Frame-relay map question

I've been having troubles understanding this command and the frame-relay interface-dlci commands, but I think I've finally figured them out.

My question is -- when I was googling to try finding directions on setting up the frame-relay switch for labbing, the directions I ended up following were Chris Bryant's. One of his commands turns off inverse arp (necessitating the use of the frame-relay map command on the routers, if my understanding is correct), and his comments (on his youtube video at YouTube - CCNA CCNP Frame Relay Switch Config 2/2 (it's at 3:23) say he does that command 'out of habit'. I'm curious if there's any reasoning behind his preferring to manually map the DLCIs? Odom's book said in a production environment you'd probably just let inverse arp take care of the mapping.

[EDIT]
ok; now that I'm labbing I find I don't understand the 2 quite as well as I thought.

When not using subinterfaces (meaning you have a full-mesh with common subnet) you can just map the DLCI's to the destination IP.

When using point-to-point subinterfaces you use the frame-relay interface-dlci, since there's only 1 router on the other end of the subinterface you don't need to map the remote IP address to the appropriate DLCI. I'm guessing the FR switch sees the frame coming in on DLCI 105 and throws it out the appropriate serial port based on it's settings (although this leads me to question the necessity of the frame-relay map command, if the FR switch can route correctly based on the DLCI alone).

When using multipoint subinterfaces, Odom's book uses the frame-relay interface-dlci command also, but I'm wondering why you couldn't use the frame-relay map command there, since they seem to be acting more like the first (non-subinterface) example - with multiple "routes" out that (sub)interface. Is it only for consistency? Physical interfaces use map while sub interfaces use interface-dlci?

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Comments

bermovick

OK sorry; I know replying to my own post is a bit of a faux pas, but I'm trying to find my answers through labs and they're only messing me up worse. I'm testing using both frame-relay map and frame-relay interface-dlci commands on various (sub)interface types to try getting better clarification.

This is a partial mesh with some full mesh. R9 connects to R2, R3, subnet R4-R6 and subnet R7-R8

Case 1: Router 5, attempting to use interface-dlci rather than map (fails). This didn't tell me anything except that apparently interface-dlci doesn't work for the physical interface.

R5(config-if)#do sh run
(snip)
interface Serial0/0
ip address 192.168.5.5 255.255.255.0
encapsulation frame-relay
no fair-queue
clock rate 2000000
frame-relay map ip 192.168.5.1 109 broadcast
frame-relay interface-dlci 109
no frame-relay inverse-arp
(snip)
R5(config-if)#no frame-relay map ip 192.168.5.1 109 broadcast
R5(config-if)#do ping 192.168.5.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.5.1, timeout is 2 seconds:
.....
R5(config-if)#no frame-relay interface-dlci 109
R5(config-if)#frame-relay map ip 192.168.5.1 109 broadcast
R5(config-if)#do ping 192.168.5.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.5.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 12/26/40 ms

Case 2: Using interface-dlci on a multipoint subinterface. This apparently is supposed to work, but for some reason doesn't. (Odom's book uses interface-dlci on all subinterfaces in his examples).

R9(config)#do sh run
(snip)
interface Serial0/0.4 multipoint
ip address 192.168.4.1 255.255.255.0
frame-relay interface-dlci 107
frame-relay interface-dlci 108
no frame-relay inverse-arp
(snip)
R9(config)#do ping 192.168.4.7

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.4.7, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
R9(config)#int s0/0.4
R9(config-subif)#frame-relay map ip 192.168.4.7 107 broadcast
R9(config-subif)#do ping 192.168.4.7

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.4.7, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/16/40 ms
R9(config-subif)#

What in the world am I doing wrong???

[EDIT] Wow; this is like my own personal thread; again I apologize, but I think I may have it! Let me know if this is correct.

frame-relay map is REQUIRED when reverse-arp is disabled; like a static route is needed if no routing protocols are enabled. My interface-dlci's were only not working because I had reverse arp disabled everywhere. So no reverse arp, map is needed.

If reverse arp is on (default), nothing needs done on physical interfaces, since a physical interface would only have a single DLCI; there's nothing further that needs done.

With subinterfaces - again, without reverse arp, you HAVE to use frame-relay map. BUT if reverse arp is enabled, you can use either frame-relay map which creates a 'route' as well as assigning that DLCI to that subinterface (I would guess this overrides the reverse arp, although the 2 should be the same), or frame-relay interface-dlci which only assigns the dlci to the subinterface, letting Rarp take care of the mapping.

[EDIT2]: Bad wording 2 paragraphs up. The physical interface can have more than 1 DLCI, but since there's only a single interface, you don't need to use interface-dlci since all dlci's are going to the 1 interface.

wbosher

Some one may need to correct me here, but I think that you have to use frame map statements on multi-point interfaces.

The interface-dlci commands can only be used on point-to-point connections, because you don't need to specify the destination IP address on a p-to-p link.

typesh

Hey bermovick,

Have a look at Page 500. The first paragraph under the title "Assigning a DLCI to a Particular Subinterface." This might help distinguish between the two commands.

It says that using the frame-relay map command is an alternative to using frame-relay interface-dlci dlci number

It looks like the examples used both methods during the configuration.

bermovick

See, that's what I thought, which was why I was going to try using both for this lab (notice I have 2 point-to-point and 2 multilinks on R9). I was getting confused as to why the interface-dlci wasn't working when I thought it should. I think I figured it out though. Notice all of Odom's examples (except 1) allow Inverse Arp, so either would work.

I double-checked all my running-configs because I was CERTAIN interface-dlci worked on one of the routers and found it:

interface Serial0/0
no ip address
encapsulation frame-relay
clock rate 2000000
no frame-relay inverse-arp
!
interface Serial0/0.2 point-to-point
ip address 192.168.2.1 255.255.255.0
frame-relay interface-dlci 102
!

I had been curious when entering the commands if the subinterface inherited the no frame-relay inverse-arp command from the physical interface (like it does the encapsulation frame-relay). I notice I did NOT enter it in the subinterface commands on this router, and a quick google search verifies they do NOT - Frame Relay Subinterfaces Do Not Inherit Inverse-ARP Configuration CCIE Pursuit Blog