Frame Relay Addressing
Frame Relay Layer 2 addresses are DLCIs. I mentioned previously that the WAN DLCIs are analgous to the LAN MACs. ARP is the method I discussed for mapping IP to MAC in the LAN chapters and RARP is just the opposite of mapping MAC to IP. Frame uses Inverse ARP. Routers learn remote IP addresses to map to local DLCIs via Inverse ARP or static map statements as illustrated in Figure 8-9. Use the picture and steps covered in the next section to assist you with troubleshooting Frame Relay Inverse ARP issues.
Inverse ARP
As you have witnessed in the debug lmi commands, about once a minute or every six keepalives a router requests a full status response from a Frame Relay network with active DLCIs. This is the signaling or LMI between the local router and the local Frame Relay switch. The router then sends an Inverse ARP out on each PVC. The remote routers respond with their respective IP addresses. The original router then maps the IP addresses to the right DLCIs. Understanding this is critical to anyone troubleshooting Frame Relay networks. Follow along with the steps in Figure 8-9:
1. | After the Physical Layer is up, the router sends a status inquiry to the local frame switch. The default keepalive activity between the router and the switch is occurring every 10 seconds. Every sixth poll (60 seconds), the router requests a full status. |
2. | The local frame switch replies with a status update message to the router every 10 seconds with a full update including DLCI information every 60 seconds, specifically, every sixth LMI inquiry. |
3. | Now that the local router knows about a PVC through an active DLCI, it can send its IP address to the other end. This is an Inverse ARP packet. |
4. | The remote router knows which local DLCI it received the address on and sets up a corresponding map. It is important to note that the local router uses its own DLCI, which may not be the same as the remote router is using. |
5. | Keepalive (LMI) activity continues to occur by default every 10 seconds between the router and the local frame switch with a full status message every 60 seconds. This dynamic nature ensures that any changes are accounted for as they occur. |
NOTE
Decrease your keepalive activity by one until they stabilize if you are having trouble with flapping links.
See the previous examples for show frame-relay map output, but Example 8-19 and 8-20 display a couple of debug commands that can assist you capture the Inverse ARP activity.
Example 8-19. Inverse ARP and debug frame-relay events
r1#debug frame-relay events Frame Relay events debugging is on r1#configure terminal r1(config)#interface s1 r1(config-if)#shut r1(config-if)#no shut 01:48:12: %LINK-3-UPDOWN: Interface Serial1, changed state to up 01:48:13: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed state to up 01:48:22: %FR-5-DLCICHANGE: Interface Serial1 - DLCI 104 state changed to ACTIVE 01:48:23: Serial1: FR ARP input 01:48:23: datagramstart = 0xE3B384, datagramsize = 30 01:48:23: FR encap = 0x18810300 01:48:23: 80 00 00 00 08 06 00 0F 08 00 02 04 00 08 00 00 01:48:23: C0 A8 05 06 18 C1 00 00 00 00 01:48:23: r1(config-if)#end r1#ping 192.168.5.6 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 192.168.5.6, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 56/58/60 ms r1#undebug all |
Example 8-19 shows the Inverse ARP activity when you bring up an interface. Although not recommended in a practical environment, compare the previous events received to the debug frame-relay packet output in Example 8-20.
Example 8-20. Inverse ARP and debug frame packet
r1#debug frame-relay packet Frame Relay packet debugging is on r1#configure terminal r1(config)#interface s1 r1(config-if)#shut r1(config-if)#no shut 01:49:46: %LINK-3-UPDOWN: Interface Serial1, changed state to up 01:49:47: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed state to up 01:49:56: %FR-5-DLCICHANGE: Interface Serial1 - DLCI 104 state changed to ACTIVE 01:49:56: Serial1(o): dlci 104(0x1881), pkt encaps 0x0300 0x8000 0x0000 0x806 (ARP), datagramsize 30 01:49:56: FR: Sending INARP Request on interface Serial1 dlci 104 for link 7(IP) 01:49:56: broadcast dequeue 01:49:56: Serial1(o):Pkt sent on dlci 104(0x1881), pkt encaps 0x300 0x8000 0x0 0x806 (ARP), datagramsize 30 01:50:03: Serial1(i): dlci 104(0x1881), pkt encaps 0x0300 0x8000 0x0000 0x806 (ARP), datagramsize 30 01:50:03: Serial1: frame relay INARP received 01:50:03: FR: Sending INARP Reply on interface Serial1 dlci 104 for link 7(IP) r1(config-if)#end r1#ping 192.168.5.6 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 192.168.5.6, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 64/64/64 ms r1# 01:50:37: Serial1(o): dlci 104(0x1881), pkt type 0x800(IP), datagramsize 104 01:50:37: Serial1(i): dlci 104(0x1881), pkt type 0x800, datagramsize 104 01:50:37: Serial1(o): dlci 104(0x1881), pkt type 0x800(IP), datagramsize 104 ... 01:50:38: Serial1(i): dlci 104(0x1881), pkt type 0x800, datagramsize 104 r1#undebug all |
The debug frame packet command is excellent to watch the Inverse ARP activity for understanding. On the other hand, like any debug packet command, it is not too forgiving in the production environment.
As you can see, commands such as show interfaces, show frame-relay lmi, show frame-relay pvc, show frame-relay map, show frame-relay route, debug frame-relay lmi, debug frame-relay events, and debug frame packet are quite beneficial in troubleshooting Frame Relay. How many times have you made a typo on an IP address or DLCI assignment? Mistakes certainly stand out with these commands, especially if you quickly compare them to show ip interface brief. So far on the end-user side of the frame you have been using Inverse ARP for the Layer 2-to-Layer 3 mapping. Now I want to turn your attention to using static maps.
Static Map Statements
Static map statements in Frame Relay disable Inverse ARP. You can think of this like your routing protocols having a higher administrative distance than your static routes and the latter taking precedence. Actually, routing protocols are a good topic to discuss with Frame Relay and I cover them in the “Shooting Trouble with Frame Relay” section.
NOTE
Frame Relay is an NBMA technology, so remember to use the broadcast keyword with static maps so that the Layer 3 routing updates get forwarded. It allows broadcasts and multicasts over the PVC and in effect turns the broadcast into a unicast to send it out so that the other side gets the routing updates.
With Inverse ARP, the show frame-relay map command displays not only dynamic for the method of learning about the map, but also broadcast. However, this is not the default when you define your own static map statements.
Now I want you to turn off Inverse ARP and use static mappings for your hub-and-spoke topology from r2 to r1 and r3 as in Example 8-21.
Example 8-21. Static Map Statements
r1(config)#interface s1 r1(config-if)#frame map ip 192.168.5.6 ? <16-1007> DLCI r1(config-if)#frame map ip 192.168.5.6 104 ? broadcast Broadcasts should be forwarded to this address cisco Use CISCO Encapsulation compress Enable TCP/IP and RTP/IP header compression ietf Use RFC1490/RFC2427 Encapsulation nocompress Do not compress TCP/IP headers payload-compression Use payload compression rtp RTP header compression parameters tcp TCP header compression parameters <cr> r1(config-if)#frame map ip 192.168.5.6 104 broadcast r1(config-if)#end r1#copy running-config startup-config r3(config)#interface s0/0 r3(config-if)#frame map ip 192.168.5.5 108 broadcast r3(config-if)#end r3#copy running-config startup-config |
Configuring the static map statements automatically turned off Inverse ARP as Example 8-22 illustrates. Verify that r3 can ping the address mapped to DLCI 108.
Example 8-22. Viewing the Static Map Configurations
r1#show frame-relay map Serial1 (up): ip 192.168.5.6 dlci 104(0x68,0x1880), static, broadcast,CISCO, status defined, active r3#show frame-relay map Serial0/0 (up): ip 192.168.5.5 dlci 108(0x6C,0x18C0), static, broadcast,CISCO, status defined, active r3#ping 192.168.5.5 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 192.168.5.5, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 56/58/60 ms |
NOTE
Inverse ARP is enabled by default, but turned off automatically when you put in a static map statement. To select Inverse ARP for a particular protocol, use the following interface configuration command: frame-relay inverse-arp protocol dlci#. This is not necessary on a point-to-point subinterface.
Other options that are available while configuring static map statements include compression, and from a support standpoint what you are doing on one end better match what you are doing on the other. An example of the command is frame-relay map ip 192.168.5.6 104 broadcast payload-compress packet-by-packet. Feel free to experiment with compression with Frame Relay, but remember to remove it from your configurations before you continue on with Frame Relay at the Physical Layer.