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no arp frame-relay vs no frame-relay inverse-arp

Hi, does anyone know the difference between

no arp frame-relay

and

no frame-relay inverse arp?

Thanks,

Lisa G

4 REPLIES

Re: no arp frame-relay vs no frame-relay inverse-arp

Hi Lisa,

Yes, the difference is fairly simple, but has interesting consequences.

no frame-relay inverse arp says that I shall not ask the other end what his IP address is.

no arp frame-relay means that if the other end asks me my IP address, I shall not answer him.

In a CCIE practice lab, they usually do no frame-relay inverse-arp, but the other on can be useful too.

Kevin Dorrell

Luxembourg

Re: no arp frame-relay vs no frame-relay inverse-arp

no frame-relay inverse would also disable sending unsolicited announcement of router's own mapping, but nevertheless there's still remains one issue against neither of those commands protects - router will still happily accept unsolicited announcement from remote side and use them.

I've just checked this in the lab:

*Oct 23 14:15:51.181: Serial6/0:0(i): dlci 205(0x30D1), pkt encaps 0x0300 0x8000 0x0000 0x806 (ARP), datagramsize 34

*Oct 23 14:15:51.181: Serial6/0:0.1: frame relay INARP received

*Oct 23 14:15:51.181: FR-ADJ: dlci 205: 192.168.100.2: adding adjacency

*Oct 23 14:15:51.181: FR: Sending INARP Reply on interface Serial6/0:0.1 dlci 205 for link 7(IP)

while interface config has both commands:

Current configuration : 275 bytes

!

interface Serial6/0:0.1 multipoint

ip address 192.168.100.1 255.255.255.224

ip ospf network broadcast

no arp frame-relay

no frame-relay inverse-arp

end

And here is the mapping:

Serial6/0:0.1 (up): ip 192.168.100.2 dlci 205(0xCD,0x30D0), dynamic,

broadcast,

CISCO, status defined, active

Also looks like router was provoked to send InARP reply even if 'no arp fr' configured.

Did I miss something?

Hall of Fame Super Bronze

Re: no arp frame-relay vs no frame-relay inverse-arp

It sounds like you configured that interface while active. When changing to no frame-relay inverse-arp, you need to shutdown the interface, then make the change.

Based on your frame map, your router is learning its mapping dynamically.

Re: no arp frame-relay vs no frame-relay inverse-arp

Indeed, the mapping is dynamic and that's what I wanted to show - that 'no frame-relay inv' and 'no arp fr' do not protect router from accepting dynamic mapping.

I've shut the interface, I tried to reboot the router. As long as the other side doesn't send anything, the router doesn't create dynamic mapping. But as soon as the router receives uncolicited InARP reply (note that it's not INARP request, but InARP reply) from the other side, the router creates dynamic mapping and send InARP reply.

It might be that commands were intended to block InARP completely, but during past 12 years I haven't seen any IOS release where it would have been working. Since workaround is rather simple, there's hardly any chance that such behaviour will be changed.

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