2.7 Hub-and-Spoke & Inverse-ARP

As per this note in WB 1 : "Frame Relay Inverse-ARP packets are link-local to the layer 2 process, and

cannot be forwarded between different circuits. This means that when R1 sends

an Inverse-ARP request out to R5 on DLCI 105, R5 cannot turn around and

forward this request to R3 on DLCI 503. The result of this is that R5 can resolve

R1 and R3 via Inverse-ARP, R1 and R3 can resolve R5 via Inverse-ARP, but R1

and R3 cannot resolve each other through Inverse-ARP. To gain reachability

between these devices a static frame-relay map command must be used to

populate the layer 3 to layer 2 resolution table."  : R1 & R3 can't resolve each other by inverse-arp but as per following output it looks like R1 & R3 are able to resolve each other. do you guys know why they are able resolve each other by inverse-arp? what's convergence time for inverse-arp? sometimes I've to wait upto 4-5 mins. btw, I'm using GNS3 + GNS3's builting FR switch.

 

Rack1R1#sh run int s0/0

Building configuration...

 

Current configuration : 110 bytes

!

interface Serial0/0

 ip address 155.1.0.1 255.255.255.0

 encapsulation frame-relay

 clock rate 2000000

 

Rack1R1#sh frame map

Serial0/0 (up): ip 155.1.0.3 dlci 103(0x67,0x1870), dynamic,

              broadcast,, status defined, active

Serial0/0 (up): ip 155.1.0.5 dlci 105(0x69,0x1890), dynamic,

              broadcast,, status defined, active

 

 

Rack1R3#sh run int Serial1/0

Building configuration...

 

Current configuration : 148 bytes

!

interface Serial1/0

 ip address 155.1.0.3 255.255.255.0

 encapsulation frame-relay

 serial restart-delay 0

 

Rack1R3#sh frame map

Serial1/0 (up): ip 155.1.0.1 dlci 301(0x12D,0x48D0), dynamic,

              broadcast,, status defined, active

Serial1/0 (up): ip 155.1.0.5 dlci 305(0x131,0x4C10), dynamic,

              broadcast,, status defined, active

 

 

Rack1R5#sh run int s0/0

Building configuration...

 

Current configuration : 110 bytes

!

interface Serial0/0

 ip address 155.1.0.5 255.255.255.0

 encapsulation frame-relay

 clock rate 2000000

 

Rack1R5#sh frame map

Serial0/0 (up): ip 155.1.0.1 dlci 501(0x1F5,0x7C50), dynamic,

              broadcast,, status defined, active

Serial0/0 (up): ip 155.1.0.3 dlci 503(0x1F7,0x7C70), dynamic,

              broadcast,, status defined, active

 

Comments

  • Hi,

    The problem seems to be related to how your Frame-Relay switch is configured. Inverse-arp is link-local only, so R1 should not be able to dynamically map R3's address.

    Looks like your FR switch is configured as a full-mesh instead of hub-spoke.

  • HI,

    Thank you. I followed INE's "How to build CCIE Rack" to build up my topology:

     

    http://www.ine.com/topology.htm

     

    is it full mesh in above link(looks like it is)? I thought I would I'm following ine topology to build my lab and INE workbook so I should get same results as shown in the WB (which I won't cuz looks like FR is full mesh as you said).

     

  • Hi,

    Yes; if you are using those mappings it is indeed full mesh and this is the expected result.

    HTH

    Good luck!

  • Thank you. one more question :)

     

    what's convergence time for inverse-arp or frame-relay? sometimes I've to wait upto 4-5 mins. btw, I'm using GNS3 + GNS3's builting FR switch.

  • Hi,

    This is a pretty good question, if I remember correctly inverse-arp happens every 10-12 minutes. I guess you can do a clear frame-relay inarp or shut/no shut the interface to trigger invese-arp.

    Good luck!

  • on R1 create point -to-point interface and assign dlci 105

    on R5 create point -to-point interface and assign dlci 305

    If you have your own lab you probably configured the frame-relay as full mush. as you know as soon as you assign ip address to the main interface the router send Inverse arp. You can stop the inverse arp that your router send by using static map for the Dlci or you can use no frame-relay inverse arp dlci , but you cannot stop the router to reply to an inverse arp that was received from another router.

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