Thursday, May 15, 2008

Frame-Relay Inverse-Arp

Frame-Relay (a Layer 2 protocol) uses Inverse-Arp to map a know Layer 2 Address (DLCI) to a unknow Layer 3 Address (for example, IP).

If we would ever compare Ethernet ARP Request, with Frame-Relay Inverse ARP, we would have the following:

FR2

As you can see, both of them perform pretty much the same thing, but Ethernet ARP Request knows the Layer 3 Address, and requests the Layer 2 Address (MAC), on the other hand Frame Relay Inverse ARP knows the Layer 2 Address (DLCI) and request the Layer 3 Address (Next-Hop IP Address).

Once the interface is enabled, the router will send Inverse-Arp requests out all DLCIs learned via LMI for all protocols configured on the interface.

You can rely on Inverse-Arp to map the DLCI to the IP Address of your Routers, or if you want (or the situation asks for), you can create static maps.

Check this example network:

FR

Router R1 will rely on Inverse-Arp to create it´s Frame-Relay Maps.

R2 and R3 will use Static Mapping, with Inverse-Arp disabled. The configuration will look like that:

R1:

interface Serial1/0
ip address 172.16.1.1 255.255.255.0
encapsulation frame-relay

R2:

interface Serial1/0
ip address 172.16.1.2 255.255.255.0
encapsulation frame-relay
frame-relay map ip 172.16.1.3 201
frame-relay map ip 172.16.1.1 201 broadcast
no frame-relay inverse arp

R3:

interface Serial1/0
ip address 172.16.1.3 255.255.255.0
encapsulation frame-relay
frame-relay map ip 172.16.1.1 301 broadcast
frame-relay map ip 172.16.1.2 301
no frame-relay inverse arp

 

Frame-Relay Inverse ARP is not required on point-to-point interface, and, if needed it can be disabled with the following command under the interface you need to do it:

no frame-relay inverse arp

However, Inverse ARP Reply can NOT be disabled, that means, even if you disable your Inverse ARP in an interface, if this interface receives any Inverse ARP Request it´ll respond to the request!

image

Tip: If you see mapping to 0.0.0.0 (show frame-relay map). Try “clear frame-relay inarp”. If it still doesn’t work, save config & reload.

 

We can check which type of mapping was configured with the command "show frame-relay map":

- dynamic means the mapping was done using Inverse-Arp;

- static means the mapping was done manually (by command-line-interface).

Just to prove that, check the status of our example Routers R1, R2 and R3 like that:

R1#sh frame-relay map
Serial1/0 (up): ip 172.16.1.3 dlci103(0x67,0x1870), dynamic,
              broadcast,, status defined, active
Serial1/0 (up): ip 172.16.1.2 dlci102(0x66,0x1860), dynamic,
              broadcast,, status defined, active

R2#sh frame-relay map
Serial1/0 (up): ip 172.16.1.3 dlci 201(0xC9,0x3090), static,
              CISCO, status defined, active
Serial1/0 (up): ip 172.16.1.1 dlci 201(0xC9,0x3090), static,
              broadcast, CISCO, status defined, active

R3#sh frame-relay map
Serial1/0 (up): ip 172.16.1.1 dlci 301(0x12D,0x48D0), static,
              broadcast, CISCO, status defined, active
Serial1/0 (up): ip 172.16.1.2 dlci 301(0x12D,0x48D0), static,
              CISCO, status defined, active

 

More info can be found at IEWB Vol.1 and also on Cisco´s Website under the following link:

http://www.cisco.com/en/US/tech/tk713/tk237/technologies_tech_note09186a008014f8a7.shtml

Not that difficult, right?! :)

3 comments:

ryl0 said...

thank you for that excellent clarification!

Anonymous said...

nice

Jatin said...

Brilliant!! That helps.