What is Difference

[IrfanKhan4904]

what is the difference and which is best for fast network?

interface GigabitEthernet0/25
switchport trunk encapsulation dot1q
switchport mode trunk

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interface GigabitEthernet0/25

switchport mode access

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interface GigabitEthernet0/25

[Martin L · Accepted solution]

well, mode trunk is telling switch port to be a trunking port while mode access is access port for particular vlan x (switchport access vlan x command is missing). by default, access port is access port for vlan 1.  so, your 2nd example with missing vlan x tell switch to use port as access for vlan 1. 

missing any command - your 3rd example- usually tell switch to be an access port for vlan 1 if you connect end device like a PC or a trunk port if you connect another switch.  this is sort of hidden auto connect command as Cisco still uses switches to became L2 or L3 automatically. note that this "auto port operation" could depend on switch model.  The idea is to connect PCs into switch and they can communicate  without any configs on switch itself.

Regards, ML
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[balaji.bandi · Accepted solution]

nterface GigabitEthernet0/25
switchport trunk encapsulation dot1q
switchport mode trunk                               <--- this means port in trunk mode, this will be used most case to connect to any other device which required to pass all the vlan from 1-4XXX

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interface GigabitEthernet0/25

switchport mode access   <-- this means this port act as accessport -you can connect to any end device - no VLAN mentioned means take as vlan 1 as default.

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interface GigabitEthernet0/25    < - no configuration on the port means by default belong to VLAN 1 - factory default status

[Joseph W. Doherty · Accepted solution]

As the other posters have already noted, the difference is whether the port will be a trunk port, which supports tagged frames, or an access port, which does not support tagged frames.

As to the "fast" issue, one could argue that the untagged access port, not having to ever transmit the additional VLAN tagged header bits, would be "faster", although, at least on Cisco trunk ports, they also support a "native" VLAN, i.e. those frames are also not tagged.

[Richard Burts]

The original post asks 2 questions. We have had several good responses that address what is the difference. I would join @Joseph W. Doherty in addressing which is best for fast network?. The question asks about a Gig  Ethernet interface and I would say that the speed of the interface is the most important thing for fast network. Joseph makes an interesting point about the fact that trunk ports would transmit some frames with vlan tags, which makes the frame very very slightly larger. If you had exceptionally sensitive measuring equipment you might detect the difference in transmission of tagged frames vs untagged frames. But from a practical perspective I wold say that there is not any noticeable difference between access port and trunk port for the speed of the network.

[Joseph W. Doherty]

"But from a practical perspective I wold say that there is not any noticeable difference between access port and trunk port for the speed of the network."

For the most part, I agree with Rick that normally there would be no noticeable difference.  However, do keep in mind the impact would vary based on frame size.  The impact would be most significant for minimum sized frames and least significant for maximum size frames.

BTW, this is somewhat like discussing the impact of using standard Ethernet vs. jumbo Ethernet.  The former has more "overhead" for its MTU sized frames than the latter.

Also, another point about speed, whether Ethernet is 10 Mbps or 100 Gbps, electrical or light "speed" across the medium doesn't vary based on the "speed" of the interface.  What does vary is how quickly bits can be transmitted.  I.e. faster ports take less time to transmit the same amount of data.

The last point is often why upgrading the "speed" of a WAN link doesn't have the same benefit as upgrading the "speed" LAN link, if the application deals with to and from data transmissions.  For example, this effect might be noticed with TCP.