Ask the Expert: LAN Switching

[ciscomoderator]

With Matt Blanshard

Welcome to the Cisco Support Community Ask the Expert conversation. This is an opportunity to ask your toughest layer 2 questions to two of the technical leaders of the San Jose LAN Switching team, Matt Blanshard. Learn more about Spanning Tree, VTP, Trunking, Resilient Ethernet Protocol, IGMP Snooping, Private VLANS, Q-in-Q Tunneling, QoS, various switching platforms including all desktop switches, Metro Ethernet switches, 4500 and 6500 switches, Blade Center switches, and Nexus 7000 switches. 

Matt Blanshard began his Cisco career as an intern in 2007.  He is now a technical leader at the Cisco Technical Assistance Center on the LAN Switching team. He holds a bachelor's degree from the University of Phoenix in computer science, and has CCNA certification. 

Remember to use the rating system to let Matt know if you have received an adequate response. 

Matt might not be able to answer each question due to the volume expected during this event. Remember that you can continue the conversation on the discussion forum shortly after the event. This event lasts through March 23rd, 2012. Visit this forum often to view responses to your questions and the questions of other community members. 

[QFX527518]

on the cat6500,can see the client's MAC,but a-client1 cant see b-client1's arp.now,if change client vlan id 87 to vlan 1,a-client1 can ping b-client1.

[Matthew Blanshard]

Can you send the output of show int po3 trunk and show int po5 trunk?  Is VTP pruning turned on?

-Matt

[QFX527518]

[Matthew Blanshard]

This is going to require more in-depth troubleshooting then we can do here.  I would suggest opening up a TAC case and PM me the case number and I can grab it and work with you on it.

-Matt

[johnnylingo]

Matt,

Thanks for the info on scalabiltiy with R-PVST vs. MST.  I was aware of the limitations with regards to the BPDUs, but having hard numbers helps a lot there, so thank you very much!

My other question might be more geared towards product mangers but perhaps you know the answer.  I noticed the Nexus 3000 switches do not support PAgP.  Is this is specific move just for the Nexus platform, or an indication of long-term support?  

To my knowlege, there are no real advantages of running PAgP over LACP, so it would make sense to phase out PAgP in new products, similar to what was done with ISL years ago.

[Matthew Blanshard]

You hit the nail right on the head.  LACP is the preferred way to go and any new platforms coming out are unlikely to support PAGP in the future.  Since LACP is an IEEE standard it provides many advantages over PAGP in terms of interoperability. 

-Matt

[Peter Paluch]

Hello Matt,

This is an interesting information. However, for 6500 VSS, the PAgP+ was recommended for active-active detection. At the time, I had the feeling that Cisco did not implement a similar functionality into LACP. So for VSS, are we stuck with PAgP+ or is the active-active detection already available with LACP as well?

Thank you!

Best regards,

Peter

[Matthew Blanshard]

Hello Peter,

Unfortunately there is no dual active detection implemented into LACP.  If you can't implement PAgP+ via an older switch the alternative to PAgP+ is to use the fast hello method.  You can achieve the same level of response time with fast hello as you can wiht PAgP+ to a dual-active scenario.

-Matt

[DriJones01]

Inherited an SMB network with a Catalyst 3650, some Express 500 switches and NEC DTerm IP phones running PoE. No passwords or documentation of config.

Took one of the spare 500 switches, reset to factory, and setup IP+Desktop smartports on the copper ports, added the Cisco-Voice VLAN.

Phones are powering, but not finding the Voice VLAN or DHCP server for the phones.

Is there a way to discover (Wireshark or otherwise) what VLAN I need on the 500 switch so that the IP phones will work sjhort of wiping everything and starting over?

[Matthew Blanshard]

Hello Indy,

This is going to be a tough one.  Is there a way you can find out the vlan id on the phone?  On Cisco phones you can find out the voice vlan id on the network configuration screen.  Does the NEC phone offer something similar? Without that doing a wireshark is going to be difficult unless your PC supports vlan tagging on the NIC. 

-Matt

[DriJones01]

Matt,

Found a way to get into the phone, and the configuration doesn't yield much other than the VLAN is 5.

Took a spare Catalyst 500 and added a VLAN ID=5.

In trying to configure a port to VLAN 5, the Cat 500 inists to keep the Cisco-Voice VLAN as the voice LAN. So .... recreated the Cisco-Voice VLAN as VLAN ID=5.

Is that the correct configuration that the Cat 500 is looking for?

[Matthew Blanshard]

Yes that is exactly the info we were looking for.  It should work with the configuration you specified.

-Matt

Please excuse typos sent from my android phone.

[hobbe]

Hi

I have a question about the mtu size.

This is not a question about what the mtu size is as much as a reflektion over why it is what it is.

If you set the MTU size of a 3750x fx you will have a max MTU size of 9198

System jumbo mtu 9198

Same with the 4500

That is all fine and well BUT

Why are the NEXUS and the 6500 max mtu size 9216 ?

How is this calculated ? where does the extra 18 come from and why ?

Thanx

/Hobbe

[Matthew Blanshard]

Hello Hobbe,

The difference is due to the 18 byte ethernet header.  On the 6500 the 9216 includes the 18 byte ethernet header already, on the 4500 it does not, so it gets added afterwards.

-Matt

[hobbe]

Thank you very much for the explanation.

You do not happen to know any references or reasons why they calculate that way ?

why not being consistant ?

/Hobbe