Configuring Frame Relay Inverse ARP
The OSI Model dictates that in order for L3 addresses to communicated with other L3 addresses it must first go through Layer 2. In the case of Frame Relay, you must have Layer 2 to Layer 3 Mappings known as the ARP table. This lab will discuss and demonstrate the configuration of frame relay inverse ARP.
Real World Application & Core Knowledge
As a CCNA candidate you are REQUIRED to have an understanding of Frame Relay Inverse ARP (Address Resolution Protocol). In the previous lab you created a Frame Relay Map. These static maps will map an IP address to a DLCI # however; Inverse ARP does this automatically. Inverse ARP will attempt to learn its neighboring devices IP addresses and automatically create a dynamic map table.
This feature is often frowned upon and 99% of the time disabled by network engineers to prevent “unexpected issues” for example an ISP brings up another DLCI for your head quarters and this DLCI should be a point to point DLCI and require a security policy but when this DLCI is activated and if the router is already installed with the IP address and DLCI information the HQ router will automatically map it via Inverse ARP causing a security problem.
Other potential problems could arise from this feature such as routing loops or sub-optimal routing. This lab will be similar to the previous Lab 5-4 but the hub router will use Inverse ARP instead of static frame-relay map statements.
Configuring Frame Relay Inverse ARP is actually quite simple. It is on by default on all Frame Relay multipoint interfaces.
In this lab you’ll familiarize yourself with the following commands;
| Command | Description |
|---|---|
| no frame-relay inverse-arp | This command under Serial interface configuration mode will disable frame-relay inverse arp. Doing so will require static frame-relay map statements. |
| clear frame-relay inarp | This command when executed from privileged mode will clear the dynamically learned Inverse ARP mappings. |
Lab Prerequisites
- If you are using GNS3 than load the Free CCNA Workbook GNS3 topology than start devices; R1, R2 and R3.
- Establish a console session with devices R1, R2 and R3 than configure the devices respected hostname(s).
- Configure R1’s Serial0/0 interface with the IP address of 10.55.123.1/29 and use Frame Relay encapsulation.
- Configure R2’s Serial0/0.221 as a point-to-point sub-interface with the IP Address of 10.55.123.2/29 and the Frame Relay interface DLCI of 221
- Configure R3’s Serial0/0.321 as a point-to-point sub-interface with the IP Address of 10.55.123.3/29 and the Frame Relay interface DLCI of 321
Lab Objectives
- After you’ve completed the Lab Prerequisites, view the current frame-relay map table on R1 to verify is R2 and R3’s IP to DLCI Mappings have been dynamically learned.
- Once R1 has learned R2 and R3’s frame-relay map’s dynamically via inverse arp. Ping both R2 and R3 from R1 then verify that R2 can ping the other poke R3.
- Disable Frame Relay Inverse ARP on R1’s Serial0/0 interface and clear the frame relay inverse arp table using the clear frame-relay inarp command then verify IP connectivity by pinging R2 and R3.
- Create a static Frame Relay MAP on R1’s Serial0/0 interface for IP to DLCI Mappings for traffic destined to R2 and R3. Verify connectivity by pinging R2 and R3 from R1.
Lab Instruction
Objective 1. – After you’ve completed the Lab Prerequisites, view the current frame-relay map table on R1 to verify is R2 and R3’s IP to DLCI Mappings have been dynamically learned.
R1#show frame-relay map
Serial0/0 (up): ip 10.55.123.2 dlci 122(0x7A,0x1CA0), dynamic,
broadcast,, status defined, active
Serial0/0 (up): ip 10.55.123.3 dlci 123(0x7B,0x1CB0), dynamic,
broadcast,, status defined, active
R1#
Objective 2. – Once R1 has learned R2 and R3’s frame-relay map’s dynamically via Inverse ARP. Ping both R2 and R3 from R1 then verify that R2 can ping the other poke R3.
R1#ping 10.55.123.2 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 10.55.123.2, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 72/95/124 ms R1#ping 10.55.123.3 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 10.55.123.3, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 76/105/172 ms R1#
R2#ping 10.55.123.3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.55.123.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 156/215/340 ms
R2#
Objective 3. – Disable Frame Relay Invese ARP on R1’s Serial0/0 interface and clear the frame relay inverse arp table using the clear frame-relay inarp command then verify IP connectivity by pinging R2 and R3.
R1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. R1(config)#interface Serial0/0 R1(config-if)#no frame-relay inverse-arp R1(config-if)#end R1# %SYS-5-CONFIG_I: Configured from console by console R1#clear frame-relay inarp R1#ping 10.55.123.2 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 10.55.123.2, timeout is 2 seconds: ..... Success rate is 0 percent (0/5) R1#ping 10.55.123.3 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 10.55.123.3, timeout is 2 seconds: ..... Success rate is 0 percent (0/5) R1#
Objective 4. – Create a static Frame Relay MAP on R1’s Serial0/0 interface for IP to DLCI Mappings for traffic destined to R2 and R3. Verify connectivity by pinging R2 and R3 from R1.
R1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. R1(config)#interface Serial0/0 R1(config-if)#frame-relay map ip 10.55.123.2 122 broadcast R1(config-if)#frame-relay map ip 10.55.123.3 123 broadcast R1(config-if)#end R1# %SYS-5-CONFIG_I: Configured from console by console R1#ping 10.55.123.2 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 10.55.123.2, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 36/106/200 ms R1#ping 10.55.123.3 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 10.55.123.3, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 76/120/256 ms R1#
