Would PC1 be able to ping PC2 given the following...

[andrewmorris]

Hi There,

The following is driving me insane whilst preparing for my CCNP switching exam.

Given the following scenario of two Catalyst switches physically connected by their Gigabit ethernet connections:

Catalayst A:

Interface gig0/1

switchport mode access

switchport access vlan 2

Interface fast0/1

switchport mode access

switchport acces vlan 2

Catalayst B:

Interface gig0/1

switchport mode access

switchport access vlan 10

Interface fast0/1

switchport mode access

switchport acces vlan 10

PC1 attaches to Catalyst A switchport fast0/1 and has an IP address of 10.2.2.1

PC2 attaches to Catalyst B switchport fast0/1 and has an IP address of 10.2.2.2

Will PC1 be able to ping PC2 given the config above and without the intervention of a layer 3 router?

As many answers would be hugely appreciated please

Andrew

[a-trinidad]

Hello,

I think there is an error of config on giga-eth interface, I think it should be :

interface gig0/1

switchport trunk allowed vlan 2,10

switchport mode trunk

no ip address

But anyway, as PC1 and PC2 belong to two differents VLANs, they will not be able to communicate with each other without being routed by a router.

[andrewmorris]

Hi There,

The config is correct as given in the book. Both the gig interfaces and fast interfaces are configured to be access links.

I personally agree with you that PC1 should not be able to ping PC2, as you would need either a layer3 device or an external layer2 bridge.

However, the author answers that this is possible because:

1. All links are access-links

2. The gig interfaces are physically connected

therefore the PC's not knowing about the VLANS will be able to communicate with each other. The PC's are within the same subnet, 255.0.0.0

I'm still confused!

[Georg Pauwen]

Hello,

no, with the current configuration, the two machines will not be able to ping each other, since the ports are assigned to different VLANs. Put all the ports in the same VLAN and both PCs will be able to talk to each other.

Regards,

GP

[andrewmorris]

Hi There,

I also think it shouldn't work but according to the author of the CCNP Switching Book, David Hucaby, he insists that it will work giving the explanation I gave above.

Do you reckon he is wrong? He seems to be spot on with everything else.

Regards

Andrew

[dnewell24]

My answer to you is yes, they will be able to ping each other. I will try to explain.

Layer 1

We can easily assume there is good physical layer connectivity between the hosts. They are speaking the same language here.

Layer 2

All devices are using Ethernet, same media access method, and similar frame formats. So there is no need for another device to change frame formats.

Spanning tree will have no problems with this. There is nothing in a BPDU that specifies VLAN info. With the switchport commands specified, there is no frame tagging. So the switches will do the root bridge battle and once decided the GIG links will go into forwarding state.

As a matter of fact, none of the frames are going to be tagged with VLAN info. The switches will send all traffic as normal.

The broadcast domain, for the VLANs, has been combined by physically connecting the interfaces.

Layer 3

The host IP's appear to be in the same subnet.

This should be the sequence of events when PC1 tries to ping PC2.

1. On PC1 a user initiates the ping to PC2.

2. PC1 compares the destination address to his own to determine if it's in his subnet.

3. Since PC2 is in the same subnet, PC1 will check the arp table to see if the MAC address is there.

4. Lets assume the MAC address is not there. PC1 will arp for the MAC address.

5. When the frame hits switch 1, it will notice that this is a broadcast frame and proceed to forward it out all vlan 2 interfaces.

6. One of these interfaces is switch B's VLAN 10. Again since the frame is not tagged it will take the frame and forward it out all VLAN 10 interfaces.

7. PC2 will receive the frame and do its part in the ARP process.

8. The frame will go back to PC1 and the ICMP packets will flow. The switch’s mac address table has the right info. The PC's arp table will have the right info. Everything will flow accordingly.

[a-trinidad]

Hello,

Just to check if I understood well. If we had configured a trunk access between the switches, the frame would be tagged and PC1/PC2 would not be able to ping each other ?

[dnewell24]

Yes you are correct.

[andrewmorris]

Thanks!

You are spot on, I understand that perfectly, many thanks.

Andrew Morris

[terryyau]

we have 2 cisco 3550, all ports belong to vlan 10.

we have a cisco 2950, all ports belong to vlan 1.

I attach cisco 2950 to each cisco 3550 by 2 cross-cables, the server on cisco 2950 can ping to that on cisco 3550 and vice versa.

any comment?

[terryyau]

furthermore, according to cisco 3550 manual about access port:

Access Ports

An access port belongs to and carries the traffic of only one VLAN (unless it is configured as a voice

VLAN port). Traffic is received and sent in native formats with no VLAN tagging. Traffic arriving on

an access port is assumed to belong to the VLAN assigned to the port. If an access port receives a tagged

packet (Inter-Switch Link [ISL] or 802.1Q tagged) for the VLAN assigned to the port, the packet is

forwarded. If the port receives a tagged packet for another VLAN, the packet is dropped, the source

address is not learned, and the frame is counted in the No destination statistic.

[andrewmorris]

Thanks for ALL of your help. This excercise has really cemented in how trunking and access-links operate in addition to making me understand the significance of ISL and 802.1q as tagging protocols.

Thanks again all

Regards

Andrew Morris