Frame relay scenario

Answered Question
Aug 25th, 2009

Hi every body.

I have this scenario and possible solution :

There are one main side,R1 and two sides, R2 and R3 . Main side R1 is connected to both sides using frame relay in hub and spoke topology.

R1 is connected to frame relay network by s0. R1 and R3 are connected to Frame relay network by sub interfaces point-to point type s0/0.2 and s0/0.3 respectively.

1) Will R2 map the dlci 1 to int s0/0.2?

Here is my understanding, as soon as link between frame relay switch and s0 int of R2 comes up, R2 learns Vc is up and using dlci 2. Since this is only dlci R2 receives from FR switch and there is only one frame relay enabled int s0/0.2, R2 simply concludes this dlci is for int s0/0.2.

Had there been more than two or more dlci on R2 and more than one sun interface point-to-point type, then we would have needed to tell R2 which dlci is for which interface by using the command:

frame-relay interface-dlci

Is my understanding correct?

=========================================

For my second question, please consider the following with the same above mentioned topology in mind.

R1 config

int s0

10.10.10.1/24

frame-relay encapsulation

R2 config

int s0

frame-relay encapsulation

int s0/0.2 point-to-point

ip address 10.10.10.2/24

R3:

int s0

encapsulation frame-relay

int s0/0.3 point-to-point

10.10.10.3/24

==========================

On R1

Vc between R1 and R2 uses dlci dlci 2

Vc between R1 and R3 uses dlci 3

On R2

vc between R2 and R1 uses dlci 1

On R3

Vc between R3 and R1 uses dlci 1

Please note that there is no vc running between R2 and R3.

If i use the command on R2 " ping 10.10.10.3" , it should work. Below are my reasons:

1) When i issue the command ping 10.10.10.3, it matches the entry in routing table at R2

C 10.10.10.0/24 directly connected,s0/0.2

R2 finds the s0/0.2 is sub interface of point-to-point type, therefore only one vc or Dlci is expected on this interface.

Since R2 has dlci 1 on s0/0.2, therefore R2 simply uses this dlci and forwards this ping packet to R1 which in turn forwards it to R3.

Is my understaning correct?

thanks a lot.

I have this problem too.
0 votes
Correct Answer by Lucien Avramov about 7 years 3 months ago

You are correct Sarah.

With inverse arp disabled it works as described as it's point to point.

Here is an output from the router:

R1#show frame map

Serial0/0/0.1 (up): point-to-point dlci, dlci 102(0x66,0x1860), broadcast

status defined, active

And the debug when a ping goes:

*Aug 26 18:54:32.392: Serial0/0/0.1(o): dlci 102(0x1861), pkt type 0x800(IP), datagramsize 104

*Aug 26 18:54:32.448: Serial0/0/0(i): dlci 102(0x1861), pkt type 0x800, datagramsize 104

*Aug 26 18:54:32.452: ICMP: echo reply rcvd, src 10.0.0.2, dst 10.0.0.1

*Aug 26 18:54:32.452: Serial0/0/0.1(o): dlci 102(0x1861), pkt type 0x800(IP), datagramsize 104

*Aug 26 18:54:32.508: Serial0/0/0(i): dlci 102(0x1861), pkt type 0x800, datagramsize 104

*Aug 26 18:54:32.508: ICMP: echo reply rcvd, src 10.0.0.2, dst 10.0.0.1

*Aug 26 18:54:32.508: Serial0/0/0.1(o): dlci 102(0x1861), pkt type 0x800(IP), datagramsize 104

*Aug 26 18:54:32.564: Serial0/0/0(i): dlci 102(0x1861), pkt type 0x800, datagramsize 104

*Aug 26 18:54:32.564: ICMP: echo reply rcvd, src 10.0.0.2, dst 10.0.0.1

*Aug 26 18:54:32.568: Serial0/0/0.1(o): dlci 102(0x1861), pkt type 0x800(IP), datagramsize 104

*Aug 26 18:54:32.624: Serial0/0/0(i): dlci 102(0x1861), pkt type 0x800, datagramsize 104

*Aug 26 18:54:32.624: ICMP: echo reply rcvd, src 10.0.0.2, dst 10.0.0.1

*Aug 26 18:54:32.624: Serial0/0/0.1(o): dlci 102(0x1861), pkt type 0x800(IP), datagramsize 104

*Aug 26 18:54:32.680: Serial0/0/0(i): dlci 102(0x1861), pkt type 0x800, datagramsize 104

Correct Answer by Lucien Avramov about 7 years 3 months ago

I just tested it, you need the frame-relay interface-dlci only on R2 and R3. And then it does work in the second scenario.

Correct Answer by Lucien Avramov about 7 years 3 months ago

Yes it does work pretty much like you describe.

You dont need manual mapping on R1, it will find it from inverse-arp.

And yes, then R2 can ping R3 via R1.

One need you need on R2 and R3 is to tell them which DLCI you use as I mentioned in my previous post, the command is:

frame-relay interface-dlci 1

Correct Answer by Giuseppe Larosa about 7 years 3 months ago

Hello Sarah,

frame-relay interface-dlci is needed to be able to associate a DLCI to a specific subinterface otherwise the router can learn from LMI of the existence of the DLCI but it cannot associate it to the correct subinterface.

So I don't think it works also the second scenario.

Here you hit a different aspect:

Inverse ARP can handle the OSI L3 resolution problem but it cannot handle the DLCI to subinterface association.

This needs an explicit configuration on the router or all DLCIs will be associated to main interface (I guess)

Hope to help

Giuseppe

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Correct Answer
Giuseppe Larosa Tue, 08/25/2009 - 23:06

Hello Sarah,

frame-relay interface-dlci is needed to be able to associate a DLCI to a specific subinterface otherwise the router can learn from LMI of the existence of the DLCI but it cannot associate it to the correct subinterface.

So I don't think it works also the second scenario.

Here you hit a different aspect:

Inverse ARP can handle the OSI L3 resolution problem but it cannot handle the DLCI to subinterface association.

This needs an explicit configuration on the router or all DLCIs will be associated to main interface (I guess)

Hope to help

Giuseppe

Correct Answer
Lucien Avramov Tue, 08/25/2009 - 23:16

I just tested it, you need the frame-relay interface-dlci only on R2 and R3. And then it does work in the second scenario.

Correct Answer
Lucien Avramov Tue, 08/25/2009 - 23:14

Yes it does work pretty much like you describe.

You dont need manual mapping on R1, it will find it from inverse-arp.

And yes, then R2 can ping R3 via R1.

One need you need on R2 and R3 is to tell them which DLCI you use as I mentioned in my previous post, the command is:

frame-relay interface-dlci 1

sarahr202 Wed, 08/26/2009 - 00:03

Thanks Lucien

My understanding is we don't need inverse arp on sub int ptp. Because there is only one dlci mapped to the interface.

For example in my earlier post, i described a topology.

If i use the command ping 10.10.10.3 on R2, r2 will match it to entry;

C 10.10.10.0/24 directly connected s0/0.2

Next R2 checks which dlci is used on s0/0.2 which is dlci 1

R2 simply sends the ping packet using dlci 1 to R1 which then forwards it to R3.

So i don't see any reason to have inverse arp enabled on s0/0.2 on R2 for ping 10.10.10.3 to work.

Thanks and have a nice day.

Correct Answer
Lucien Avramov Wed, 08/26/2009 - 10:58

You are correct Sarah.

With inverse arp disabled it works as described as it's point to point.

Here is an output from the router:

R1#show frame map

Serial0/0/0.1 (up): point-to-point dlci, dlci 102(0x66,0x1860), broadcast

status defined, active

And the debug when a ping goes:

*Aug 26 18:54:32.392: Serial0/0/0.1(o): dlci 102(0x1861), pkt type 0x800(IP), datagramsize 104

*Aug 26 18:54:32.448: Serial0/0/0(i): dlci 102(0x1861), pkt type 0x800, datagramsize 104

*Aug 26 18:54:32.452: ICMP: echo reply rcvd, src 10.0.0.2, dst 10.0.0.1

*Aug 26 18:54:32.452: Serial0/0/0.1(o): dlci 102(0x1861), pkt type 0x800(IP), datagramsize 104

*Aug 26 18:54:32.508: Serial0/0/0(i): dlci 102(0x1861), pkt type 0x800, datagramsize 104

*Aug 26 18:54:32.508: ICMP: echo reply rcvd, src 10.0.0.2, dst 10.0.0.1

*Aug 26 18:54:32.508: Serial0/0/0.1(o): dlci 102(0x1861), pkt type 0x800(IP), datagramsize 104

*Aug 26 18:54:32.564: Serial0/0/0(i): dlci 102(0x1861), pkt type 0x800, datagramsize 104

*Aug 26 18:54:32.564: ICMP: echo reply rcvd, src 10.0.0.2, dst 10.0.0.1

*Aug 26 18:54:32.568: Serial0/0/0.1(o): dlci 102(0x1861), pkt type 0x800(IP), datagramsize 104

*Aug 26 18:54:32.624: Serial0/0/0(i): dlci 102(0x1861), pkt type 0x800, datagramsize 104

*Aug 26 18:54:32.624: ICMP: echo reply rcvd, src 10.0.0.2, dst 10.0.0.1

*Aug 26 18:54:32.624: Serial0/0/0.1(o): dlci 102(0x1861), pkt type 0x800(IP), datagramsize 104

*Aug 26 18:54:32.680: Serial0/0/0(i): dlci 102(0x1861), pkt type 0x800, datagramsize 104

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