DLCI / Subinterface / IP clarification plz

Hi, everyone.

I'm having trouble understanding the following sentence from the Cisco Press ICND2 book:

"Mapping via Inverse ARP or static frame-relay map statements is needed only when more than two VCs terminate on the interface or subinterface, because those are the only instances in which confusion about which DLCI to use might occur."

So my main question is, why "more than two"? The whole IPs to DLCIs to subinterfaces thing is kind of confusing, but I thought I was understanding it up until this point...

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Comments

Monkerz

It means you will need to use "iARP" or "frame-relay map" to statically assign DLCI map statements when setting up a configuration that uses a single interface/subinterface for multiple DLCIs. Thats a long way of saying a multipoint frame relay setup.

Look at the differences in a Point-to-Point setup and a Multi-Point setup. You can see where the "frame-relay map" statements are used on a single interface that is the gateway for two different VCs.

--Multi-Point Frame Relay:
*R1 (HUB)
conf t
int serial 0/1/0
ip add 192.168.1.1 255.255.255.0
encapsulation frame-relay
no shut
frame-relay map ip 192.168.1.2 102 broadcast
frame-relay map ip 192.168.1.3 103 broadcast

*R2
conf t
int serial 0/0
ip add 192.168.1.2 255.255.255.0
encapsulation frame-relay
no shut
frame-relay map ip 192.168.1.1 201 broadcast
frame-relay map ip 192.168.1.3 201 broadcast

*R3
conf t
int serial s0
ip add 192.168.1.3 255.255.255.0
encapsulation frame-relay
no shut
frame-relay map ip 192.168.1.1 301 broadcast
frame-relay map ip 192.168.1.2 301 broadcast

--P2P Frame Relay:
*R1 (Hub)
conf t
int s0/1/0
encapsulation frame-relay
int s0/1/0.102 point-to-point
ip address 192.168.1.1.255.255.255.0
frame-relay interface-dlci 102
int serial 0/1/0.103 point-to-point
ip address 192.168.2.1 255.255.255.0
frame-relay interface-dlci 103
int s0/1/0
no shut

*R2
conf t
int serial 0/0
encapsulation frame-relay
int serial 0/0.201 point-to-point
ip add 192.168.1.2 255.255.255.0
frame-relay interface-dlci 201
inter serial 0/0
no shut

*R3
conf t
int s0
encapsulation frame-relay
int s0.301 point-to-point
ip add 192.168.2.2 255.255.255.0
frame-relay interface-dlci 301
int s0
no shut