fabricpath ISIS adjacency can't be formed

[kerim mohammed]

HI,

I have four Nexus 7018s fabricpath VDCs. one of the VDCs can't form adjacency with the others. this is despite the fact that the port-channel interface mode is set to fabricpath and this switch sees the other switches as CDP neighbors. the other three formed adjacencies with no problem. what can be causing this? please see the attached diagram. the diagram below is based on "show fabricpath isis adj" command output.

[Steve Fuller]

Hi,

First of all, are there any error messages on any of the switches? Either from the switch shown as X=2 on your diagram or any of the other three?

On the switch X=2, do you see IS-IS neighbours in any state i.e., INIT? Can you post the output of the show fabricpath isis adjacency from this switch? Also the output of the show fabricpath isis internal event-history adjacency command might be useful.

So we can figure out what is physically connected to where, can you post the physical topology, or otherwise show cdp neigh from each switch, and then show fabricpath switch-id and show fabricpath isis interface brief from each each of the switches.

A couple of things to consider:

- If you've changed the interface MTU from defaults have you done so consistently across all the interfaces? I've seen issues with IS-IS used with OTV where we had adjacencies that wouldn't form due to MTU mismatch.

- If you're manually setting the FabricPath switch ID, then ensure you haven't a typo and configured the same ID on two switches.

Regards

[kerim mohammed]

          Below is the ouput from Switch1 (X=1) and Switch 2 (X=2) 

[Steve Fuller]

Hi,

I don't see the show fabricpath isis interface brief from the BM02 switch. Can you post that and a show run fabricpath from both the BM01 and BM02 switches?

Regards

[kerim mohammed]

    Here...the only difference i see is " System default switchport fabricpath" command is missing on SwitchID 32 (X=2), But i really don't think that could be the issue here.

thanks,

Kerim

[Steve Fuller]

Hi,

So BM02 doesn't appear to be running FabricPath on any interfaces. Do you have the command switchport mode fabricpathconfigured under each of the physical interfaces that are part of port-channel 2612 that connects between the BM01 and BM02 switch e.g., eth16/1, eth16/2 etc?

I just noticed your comment about the system default switchport fabricpath comamnd and think that's exactly the problem. The command switchport mode fabricpath must be configured under each physical interface as well. On the BM01 switch it doesn't need this as it's enabled by default. On the BM02 switch, the ports will be acting as plain Classic Ethernet. Add the command switchport mode fabricpath under each physical interface or the system default switchport fabricpath command and I think you'll be OK.

Regards

[kerim mohammed]

It is configured on the port-channel interface. "System default switchport fabricpath" is to enable fabricpath on all ports so we don't have to explicitly enable  it on each port. conversly, if you enable it on ports, you don't have to config "System Default switchport fabricpath" globaly. Let me know if I am missing something:

[Steve Fuller]

Hi,

"System default switchport fabricpath" is to enable fabricpath on all ports so we don't have to explicitly enable it on each port.

That's correct. If you use the global system default switchport fabricpathcommand all physical interfaces will run as FabricPath interfaces by default. This is what we have on BM01 so we don't need to specify the switchport mode fabricpath command under the physical interface.

Conversly, if you enable it on ports, you don't have to config system Default switchport fabricpath" globaly

Again that's correct.

What we are missing though is that BM02 has neither of these commands configured. You either need to the global

system default switchport fabricpath command or add switchport mode fabricpath command under each physical interface.

Regards

[kerim mohammed]

so setting switchport mode to fabricpath on a port-channel will not set the port mode to fabricpath on memeber(physical) ports. Because, it is enabled on the port-channel interface on BDM02 (po2621, shown above).

[Steve Fuller]

I don't believe so. In the output of the show running fabricpath for BM02 there's no physical interface shown as part of the FP config. When I run the same command on one of my switches, where I'm using the switchport mode fabricpath command under the physical interface, I see the physical interface as part of the show running fabricpath configuration.

If you run the command show interface eth16/1 switchport I believe the Operational mode won't show as FabricPath as you'll see on the BM01 switch.

Regards

[kerim mohammed]

Steve, you raised a very important point. But, the port-channel interface po2621 on BM02 is configured for fabricpath. however, this didn't take effect on the physical interfaces, why? the port-channel operational mode is fabricpath but the member port's operational mode is access and yet they are still members of the port-channel(not suspended for mismatch), why? in case of BM01 both the port-channel and memeber ports are operationing  in fabricpath mode. I also attached the screenshot from fabricpath interface config guide which states "system default switchport fabricpath" is optional as long as fabricpath is enabled on port-channel or one of the memeber ports (stress on or). Let me know what I am missing. thanks! 

[Steve Fuller · Accepted solution]

But, the port-channel interface po2621 on BM02 is configured for fabricpath. however, this didn't take effect on the physical interfaces, why?

I can't find any information in the configuration guide that states that the switchport mode command applied to the port-channel interface is copied to the physical interface. What I have found is the following statement in the configuration guide:

If one member of the port channel is in FabricPath mode, all the other members will be in FabricPath mode.

The way I would understand this is that at least one of the physical interfaces has to be configured with switchport mode fabricpath, and then this will be copied to the other physical interfaces that are members of the same port-channel. Note though that I've not tried this so can't confirm this is actually the behavior.

the port-channel operational mode is fabricpath but the member port's operational mode is access and yet they are still members of the port-channel(not suspended for mismatch), why?

Sorry, but at this point I don't know why the physical interface isn't suspended, or at least some syslog message generated to indicate it's operating in a different mode to the port-channel.

in case of BM01 both the port-channel and memeber ports are operationing  in fabricpath mode.

This is because you have the system default switchport fabricpath command configured, and as such all FabricPath capable interfaces operate in FapricPath mode. On the BM02 switch, where you don't have the system default switchport fabricpath command configured, all FP capable interfaces will operate as access ports. This is the default as per the second note shown on the page of the configuration guide you posted.

I also attached the screenshot from fabricpath interface config guide which states "system default switchport fabricpath" is optional as long as fabricpath is enabled on port-channel or one of the memeber ports (stress on or).

I presume you're referring to the Step 2 part showing "interface [ethernet slot/port | port-channel channel-no]". I personally wouldn't take this literally.

The way I understand system default switchport fabricpath is that it is optional provided the F series module interfaces that you want to operate in FabricPath mode are specifically configured with the switchport mode fabricpathcommand i.e., Step 4 is optional if you've already configured the Ethernet interface as per Step 2 and Step 3.

The reality is that both physical and port-channel interfaces mustbe enabled for FabricPath. Whether you achieve that using the interface switchport mode fabricpath comand or the global system default switchport fabricpath command is a decision to be made based on the environment. I personally prefer to use the switchport mode command under the interface as it give's a better control.

Regards

[kerim mohammed]

thanks Steve..