Frame Relay Config Question

waru

Hi,

I was reading through an article on configuring subinterfaces for frame relay and noticed the following config:

interface Serial4/2
ip address 172.16.1.1 255.255.255.248
encapsulation frame-relay
frame-relay map ip 172.16.1.2 103 broadcast


Obviously this is not a subinterface but simply a point to point connection right? I thought that you didnt need to worry about the frame-relay map command on a point to point connection?

What would be the difference between the above config and:

interface Serial4/2
ip address 172.16.1.1 255.255.255.248
encapsulation frame-relay
frame-relay interface-dlci 103

I know Im missing something obvious here so any help would be greatly appreciated!!

Cheers
waru

dtlokee

Physical interfaces are treated as multipoint interfaces, they use inverse arp (or static mappings) to resolve the DLCI's to remote end IP addresses which is required becaue the Multipoint interface acts as multiaccess and there can be more than 2 devices on the subnet. Using the second example would treat the DLCI as point to point and therefore only one other host could exist on the subnet.

If the frame-relay pvc's are not fully meshed between all your routers you will want to turn off inverse-arp and use static mappings. When you do this remember to use the broadcast keyword on any mappings to IP addresses at the other end of the PVC (not the ones that are 2 PVC's away in the case of a hub and spoke topology)

HTH

waru

Thanks dtlokee. That would make sense if physical interfaces are treated as multipoint subinterfaces however I have seen the second example I gave used on a physical interface so what is the difference between using frame-relay map and frame-relay interface dlci 103

EdTheLad

waru wrote:

Thanks dtlokee. That would make sense if physical interfaces are treated as multipoint subinterfaces however I have seen the second example I gave used on a physical interface so what is the difference between using frame-relay map and frame-relay interface dlci 103

As stated the physical interface will work in multipoint mode.Inverse-arp is enabled by default on the physical interface so when you assign a dlci to the physical interface using "frame-relay interface dlci 103" the router can discover the remote ip address which corresponds to 103 if the end to end path is established and the farend configured correctly.You can also assign this mapping manually using the "frame-relay map" command.In the ccie lab you are supposed to disable inverse-arp and assign everything manually using the map command.Inverse-arp can save you the hassle of configuration but it can be messy if you are not in control of the neighboring router, it just adds another step in troubleshooting if things don't workout as you planned.

waru

Ahh so frame-relay interface dlci 103 is only used when inverse arp is turned on. All makes sense now!! Thanks very much!