Question regarding Packet tracer 5

Configuration tested

Network A 10.0.0.0/24
Network B 10.1.5.0/24

Router0
FA0/0 10.1.5.1/24
FA0/1 10.0.0.1/24

Server B-1 10.1.5.10/24 GW 10.1.5.1
Server B-2 10.1.5.11/8 GW 10.1.5.1

Server B-2 should be able to ping server B-1 and the default Gateway, however it should not be able to communicate with devices on network A as it would think (rightfully so) that they could be reached without using the gateway.

Router0 will block broadcast traffic originating on network B from reaching network A.

Question: why then does the ping from server B-2 to the devices on network A succeed using packet tracer5? or am I missing something here?

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Comments

aordal

Well first of all, a ping isn't a broadcast. And the server is simply sending a packet to the defualt GW with a Destination IP address of a host on Network A. The router looks at its routing table, see's that it has a route and forwards the packet out its other interface and voila. Successful.

Now if you had a switch on your Network B, it wont have MAC addresses in its table for hosts on Network A, because a host does send out broadcasts to learn MAC addresses. Also your server's arp table won't have a MAC for any hosts on Network A. But you can still ping it, that's what routers do.

Highspade

Yes, but...

Since host B-2 has an incorrect subnet mask it would think that network A resides on its local network and never attempt to send a request to the default GW, or am I wrong on that?

nevolved

Aordal that is not quite right. When a host goes to send a packet it first checks if it is on the same subnet as the receiving host (by comparing it's subnet mask and network ID to determine the hosts that are withing that subnet...if the receiving IP address is within that range then it will NOT send it to a default gateway). The only time a default gateway is used is if the receiving IP address is on another subnet.

Packettracer is just a simulation, if something doesn't work as intended then that is to be expected. It would be easier to determine the answer to your original question with a network diagram though, to see if it is a problem with PT5.

The router would never receive the packet, and even if it did the directly connected route would have a metric of 0 which would keep it on the local subnet.

aordal

Yeah i didnt see that /8. Dirty wrong subnet.

BennyLava

I'm guessing that you have proxy ARP enabled on Router0's FA0/0 interface. If Router0 receives an ARP from Server B-2 for a host on network A with proxy ARP enabled, it will reply with it's own FA0/0 MAC. Try disabling proxy ARP on FA0/0 with 'no ip proxy-arp' and it should behave like you initially expected.

tech-airman

Highspade,

Highspade wrote: »

Configuration tested

Network A 10.0.0.0/24
Network B 10.1.5.0/24

Router0
FA0/0 10.1.5.1/24
FA0/1 10.0.0.1/24

Server B-1 10.1.5.10/24 GW 10.1.5.1
Server B-2 10.1.5.11/8 GW 10.1.5.1

Server B-2 should be able to ping server B-1 and the default Gateway, however it should not be able to communicate with devices on network A as it would think (rightfully so) that they could be reached without using the gateway.

Why do you think "...(rightfully so)?"

Highspade wrote: »

Router0 will block broadcast traffic originating on network B from reaching network A.

Yes, but if you say "ping 10.0.0.2" from Server B-2, then that traffic is NOT a broadcast but a unicast so the router shouldn't block that traffic.

Highspade wrote: »

Question: why then does the ping from server B-2 to the devices on network A succeed using packet tracer5? or am I missing something here?

Please explain why a "...ping from Server B-2 to [a] device on network A..." should fail?

Highspade

BennyLava wrote: »

I'm guessing that you have proxy ARP enabled on Router0's FA0/0 interface. If Router0 receives an ARP from Server B-2 for a host on network A with proxy ARP enabled, it will reply with it's own FA0/0 MAC. Try disabling proxy ARP on FA0/0 with 'no ip proxy-arp' and it should behave like you initially expected.

I will check that and try the test again.

Highspade

tech-airman wrote: »

Highspade,

Why do you think "...(rightfully so)?"

I explained why I thought so immediately after the comment.

tech-airman wrote: »

Yes, but if you say "ping 10.0.0.2" from Server B-2, then that traffic is NOT a broadcast but a unicast so the router shouldn't block that traffic.

Please explain why a "...ping from Server B-2 to [a] device on network A..." should fail?

I understand that ping is not a broadcast, but if the device that is sending the ping does not have the entry in its ARP table it would broadcast an ARP request once it determined that (at least according to its subnet mask) the device should be on its local network.

I was not aware of the proxy ARP setting so that might be confusing me here.

Neeko

Proxy ARP [IP Addressing Services] - Cisco Systems

Take a look at that link about proxy ARP, it has an example which in essence is basically the same as what you're describing.