature will cause R1 to respond to R5s ARP request but R1 will insert its own MAC address as the destination MAC address that R5 will then use for the ping.

    (R1 does this because it has a route in its routing table to as it is a directly connected network.)

    The default route on SW1 is required to ensure there is a return path for the ping replies.

    Bonus question answer:

    R1(config)#prompt config hostname-length 0

    • ccie4all says:

      Hello John !

      Brilliant ! This is exactly what needs to be done step by step in order for R5 to communicate with SW1 !
      In case proxy arp is disabled on R1 the solution would be to statically configure an arp entry on R5
      arp c400.18a4.0001 ARPA
      The mac address in the above arp entry would be R1 fa0/1 mac address

      I must admit as I was going through various Cisco Gold Labs and came across this question in one of their labs , it did take me a while to figure it out !

      Bonus question : correct answer !


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