frame-relay scenario

Answered Question
Sep 3rd, 2009

Hi every body

I have this question.

Let say we have two routers,r1 and r2. r1 is connected by s0 to r2. r2 is connected to r1 by s0/0.1, sub interface point to point type.

--e0-r1s0------------------s0/0.1r2

r2:

int s0

encapsulation frame-relay

int s0/0.1

ip address 198.198.198.2/24

frame-relay interface-dlci 1

ip route 10.0.0.0 255.0.0.0 198.198.198.2

Assume r1 has all proper configuration, if i use ping on r2 " ping 10.10.10.1,

Will r2 be able to send this ping packet to r1 using dlci 1? I understand if i had configured ip route 10.0.0.0 255.0.0.0 s0/0.1, r2 will use the dlci 1 of s0/o.1 to send the ping packet to r1. But i am just curious what would happen if i configure ip route 10.0.0.0 255.0.0.0 198.198.198.2 and issueb ping " 10.10.10.1" will r2 use the dlci 1 to send the ping packet to r1. Sorry for being repetitive , i just want make sure you guys understand what i have on my mind.

I have joined the army and heading soon to south carolina for my bootcamp, So i would not be able to post any questions for 6 months. Atleast you guys have a little break from my weird questions. I want to thank all of you, specially, Peter, Lucien, Jon marshal, Edison ,Giuseppe ,Joseph and others for their help and kindness.

you guys have a good day.

Correct Answer by Giuseppe Larosa about 7 years 5 months ago

Hello Sarah,

>> Router at regular interval sends lmi query and frame relay switch replies by lmi status message. When router does send this lmi query message to frame relay switch at regular interval, what does the router want to inquire about? is it about the status of vc?

Your understanding is correct however only 1 every 6 FR switch answers contains detailed information about the state of VCs.

FR standards require that FR switch detailed answer is confined to a single PDU so this leads to limits in the number of PVCs that can be defined on a single link that depends from link MTU and from FR encapsulation in use (each of them uses more or less bytes to describe a VC).

As noted in the idea center there are persons like you with a gift for putting questions.

When I say we have seen your progress is because we see that your questions are becoming more difficult to answer, and that you propose scenarios.

Best Regards

Giuseppe

Correct Answer by Peter Paluch about 7 years 5 months ago

Hello,

Just a couple of small comments.

Your s0/0.1 interface on R2 misses the type indicator (multipoint or point-to-point). I assume that it is point-to-point but it would need to be specified when first creating that interface.

The "frame-relay interface-dlci 1" command would not be accepted, as the DLCIs from 0 till 15 are reserved. I know that you were just writing a schematic of the configuration, though.

If you entered the "ip route" command on the R2 exactly as entered, the router would reject it with the message "%Invalid next hop address (it's this router)". So you would not even be able to enter such route in the routing table.

But if your routing table ultimately says - go out the s0/0.1, the packet will be routed out that interface, and because it is a point-to-point subinterface, it will be sent out using the only DLCI that is assigned to this subinterface. So ultimately, the packet will reach R1.

The question is whether the R1 will be able to respond. Rick has very correctly pointed out that if you ran a routing protocol over this pair of interfaces you could come into trouble because the routing protocol may behave differently over a point-to-point interface and differently on a multipoint interface (just consider the OSPF and its default timer values, not mentioning other key differences). For the sake of simplicity, assume only the static routing - in that case, it depends on R1's IP/DLCI mappings - whether there is a DLCI mapping for the next-hop address back to R2 in R1's mapping table.

There seems to be some persistent confusion regarding the InverseARP operation on a pair of ports where one is multipoint and the other is single point. The fact is that the point-to-point interface does not need any mappings, static or dynamically detected by InverseARP. It just sends everything out that single DLCI. A point-to-point interface does not send InverseARP queries. However, if it recevies a query it replies to it. Multipoint interfaces need the mappings for obvious reasons and they run InverseARP by default so they both send and answer InverseARP queries. So from the viewpoint of the InverseARP, you can freely combine a multipoint and point-to-point interfaces.

Regarding your army acceptance - good luck! It has been and it always be a pleasure to meet you here. You will certainly be missed here on NetPro for the next 6 months. Take care.

Best regards,

Peter

Correct Answer by Richard Burts about 7 years 5 months ago

If you configure the static route with your own interface address as the next hop then the router will not forward traffic with destination 10.10.10.1.

If you configure the static route with the next hop address being the address of the neighbor on the Frame Relay then the router would forward the traffic using the DLCI of the local subinterface.

Your question suggests a mismatch between the routers with r1 operating as multipoint and r2 operating as point to point. The mismatch could have an impact if you are trying to run dynamic routing protocols. And it would impact the ability to automatically generate Frame Relay mapping on r1. But it would not impact the behavior on r2 to forward traffic using the static route.

HTH

Rick

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Correct Answer
Richard Burts Thu, 09/03/2009 - 07:26

If you configure the static route with your own interface address as the next hop then the router will not forward traffic with destination 10.10.10.1.

If you configure the static route with the next hop address being the address of the neighbor on the Frame Relay then the router would forward the traffic using the DLCI of the local subinterface.

Your question suggests a mismatch between the routers with r1 operating as multipoint and r2 operating as point to point. The mismatch could have an impact if you are trying to run dynamic routing protocols. And it would impact the ability to automatically generate Frame Relay mapping on r1. But it would not impact the behavior on r2 to forward traffic using the static route.

HTH

Rick

Jon Marshall Thu, 09/03/2009 - 11:22

Sarah

NetPro will not be the same without you.

Hope everything goes well for you and it's been a pleasure to help you out whenever i could.

Jon

sarahr202 Thu, 09/03/2009 - 11:38

Thanks Jon for your kind remarks.

I learned a lot from you guys and wish to keep learning for a long long time.

Until now the only routers and switches, i managed to get my hands on were 2500 routers and 1900 and 2900 series switches.

Hopefuly, working for big brother will help me get my hands on cisco 6500 series switches which until now i am able to see only in online pictures.

Thanks and you take care of yourself.

Correct Answer
Peter Paluch Thu, 09/03/2009 - 11:59

Hello,

Just a couple of small comments.

Your s0/0.1 interface on R2 misses the type indicator (multipoint or point-to-point). I assume that it is point-to-point but it would need to be specified when first creating that interface.

The "frame-relay interface-dlci 1" command would not be accepted, as the DLCIs from 0 till 15 are reserved. I know that you were just writing a schematic of the configuration, though.

If you entered the "ip route" command on the R2 exactly as entered, the router would reject it with the message "%Invalid next hop address (it's this router)". So you would not even be able to enter such route in the routing table.

But if your routing table ultimately says - go out the s0/0.1, the packet will be routed out that interface, and because it is a point-to-point subinterface, it will be sent out using the only DLCI that is assigned to this subinterface. So ultimately, the packet will reach R1.

The question is whether the R1 will be able to respond. Rick has very correctly pointed out that if you ran a routing protocol over this pair of interfaces you could come into trouble because the routing protocol may behave differently over a point-to-point interface and differently on a multipoint interface (just consider the OSPF and its default timer values, not mentioning other key differences). For the sake of simplicity, assume only the static routing - in that case, it depends on R1's IP/DLCI mappings - whether there is a DLCI mapping for the next-hop address back to R2 in R1's mapping table.

There seems to be some persistent confusion regarding the InverseARP operation on a pair of ports where one is multipoint and the other is single point. The fact is that the point-to-point interface does not need any mappings, static or dynamically detected by InverseARP. It just sends everything out that single DLCI. A point-to-point interface does not send InverseARP queries. However, if it recevies a query it replies to it. Multipoint interfaces need the mappings for obvious reasons and they run InverseARP by default so they both send and answer InverseARP queries. So from the viewpoint of the InverseARP, you can freely combine a multipoint and point-to-point interfaces.

Regarding your army acceptance - good luck! It has been and it always be a pleasure to meet you here. You will certainly be missed here on NetPro for the next 6 months. Take care.

Best regards,

Peter

Giuseppe Larosa Thu, 09/03/2009 - 20:08

Hello Sarah,

>> I have joined the army and heading soon to south carolina for my bootcamp, So i would not be able to post any questions for 6 months.

I wish you all the best for this new experience.

Take care of yourself.

(choice carefully the questions you will put to bootcamp instructors :-) )

As I wrote other times your questions are helpful for us too.

And we are glad to have seen your progress in the never ending learning process.

Most of it (99%) is the result of your effort and studies.

Best Regards

Giuseppe

sarahr202 Fri, 09/04/2009 - 21:30

Thanks Giuseppe for your kind remarks.

I just want to ask one last question if i may.( something for me to think about on the long flight to south carolina on monday :-)

Lmi messages also serve keep alive function.

Router at regular interval sends lmi query and frame relay switch replies by lmi status message. When router does send this lmi query message to frame relay switch at regular interval, what does the router want to inquire about? is it about the status of vc?

Thanks and most of the things I learned, I learned from you guys not from books. In fact i came to trust more you guys than books . you guys with real world experiences are very valuable learning assets for me.

The humility and kindness, i find among you guys is enviable and worth emulating.

Thanks and have a good day.

Correct Answer
Giuseppe Larosa Fri, 09/04/2009 - 22:48

Hello Sarah,

>> Router at regular interval sends lmi query and frame relay switch replies by lmi status message. When router does send this lmi query message to frame relay switch at regular interval, what does the router want to inquire about? is it about the status of vc?

Your understanding is correct however only 1 every 6 FR switch answers contains detailed information about the state of VCs.

FR standards require that FR switch detailed answer is confined to a single PDU so this leads to limits in the number of PVCs that can be defined on a single link that depends from link MTU and from FR encapsulation in use (each of them uses more or less bytes to describe a VC).

As noted in the idea center there are persons like you with a gift for putting questions.

When I say we have seen your progress is because we see that your questions are becoming more difficult to answer, and that you propose scenarios.

Best Regards

Giuseppe

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