Interesting behavior regarding ip unnumbered interface

[from88]

hello,

im having question according ip unnumbered interface, we have a network with ~50 customers

We using multiple subintefaces on the router, every ip address uses its own subinterface.

every subinterface have one /32 static route towards them, and they are unnumbered with loopback interface.

something like this:

interface Fastethernet1/0.154

!

vlan id 154

ip unnumbered loopback 0

ip route 54.254.15.13/32 interface Fastethernet1/0.154

so every client has an IP which is configured like this and everything is working pretty fine.

The interesting thing is that the PC, which uses 54.254.15.13 (ip address) can set any subnet mask and any gateway IP address (for example 8.8.8.8 or anything obsolete) and it still works. Can someone help me try to understand this behavior ? Is everything is working with bad gateway or (and) subnet mask because of unnumbered interface ?

[amigomnemonik]

It is because of the /32 mask. As this contains only one host on this segment, no default gateway and only single IP address exist.

[Peter Paluch · Accepted solution]

Kamil,

It is because of the /32 mask. As this contains only one host on this  segment, no default gateway and only single IP address exist.

Regarding the netmask, you're right. But it does not explain why using just about any IP address as the gateway work - how does the PC determine which MAC address should be used as the destination MAC when sending packets through the router?

I suspect this is caused by the router running ProxyARP. If the PC asks about a IP-to-MAC mapping, even if the router does not own the IP address on the interface where the ARP request arrives, it will respond with its own MAC address. This will allow the station to send IP packets via the router.

This assumption is easily verified: on the subinterface, enter the no ip proxy-arp command and on the PC, flush the ARP cache (for example, arp -d * or deactivate and reactivate the interface). If the network access becomes unavailable after setting an inappropriate IP gateway, this "curiosity" was indeed caused by ProxyARP.

Best regards,

Peter

[amigomnemonik]

Hi Peter,

I like your explanation.

This is what Cisco says:

"The main advantage of proxy ARP is that it can be added to a single       router on a network and does not disturb the routing tables of the other       routers on the network.

Proxy ARP must be used on the network where IP hosts are not configured       with a default gateway or do not have any routing intelligence."

Thanks.

Regards,

[from88]

thanks for input, yes router by default is running proxy arp on subinterface, tommorow i will try to disable it and tell you the reesults, but for clarity, if the proxy arp is disabled, which gateway i should be using in these conditions ?

[amigomnemonik]

Then I would think of the minimum subnet mask of /30 for two IPs: one for host, another one for default gateway.

[from88]

Yeah, u are right. I set the default gateway for example 8.8.8.8, with wireshark on. Then Proxy arp (by default) is on,my PC sends arp request to 8.8.8.8 and he gets reply.

When i disablethe proxy arp, the PC requests the ARP for 8.8.8.8 and the router is not replying. Thanks. Still a little bit confused with Proxy arp, for example 8.8.8.8 is not local subnet to my router, why he is sending arp replies...