Native VLAN and CoS

[jpl861]

Hi there,

I ran through this paragraph in CCIE v5 book and was wondering can someone help explain this? I don't get the underlined statement below. Isn't it that native VLAN frames do not have CoS values? Let's say between the switch and the router, the native VLAN is 20. If the switch sends frames into VLAN 20, it will be untagged and no CoS value will be present. Why is it saying that when a router port is configured with dot1q x native command, it allows the router to recognized both untagged and tagged frames to be on native VLAN. First time I encountered this and it's confusing me.

If the router supports native VLAN configuration on a subinterface, it is recommended to
use subinterfaces instead of putting the native VLAN configuration on a physical port.
Aside from keeping the configuration more consistent (all configuration being placed
on subinterfaces), this configuration allows the router to correctly process frames that,
despite being originated in the native VLAN, carry an 802.1Q tag. Tagging such frames
is done when using the CoS field inside an 802.1Q tag. If the native VLAN configuration
was done on a physical interface, the router would not be able to recognize that a frame
carrying an 802.1Q tag with a nonzero VLAN ID is really a CoS-marked frame in the
native VLAN. When using subinterfaces, the encapsulation dot1q vlan-id native command
allows the router to recognize that both untagged frames and CoS-marked frames
tagged with the particular vlan-id should be processed as frames in the native VLAN.

Thanks!

John

[Joseph W. Doherty]

L2 CoS fields are only found in VLAN frame headers. No VLAN tag, no frame COS. As Cisco native VLANs are untagged, they cannot have L2 CoS.

[jpl861]

That's what I know as well and I have never seen a switch port with default native VLAN (1) and a router's subinterface port on a different native VLAN (let's say 10) worked. I am confused with these statements.

"this configuration allows the router to correctly process frames that,
despite being originated in the native VLAN, carry an 802.1Q tag."

"When using subinterfaces, the encapsulation dot1q vlan-id native command
allows the router to recognize that both untagged frames and CoS-marked frames
tagged with the particular vlan-id should be processed as frames in the native VLAN."

[Joseph W. Doherty · Accepted solution]

Ah, that's because Cisco is a bit different from other vendors. Often on Brand X, a VLAN tagged port tags all frames, none are expected be untagged, in or out.

Cisco, though supports a "native" VLAN, one whose frames are not tagged. Further, Cisco will accept BOTH untagged and tagged frames for the same VLAN.

For example, if on a Cisco port you've identified VLAN 5 as native, frames sent out the port will be untagged. Frames received by the port may be either untagged or VLAN tagged where the VLAN ID is 5.

[jpl861]

Looks like this explains it. I have tried on lab wherein the the switch's native VLAN is 1 so 10 will be tagged. I intentionally did this to make sure that traffic sent out to VLAN10 will still be tagged. And then on router side, the subtinterface's native VLAN is 10.

When I did ping test from my other router (i had to hardcode ARP first), I can see that my VLAN10 ICMPs are being received on the router with native VLAN of 10 and it is still responding to ICMPs. Although since the switch's native VLAN is 1, the ICMP replies are not being received by the other end.