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Dropped packets in 3Com 5500G switch

Dropped packets in 3Com 5500G switch

Dropped packets in 3Com 5500G switch

I'm having problems with dropped packets.

I have all ports of a 3Com 5500G switch assigned to the VLAN-1. The VLAN-1 interface IP address was but the stations connected to the switch ports have addresses from the or ranges.

When I ping a PC with an IP address within the subnet from another PC within the same subnet, I lose a lot of packets (~50%).

I've added another IP to the VLAN-1 interface and It seems that all is OK (the packet drop rate has decreased)

interface Vlan-interface1
 ip address
 ip address sub

I don't understand why I have to add a new IP address to the VLAN interface (The ping failed between IP addresses from the same subnet (, there is only Layer 2 switching).

Is this working as intended? Or is it a bug?

Thank in advance,

RE: Dropped packets in 3Com 5500G switch

it sounds like you may have a high number of broadcasts going on in the original vlan.

the best way to test whether the switch hardware or the network traffic is at fault would be to create a completely seperate vlan like vlan 2 and give it its own subnet.  then aplly that vlan to a port and see if the time is decreased to ping the new vlan.  if the time is drastically less, you may want to use wireshark on the original vlan to find out whats going on.

RE: Dropped packets in 3Com 5500G switch

You should have the subnet on its own VLAN.
What is the CPU running at?
Are any of the interfaces showing high utilisation?
Do you lose the 50% of packets on a continuous ping, or are you just losing the first 2 pings out of 4 from a bunch of seperate pings?

RE: Dropped packets in 3Com 5500G switch

Thank you very much for your answers.

I lose the packets on a continuous ping (ping -t). The interface and CPU aren't showing high utilization rates.

I know that the best solution is to implement a new VLAN, but I think that one VLAN can support two different subnet addresses.

I've used wireshark and I've seen that the switch 5500G replace the source MAC address (by its own MAC address) from the packets coming from another subnet. It reduces the TTL field from these packets too. It would be a normal behaviour for Layer 3 packets but this switch is neither the default gateway of any PC nor the next hop of any router.

Any ideas?

Best regards.

RE: Dropped packets in 3Com 5500G switch

It's clearly routing between them. Or thinks it is.
I guess when it has no interface in the 2nd subnet, then it treats it as a foreign subnet and prepares to route it, according to its configured default route.
When you put a secondary IP address on the switch VLAN interface, you are telling it that that subnet is local - the switch has no real way of knowing this otherwise.
The other experiment you could do is to play around with static ARP & MAC-address table entries for the "foreign" subnet.
Yes, you *can* put multiple subnets on a single VLAN, but you really shouldn't.

RE: Dropped packets in 3Com 5500G switch

Like Vince said: the hosts think they should route the packet since and are logically different networks.

What do you have the hosts on the subnets set to use as a default gateway?  What is your gateway setup downstream?   

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