Half duplex and packet size

[Kanan Huseynli]

Hi there,

recently in one of the branches of our customer I have detected one link (from dmz switch to firewall) operates in half-duplex (thanks to autonegotiation in FA port). Actually, port of dmz switch works in half duplex, while firewall port is full.

Site topology is as below:

LAN -- CORE SW -- FIREWALL (duplex full) -- (duplex half) DMZ SW -- BORDER ROUTER -- WAN.

Note: core/ dmz switch and border router is Cisco, firewall is checkpoint

While investigating initial issue (which is resulted by half duplex), I have seen that when I ping from border router to core switch 95-100% success, but if I ping with higher packet size (1200-1400) I see more losses (20-30%).

Could anyone is explain what is relation of half duplex and packet size?

HTH,

[David Ruess]

I would attribute it to how half-duplex works in the sense that only one host can talk at a time. Full duplex both ends can talk at the same time since they use separate pairs of wires in the cable.

That being said, since the neighbor has to wait for the local device to stop sending in order to reply (in your case a ping) the bigger the packet size the longer the wait for the packet to finish transmitting so the replys will be delayed and intermittent.

You can configure full duplex on most ports:

interface <intx/x>

duplex full

Make sure this also does not break any functionality of the devices communicating.

Hope that sheds some light on it

-David

[Joseph W. Doherty]

"Could anyone is explain what is relation of half duplex and packet size?"

Larger packet, more time on wire, more time for the mismatched (i.e. one side full other half) interfaces, to have an issue processing packet.

As full duplex <> half duplex is an invalid combination, actually what I find surprising, is that any traffic gets across.  (Usually it does, but at a very slow rate due to all the errors.)

BTW if one interface is hard coded as full duplex, and other interface is hard coded as "auto" the latter will drop into half duplex mode.

Valid combinations are: auto<>auto, full<>full or half<>half. For almost any, if not all, network equipment made in this century, (Ethernet) auto<>auto should work and it's also usually what's recommended by the hardware vendors.