What is VLAN 0, and how do I get a workaround for this problem?

[jimmiejoe]

To make a long story short, we have a 6500 L3 switch as our core switch for our corporate LAN. There is a 3550 connected to it by fiber, and a 3750 is connected to the other side of it also by fiber. I can go to the 3550 in the middle and ping/telnet either way, but can't do a complete ping or telnet from either end to the other end.

This afternoon I tried a L2 traceroute back to the gateway of the vlan, and came up with this output --

switch#traceroute mac ip 10.121.1.77 10.121.1.1

Source and Destination arped on different vlans (source on vlan 0 and destination on vlan 1).

Layer2 trace aborted.

Nothing shows up regarding Vlan 0 on the 3550 when I execute 'show vlan'. VTP is set correctly, all vlans are populated, and L3 routing is turned off. Where does VLAN 0 come from, and how do I get the switch address on Vlan 1?

Thanks,

Jim

[rais]

Apparently, one of the port is not tagging your ethernet frames. Do you have access mode enabled on any of these ports?

Thanks.

[Roberto Salazar]

Where are you issuing the L2 traceroute.

Understanding Layer 2 Traceroute

The Layer 2 traceroute feature allows the switch to identify the physical path that a packet takes from a source device to a destination device. Layer 2 traceroute supports only unicast source and destination MAC addresses. It determines the path by using the MAC address tables of the switches in the path. When the switch detects a device in the path that does not support Layer 2 traceroute, the switch continues to send Layer 2 trace queries and lets them time out.

The switch can only identify the path from the source device to the destination device. It cannot identify the path that a packet takes from source host to the source device or from the destination device to the destination host.

Switches Supporting Layer 2 Traceroute

The Layer 2 traceroute feature is available on these switches:

* Catalyst 2950 switches running Release 12.1(12c)EA1 or later

* Catalyst 3550 switches running Release 12.1(12c)EA1 or later

* Catalyst 4000 switches running Catalyst software Release 6.2 or later for the supervisor engine

* Catalyst 5000 switches running Catalyst software Release 6.1 or later for the supervisor engine

* Catalyst 6000 switches running Catalyst software Release 6.1 or later for the supervisor engine

But the above might not be the issue as you said from 3550 you can ping either 3750 or the 6500, the traceroute looks like you did from 3550. What you need to do is make sure that the 3750 and the the vlan for 10.121.1.1 are in the same vlan.

[jimmiejoe]

Rais - To answer your question, the only two commands on the port are "switchport trunk mode dot1q" and "switchport mode trunk". Other than 'no shut', that's all that is on the port

The L2 traceroute is being issued on the 3550 switch. When I tried running the same command on the 3750, I got an immediate balk from it, saying something like 'it couldn't find an arp entry for that IP".

Here's the code running on the 3550-

Cisco Internetwork Operating System Software

IOS (tm) C3550 Software (C3550-I9Q3L2-M), Version 12.1(22)EA1a, RELEASE SOFTWARE (fc1)

I have attached a text document showing the switchport 'show int xx switchport' command for both gig ports, and the L2 traces for both the 3550 and 3750. The 6509 evidently doesn't have that command as a feature, or it's not enabled on this one. There were CLNS, IPX and IP traces as choices, but no L2

[Roberto Salazar]

What vlan is 10.121.1.180 in and what vlan is 10.121.1.77 in? And what vlan is 10.121.1.1 is in on 6513? Is the port from 6513 to 3550 trunking as well? Can you confirm? What does show int trunk on 3550 and 3750 look like?

[jimmiejoe]

The 10.121 addresses are all in Vlan 1, and it it a class c network. The 10.121.1.1 address is the HSRP gateway on the 6500. All ports are dot1q trunks, and are not pruned.

Here is the 'sh int trunk' output for both switches.

3550#show int trunk

Port Mode Encapsulation Status Native vlan

Gi0/1 on 802.1q trunking 1

Gi0/2 on 802.1q trunking 1

3750_sw#show int trunk

Port Mode Encapsulation Status Native vlan

Gi1/0/1 on 802.1q trunking 1

Port Vlans allowed on trunk

Gi1/0/1 1-4094

I'm still puzzled by the Vlan 0 thing. My workstation is on another vlan, so I put a L3 Vlan matching my workstation on the 3750, gave it an IP, and was able to ping/telnet to that address directly through the 3550 to the 3750, but from the 6500 I can't do an extended ping sourcing from 10.121.1.1 (HSRP GW) or 10.121.1.2 (real). Doesn't go.