inverse ARP

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Hi guys
For Frame Relay networks, DLCIs (Data Link Connection Identifiers are
used) for establishing Virtual cirucits for communication. The inverse
ARP helps in filling the table which maps IP addresses and DLCI used
for communication. For filling the tables, ARP messages are used over
the Virtual circuits for getting the IP addresses of the terminals on
the remote side.
I am consfused as how is a Virtual cirsuit established in the first
plae if one of the Comps does not know the IP address of the remote
comp.
Please clarify.
thank you
Shantanu


inverse ARP
Hi guys
For Frame Relay networks, DLCIs (Data Link Connection Identifiers are
used) for establishing Virtual cirucits for communication. The inverse
ARP helps in filling the table which maps IP addresses and DLCI used
for communication. For filling the tables, ARP messages are used over
the Virtual circuits for getting the IP addresses of the terminals on
the remote side.
I am consfused as how is a Virtual cirsuit established in the first
plae if one of the Comps does not know the IP address of the remote
comp.
Please clarify.
thank you
Shantanu


Re: inverse ARP
As i understand it, Frame relay operates at layer-2 so it does not need
an IP address to establish it's virtual circuit.
I think it finds out the remote dlci and that is all it needs to
establish.

somebody correct me if I'm wrong.

Also make sure  you know frame relay good, I got alot of frame relay on
the ccna exam.


Re: inverse ARP
genki napisa=B3(a):
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Yes FR is a Layer 2 protocol.
LMI at the switch side informs DTE about active Virtual Circuits
DTE use Inverse Arp to send its IP address via VC - the same happens=20
from the DTE on the other side.
After a while, both DTE have correct mapping VC - IP address.
You can disable Inverse Arp and manually configure appropriate mappings.

------------------------
Ern

Re: inverse ARP

genki napisał(a):
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Yes FR is a Layer 2 protocol.
LMI at the switch side informs DTE about active Virtual Circuits
DTE use Inverse Arp to send its IP address via VC - the same happens
from the DTE on the other side.
After a while, both DTE have correct mapping VC - IP address.
You can disable Inverse Arp and manually configure appropriate mappings.

------------------------
Ern

Well this confuses me also.
Normally you, as administrator, place IP addresses on interfaces.
So how does this Inverse ARP action know whats an appropriate IP address for
the DTE (router) interface?
Does this rely on some kind of DHCP server?

Obviously I'm missing something here,
Please explain a litte more.

Thanks!



Re: inverse ARP
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The mechanism by which inverse ARP works and the motivation for the
protocol is well explained in the relevant RFC (RFC 1293).

Take a look at

http://rfc.net/rfc1293.html

Cisco da Gama
http://ciscostudy.blogspot.com


Re: inverse ARP

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Well, thanks for the link I'm still not getting it.

I read for example this:

7.2.  Protocol Operation Within Frame Relay

   One case where Inverse ARP can be used is when a new virtual circuit
   is signalled.  The Frame Relay station may format an InARP request
   addressed to the new virtual circuit.  If the other side supports
   InARP, it may return a reply indicating the protocol address
   requested.How can the other side reply with the protocol address
requested?Where does it get this intelligence from?I assume they mean with
"Frame Relay station" the (DTE) router.Cisco da Gama, can you "translate"
this part of RFC 1293 into easy to understand language for a CCNA
student?Maybe this isn't possible. ;-)Thanks.



Re: inverse ARP

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 Well, thanks for the link but I'm still not getting it.

 I read for example this:

 7.2.  Protocol Operation Within Frame Relay

   One case where Inverse ARP can be used is when a new virtual circuit
   is signalled.  The Frame Relay station may format an InARP request
   addressed to the new virtual circuit.  If the other side supports
   InARP, it may return a reply indicating the protocol address
   requested.

How can the other side reply with the protocol address
 requested?
Where does it get this intelligence from?
I assume they mean with "Frame Relay station" the (DTE) router.

Cisco da Gama, can you "translate" this part of RFC 1293 into easy to
understand language for a CCNA student?

Maybe this isn't possible. ;-)

Thanks.

[edit] Sorry for the text formatting problem, Outlook Express freaked out on
me. [/edit]



Re: inverse ARP
Dude wrote:

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Let's take a concrete example.  Imagine a point-to-point FR link like
this

A (DLCI = 102) ------[ FR CLoud ]------- B (DLCI = 201)

Let's say A's IP address is 10.10.10.1 and B's IP address is
10.10.10.2.  When the circuit first comes up, B send an inverse-ARP
packet with his own IP address out onto the FR link.  A sees a packet
from 10.10.10.2 come in on his own link with DLCI 102.  So, he can set
up the following map

IP address = 10.10.10.2, DLCI = 102

Note how he is mapping the remote IP address to his local DLCI.  This
is sorta the opposite of the usual ARP with ethernet for instance and
that is why this is called "inverse" ARP.

Now, when A sees the inverse ARP packet come in, he sends out an
inverse packet of his own so that when B sees it he can set up the
following map

IP address = 10.10.10.1, DLCI = 201

Now, the layer3 address to layer2 address mapping needed is complete
and A and B can communicate with each other.  If you are familiar with
gratuitous ARP in ethernet, the section you referered to in the RFC is
similar to that mechanism for frame relay.

Cisco da Gama
http://ciscostudy.blogspot.com


Re: inverse ARP

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Sorry I've just notice this post.
I hope that this example by Cisco da Gama explains InArp ;)


------------------------
Ern

Re: inverse ARP

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Thanks for clearing this up!
Very helpful.



Re: inverse ARP
That was a very good explanation of Inverse ARP. However, I still have
a couple of lingering questions that I can't seem to get around.

Consider an example as shown below:

A -------- FR CLOUD 1 -------- B --------- FR
CLOUD 2 -------- C
IP=10.10.10.1                                      IP=10.10.10.2
                               IP=10.10.10.3

Initially lets say that A & B are active and C is down. A thus knows
that DLCI 102 can be used to reach 10.10.10.2 via FR Cloud 1. B knows
that it can reach A through DLCI 201 by traversing FR Cloud 1

C now becomes active.
QUESTION 1>> Is it possible to create DLCI 301 such that A can be
reached by traversing FR Cloud 2, Router B and FR Cloud 1? (B would be
acting like a FR switch in this case).

If it is indeed possible to create a DLCI at C as explained above, will
Inverse Arp allow C to know that the new DLCI (Say 301) maps to
10.10.10.1; i.e. will the InArp packet be allowed to traverse B?

I would really appreciate if you could kindly help me out with the
same.

Regards,

- PinkFloyd


Re: inverse ARP
Pink_Floyd wrote:
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No, this cannot be done.

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Since the answer to Q1 is "no", this does not apply.

You are unnecessarily complicating the situation here.  What is
happening is that you are trapping yourself at layer2 in thinking about
this problem.  You need to move up to layer3 now.

For the routers in your network above to communicate, routing needs to
be turned on.  With routing,  C will know that to get to networks
behind A it should use B as the next-hop and it already has a FR map to
get to B and so things will just work.  It will not need a FR map for A
or networks behind A etc.

Cisco da Gama
http://ciscostudy.blogspot.com


Re: inverse ARP
Excellent!!

Thank you Sir for your time and efforts!!

Apologies for the delayed reply!!


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