Where's the Trunk

[kbullard00]

Greetings

Still kinda confused about trunking.

NyRr1 is setup with 802.1q to allow intervlan routing.

Sw1NY is connected to this router via Fa0/10.

Now on the switch it shows this is a trunking link.

Why does it not give any output or show a trunk when I run this command on the router?

There has to be a trunk link between a router and switch for interVLAN routing because frames going to the router have VLAN tags right?

Thanks

Sw1NY#sh int trunk
Port        Mode         Encapsulation  Status        Native vlan
Fa0/10      on           802.1q         trunking      1
Fa0/12      auto         n-802.1q       trunking      1
Fa0/13      auto         n-802.1q       trunking      1

Port        Vlans allowed on trunk
Fa0/10      1-1005
Fa0/12      1-1005
Fa0/13      1-1005

Port        Vlans allowed and active in management domain
Fa0/10      1,4,8
Fa0/12      1,4,8
Fa0/13      1,4,8

Port        Vlans in spanning tree forwarding state and not pruned
Fa0/10      1,4,8
Fa0/12      1,4,8
Fa0/13      1,4,8

Sw1NY#sh cdp nei
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
                  S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone
Device ID    Local Intrfce   Holdtme    Capability   Platform    Port ID
Sw2NY        Fas 0/11         126            S       2950        Fas 0/10
Sw3NY        Fas 0/12         157            S       2950        Fas 0/12
Sw4NY        Fas 0/13         157            S       2960        Fas 0/12
NYRr1        Fas 0/10         156            R       C2800       Fas 1/0
NYRr1        Fas 0/10         156            R       C2800       Fas 1/0.4
NYRr1        Fas 0/10         156            R       C2800       Fas 1/0.8
Sw1NY#

[cadet alain]

Hi,

show int trunk won't give any output on the router because this command is not a router command.

To verify on the router you must do sh ip int br to see subinterfaces and then sh run int "subinterface" to see dot1q followed by vlan id.

Regards.

[kbullard00]

Hmm interesting.

sh int on the router indicates 802.1Q is in use but there is no mention of trunking whatsoever.

I know there is a VLAN tag on frames going from the switch to the router at least the book says so.

So the router is using both the ip address in the frame and the vlan tag to route the frame?

[Jon Marshall · Accepted solution]

kbullard00 wrote:
Hmm interesting.
sh int  on the router indicates 802.1Q is in use but there is no mention of trunking whatsoever.
I know there is a VLAN tag on frames going from the switch to the router at least the book says so.
So the router is using both the ip address in the frame and the vlan tag to route the frame?

The vlan tag associates the frame to the subinterface on the router. The ip address is extracted after that.

Jon

[kbullard00]

Great info.

There must be some kind of mapping table the router keeps that maps VLANs to IP address ranges.

But I didn't see any info in my book that indicated a command that allowed you to view it.

Interesting since they seem to have a command for just about everything else

[Jon Marshall · Accepted solution]

It doesn't need a mapping table because the subinterface has an IP address and subnet mask associated with it as well as a vlan ID so the router knows all the information from the subinterface.

Jon

[kbullard00]

Excellent! I think I finally have a handle on this.

Too bad the book doesn't offer as much good info Thx.