Trunk Mode Between 3925 and Internal Switch Module

[jaysoo]

I am trying to set up a trunk between a 3925 ISR and a 16-port ESM in one of its slots, basically a 16-port 3560 switch. As far as I understand it there are two internal gig port connections between the two devices. On the router they are G1/0 and G1/1, which in turn connect to G0/18 and G0/17  respectively on the switch. G1/0 has an IP address and I use it to session into the switch, and I can pass layer 3 traffic on that link if I set it up to.

I set g1/1 on the router as a trunk port with the switchport trunk mode command, and did the same on port G0/17 on the switch side, like so:

interface GigabitEthernet0/17

switchport trunk encapsulation dot1q

  switchport mode trunk

If I do a "show interface g0/17 trunk" I get this:

Port        Mode             Encapsulation  Status        Native vlan
Gi0/17      on               802.1q         trunking      1

Port        Vlans allowed on trunk
Gi0/17      1-4094

Port        Vlans allowed and active in management domain
Gi0/17      1,183,249

Port        Vlans in spanning tree forwarding state and not pruned
Gi0/17      1,183,249

That looks ok to me, but when I do the same command on the router for port G1/1 I get this:

Port      Mode         Encapsulation  Status        Native vlan
Gi1/1     on           802.1q         trunking      1

Port      Vlans allowed on trunk
Gi1/1     1-4094

Port      Vlans allowed and active in management domain
Gi1/1     1

Port      Vlans in spanning tree forwarding state and not pruned
Gi1/1     1

I would expect to see the same VLANs listed there. If I turn on spanning-tree debugging on the router I can see that there is trunk communication on G1/1 and there are references to the VLANs on the switch.

Does anyone have any suggestions on how I can get this to work? Thanks.

[Peter Paluch · Accepted solution]

Hello,

While I have not yet personally worked with 3925 and these ESMs, my personal take is that if these two devices behave like two Catalyst switches on their mutual interconnection then what you see is normal and consistent with how two true Catalyst switches would behave. Both the 3925 and the ESM module are autonomous devices, each of them running their own separate and independent IOS image. Just like there is no protocol or mechanism between two common Catalysts that would mediate the list of allowed VLANs on a trunk, you do not see this change reflected between your Gi1/1 on the 3925 and Gi0/17 on the ESM. The ESM only lives inside the 3925's housing but is not otherwise being controlled by it, I would presume.

I just wonder - are you sure the ESM's Gi0/17 and Gi0/18 connect to router's Gi1/1 and Gi1/0 in this exact order? I am not doubting they do, just the ordering is weird.

Best regards,

Peter

[jaysoo]

Hi,

Thanks for your reply.

You are correct, it is normal behaviour. I was expecting something different, but that was because I wasn't thinking. Once I created my VLANs everywhere as needed everything worked fine (duh). I was somewhat put off by TAC telling me not to configure that connection as a trunk and went off on a tangent.

I agree the port line up is weird, but that's how it is as you can see here:

yourname#sh cdp ne
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
                  S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone,
                  D - Remote, C - CVTA, M - Two-port Mac Relay

Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID
Switch           Gig 1/1            142             S I   SM-ES3G-1 Gig 0/17
Switch           Gig 1/0            165             S I   SM-ES3G-1 Gig 0/18

Thanks again.

[Peter Paluch]

Hello,

I thank you as well! I didn't know the internal connections in 3925 are so... messy Anyway, why did the TAC engineer tell you not to configure the interconnection as a trunk, anyway?

Best regards,

Peter

[jaysoo]

Hi again,

I don't think the tech was familiar with the product. He kept putting me on hold for long periods of time so he could read about it.

Take care.

[paul driver]

Hello

Curious as to the vtp.status of these switches

res
paul


Sent from Cisco Technical Support Android App

[jaysoo]

Hi there,

Here you go:

Router#sh vtp stat
VTP Version                     : 2
Configuration Revision          : 0
Maximum VLANs supported locally : 52
Number of existing VLANs        : 5
VTP Operating Mode              : Server
VTP Domain Name                 :
VTP Pruning Mode                : Disabled
VTP V2 Mode                     : Disabled
VTP Traps Generation            : Disabled
MD5 digest                      : 0xBF 0x86 0x94 0x45 0xFC 0xDF 0xB5 0x70
Configuration last modified by 0.0.0.0 at 0-0-00 00:00:00
Local updater ID is 10.20.20.1 on interface Vl12 (lowest numbered VLAN interface
found)

Switch#sh vtp stat
VTP Version capable             : 1 to 3
VTP version running             : 1
VTP Domain Name                 :
VTP Pruning Mode                : Disabled
VTP Traps Generation            : Disabled
Device ID                       : 7c95.f317.8680
Configuration last modified by 0.0.0.0 at 10-13-13 00:31:16
Local updater ID is 0.0.0.0 (no valid interface found)

Feature VLAN:
--------------
VTP Operating Mode                : Server
Maximum VLANs supported locally   : 1005
Number of existing VLANs          : 6
Configuration Revision            : 1
MD5 digest                        : 0x0C 0xCB 0x29 0x9C 0x13 0x03 0x6A 0xDB
                                    0xAE 0xCE 0xBA 0x7E 0x28 0xF4 0x40 0x15