Frame-Relay - Basics - Part1

Understanding Frame-relay basics;

• What happens when you encapsulate an Interface with frame-relay
• What's the effect running frame-relay on;
• the main-interface

Typical config on the main interface;

R1#sh run int s1/0

Building configuration...

Current configuration : 158 bytes

!

interface Serial1/0

 ip address 155.1.1.1 255.255.255.0

 encapsulation frame-relay

 serial restart-delay 0

 frame-relay map ip 155.1.1.5 105 broadcast

end

• a point-to-point sub-interface

Typical config on the sub-interface; 

R2#sh run int s1/0.205

Building configuration...

Current configuration : 118 bytes

!

interface Serial1/0.205 point-to-point

 ip address 155.1.25.2 255.255.255.0

 frame-relay interface-dlci 205   

end

When configuring FR on a p2p sub-interface all you need to assign is the DLCI it needs to use. There is no L3 to L2 mapping required. For a p2p sub-interface there is only one point of exit to the rest of the network.

• a multipoint sub-interface
• Checking if Frame-Relay is running
  • show commands
  • Frame-Relay types
• Why does Frame-Relay use Inverse ARP

The same way Ethernet uses ARP to find the MAC address associated with an IP address, in FR Inverse ARP is used to find a IP address connected to a local Layer 2 circuit - i.e. DLCI number.

When you un-shut an interface configured to run FR (i.e.encap frame-realy), the interface status using "show ip int brief" would show interface as up/up.  

When you un-shut an interface running FR and it the interface show as up/down that mean there is something wrong with the FR switch and LMI is not working.  

Serial1/0             unassigned      YES manual up    down

Because FR is local to the router and the FR-switch. This interface status does not tell you anything about the router you're trying to connect to.

• Effects of not running Inverse ARP

Inverse ARP is running automatically on an interface configured to run FR. Inverse ARP is process mapping the L2 circuit (DLCI) to a L3 IP address. When you configure a static map on the FR interface Inverse ARP is disabled automatically.

interface Serial1/0
ip address 155.1.1.5 255.255.255.0
encapsulation frame-relay
serial restart-delay 0
frame-relay map ip 155.1.1.1 501 broadcast
end

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R5#sh frame-relay map

Serial1/0 (up): ip 155.1.1.1 dlci 501(0x1F5,0x7C50), static,

              broadcast,

              CISCO, status defined, active

You can disable Inverse ARP per DLCI on the interface sub-command or if you don't specify any protocol or DLCI; Inverse ARP will be disabled for all protocols and DLCIs;

i.e.

R5(config-if)#no frame-relay inverse-arp [protocol]
or
R5(config-if)#no frame-relay inverse-arp
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