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TOPIC: arp table

arp table 6 years 7 months ago #34448

  • johnyb98
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Good evening.

I would like to give a scenario and ask some questions on it.
Suppose a network with:
20 terminals
one 24-port switch
one ADSL router (not a router-switch) with DHCP and no IP range (router a)
one ADSL router-switch with DHCP and no IP range (router b)

Router a is connected to the ADSL line and router b is used only as switch. No ADSL cable is connected on it.
Router a and b have as IP address the 192.168.1.1. Then there is a conflict on the network, but no error message is displayed on any pc.
It's not the same as the address conflict on network cards.
I think and would like to ask if, with this scenario, the machines on the network will lose internet connection sometimes.

I have made the following analysis:

Let's say that a user is using the internet and everything is working fine. That means tha there is an arp entry (IP+MAC address) of this PC registered on the arp table of router a. If the user of this machine stops using the internet for some time, then the entry is deleted from the arp table of router a. If after some time that user requires to open a web page, that means he needs an IP address. So, here, we have a problem, which is which router will preempt to give him an address and register him to its arp table. If the router that preempts is the wrong router (router b), with no ADSL cable connected on it, the user will not be able to load any web page. And it's not only this. The user must not make any efforts to load web pages for some time in order his arp entry be deleted from the arp table of the wrong router. And after this, when the machine of this user will be free of arp registration, the user, when will try again for a web page, must be lucky to be registered in router's a arp table.

So, with this network connection and configuration, network terminals will never lose any network connection but will lose internet connection from time to time.

What is you opinion?

Thank you for your time!!
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Re: arp table 6 years 7 months ago #34451

  • Nevins
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I have an issue with this:

Router a and b have as IP address the 192.168.1.1


Let's say that a user is using the internet and everything is working fine


To be perfectly honest I don't know of any configuration where that it is possible for the user to have connection and both routers share an ip address.
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Re: arp table 6 years 7 months ago #34454

  • Losh
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That scenario cannot work in a real network.

Router a is an ADSL device. ADSL devices are configured with the command :

Router(config-if)# ip address negotiate

this enables them to acquire settings from the ISP cable modem termination server (CMTS) via their dialer interface.

Your Router b should be configured to receive dhcp settings from Router a and therefore supply the 24-port switch to which you connect your hosts.

Router b can therefore have a back up internet connection for redundancy purposes.
This way your hosts will always have a LAN connection with a redundant WAN connection incase the ADSL connection goes down.

I hope this helps abit. Cheers.
~ Networking :- Just when u think its starting to make sense......... ~
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Re: arp table 6 years 7 months ago #34461

  • johnyb98
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First of all, thank you for your answers.

Of course this scenario cannot be possible and make a network work based on it. So, as for the question, that means that no user can work fine in the internet with two routers installed in a network sharing the same ip address.

The problem is that I was called by a company that had the problem that I analyse in this scenario. The users of the network could see the internet for e.g. 3 minutes, then they could not see it for 4 minutes, then again they could see it for 5 minutes and so on.

The problem was caused by router b, that was a router-switch but was used only as a switch. It had the same ip with router a (192.168.1.1). The problem was that on both routers DHCP service was enabled. I would like to inform you that router a (router with ADSL cable on it) was on the rack on the third floor. In this rack was also the 24-port switch. Router b was in an office of the first floor. So, two pcs were connected on two ports of router b and from a third port of router b a cable connected it with the switch of third floor.

I decided to give router a the address 192.168.1.189. An address that was commited by no pc or other network device on the network.

I adjusted pcs to be given automatically IP ddresses and not any constant addresses and I let on both routers a and b DHCP service enabled.

Now, to be covered, I removed router b from the network and replaced it with an ONLY 4-port switch and everything is working fine.

But my huge query is:
All these problems and new settings on network terminals and routers, router b replacement e.t.c. was caused because the ADSL router that the company had at first was broken. Before it was broken, it had DHCP enabled and it was giving IP addresses to terminals as they were adjusted to be given automatically IP adresses. All these were happening with router b installed on the network with the same settings:DHCP enabled and working just as a switch. The IP of this first router was 192.168.1.2 and, as I said before, router b had always ip 192.168.1.1. So, here is my huge question:How is it possible, before this first router stopped working, everything work fine, and after its replacement to have the problems?
I inform you that, after the replacement of the first router with router a, and after giving router a the ip address 192.168.1.189, the problem on the network existed. That's why I replaced router b with an only switch.

Consequently, we have the following in the company:

condition A
first router ip:192.168.1.2 DHCP enabled
router b ip:192.168.1.1 DHCP enabled
both routers on the network
network status:internet working fine on terminals

condition B
first router stopped working
replaced with router a
router a ip:192.168.1.1 DHCP enabled
router b ip:192.168.1.1 DHCP enabled
both routers on the network
network status:internet problems on terminals

condition C
router a ip:from 192.168.1.1 changed to 192.168.1.189 with DHCP enabled
router b ip:192.168.1.1 DHCP enabled
both routers on the network
network status:internet problems on terminals

condition D
first router ip:192.168.1.189 DHCP enabled
router b replaced with an only switch
network status:internet working fine on terminals

How is it possible on condition C the network not working as it is the same with condition A?

Thank you.
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