Problems with EtherChannel on Cross-Stack Catalyst 3750 switches with WAN interconnection

Answered Question
Feb 19th, 2010
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Hi to All,


need to configure EtherChannel on 2 wan site with the following enviroment:


On the 1st site there're 2 Cross-Stack Catalyst 3750


On the 2nd site there're 2 Cross-Stack Catalyst 3750


The connection between the 2 sites consist of 2 Gigabit links: the wan provider tell me MTU is end to end 9192 bytes




A) If I try to use the EtherChannel Without PAgP or LACP all is OK:


3750switchstackA(config)#interface range gigabitethernet 1/0/23 - 24

3750switchstackA(config-if)#channel-group 1 mode on

3750switchstackA(config-if)#switchport trunk encapsulation dot1q

3750switchstackA(config-if)#switchport mode trunk



B) If I try to use the EtherChannel With LACP the etherchannel NEVER go up !



3750switchstacka(config)#interface range gigabitethernet 1/0/23 - 24

3750switchst(config-if-range)#channel-group 1 mode active

3750switchst(config-if-range)#switchport trunk encapsulation dot1q

3750switchst(config-if-range)#switchport mode trunk




The question:


the problem is the 3750 interfaces MTU ?

the LACP need a bigger MTU on my 3750 switches (need to change it) ?




3750switchstackA#sh system mtu


System MTU size is 1500 bytes

System Jumbo MTU size is 1500 bytes




Regards,


Roberto Taccon

Correct Answer by Lei Tian about 7 years 1 month ago

Hi Roberto,


LACP is an L2 control protocol, there is no encapsulation done by LACP. The LACP peers send out the special PDU to remote peer to verify if the peer is alive.

You can find out whether SP allow LACP frame by turn on debug lacp on your switch. Configure LACP mode active on both ends, and turn on debug LACP. If your provider allows the LACP control frame, you should be able to see "LACP packet received" and "Send LACP-PDU packet" from the debug output.


HTH,


Lei Tian

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francisco_1 Fri, 02/19/2010 - 01:58
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MTU must be configured only on port channel interfaces and this MTU is propagated to the bundled ports. Make sure you are using LACP on both sides and the MTU match also on both sides.

ROBERTO TACCON Fri, 02/19/2010 - 02:27
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Hi,


the interface Port-channel 1 is configured with defaul MTU 1500.


How I can modify it ?

Is there any INTERFACE command to modify it ?

Need to modify the system MTU (which one MTU/routing/Jumbo) ?






3750switchstackA#sh int po1

Port-channel1 is up, line protocol is up (connected)

  Hardware is EtherChannel, address is 0013.7f59.6619 (bia 0013.7f59.6619)

  Description: 3750switchstackA

  MTU 1500 bytes, BW 2000000 Kbit, DLY 10 usec,

     reliability 255/255, txload 1/255, rxload 1/255

  Encapsulation ARPA, loopback not set

  Keepalive set (10 sec)

  Full-duplex, 1000Mb/s, link type is auto, media type is unknown

  input flow-control is off, output flow-control is unsupported

  Members in this channel: Gi1/0/25 Gi2/0/24

  ARP type: ARPA, ARP Timeout 04:00:00

  Last input 00:00:00, output 19:21:09, output hang never

  Last clearing of "show interface" counters never

  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0

  Queueing strategy: fifo

  Output queue: 0/40 (size/max)

  30 second input rate 0 bits/sec, 0 packets/sec

  30 second output rate 0 bits/sec, 0 packets/sec

     2505117 packets input, 3513326395 bytes, 0 no buffer

     Received 55871 broadcasts (55871 multicasts)

     0 runts, 0 giants, 0 throttles

     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored

     0 watchdog, 55871 multicast, 0 pause input

     0 input packets with dribble condition detected

     974779 packets output, 113948767 bytes, 0 underruns

     0 output errors, 0 collisions, 7 interface resets

     0 babbles, 0 late collision, 0 deferred

     0 lost carrier, 0 no carrier, 0 PAUSE output

     0 output buffer failures, 0 output buffers swapped out

itmil-calinf0r2-sw01DCMItoDCVR#





3750switchstackA(config)#system mtu ?

  <1500-1998>  MTU size in bytes

  jumbo        Set Jumbo MTU value for GigabitEthernet or TenGigabitEthernet interfaces

  routing      Set the Routing MTU for the system


3750switchstackA(config)#system mtu

francisco_1 Fri, 02/19/2010 - 03:15
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using system mtu global config will affect all interfaces.


Try


conf t

int Port-channel 1

mtu ?


I think you can setup mtu on the portchannel itself.

francisco_1 Fri, 02/19/2010 - 03:33
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You cannot set the mtu on interface level. you have to do it globally. OK. There is no mtu on the protchannel to decrease mtu. You cannot setup the mtu under the portchannel. What you need to do is  configure system mtu [setup your mtu) This will affect all ports under the portchannel. The port channe interface inherits the mtu from the physical interface when the interface becomes active.


Config t

system mtu

Lei Tian Fri, 02/19/2010 - 03:16
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  • Cisco Employee,

Hi Roberto,


You need to consult your service provider make sure the SP can transparently pass LACP/pagp control frame. If SP block the control frame, you will not be able to use LACP/pagp.


HTH,


Lei Tian

ROBERTO TACCON Fri, 02/19/2010 - 03:41
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The SP tell me: "the 2 wan links are transparent to the switches, the MTU is 9192 (MTU ethernet: with ethernet Header).



If the default MTU is 1500 bytes and the trunk 802.1Q requires an extra 4 bytes: the default interfaces MTU will be 1504 bytes... why the switch tell me 1500 bytes ?


How many extra bytes require LACP (are there any docs) ?


Are there any option to verify the MTU within the switch ?


How I can modify the MTU for only the Port-channel (channel of 2 Gigabit interfaces) ?




***


3750switchstackA#sh int po1

Port-channel1 is up, line protocol is up (connected)

  Hardware is EtherChannel, address is 0013.7f59.6619 (bia 0013.7f59.6619)

  Description: ** 3750switchstackA **

MTU 1500 bytes, BW 2000000 Kbit, DLY 10 usec,



***

ON THE PO1 interface there isn't a MTU command (also on the GE interface)




3750switchstackA(config)#int po1

3750switchstackA(config-if)#?

Interface configuration commands:

  arp                     Set arp type (arpa, probe, snap) or timeout

  bandwidth               Set bandwidth informational parameter

  bgp-policy              Apply policy propagated by bgp community string

  carrier-delay           Specify delay for interface transitions

  cdp                     CDP interface subcommands

  dampening               Enable event dampening

  default                 Set a command to its defaults

  delay                   Specify interface throughput delay

  description             Interface specific description

  duplex                  Configure duplex operation.

  energywise              EnergyWise interface configurations

  eou                     EAPoUDP Interface Configuration Commands

  exit                    Exit from interface configuration mode

  flowcontrol             Configure flow operation.

  help                    Description of the interactive help system

  hold-queue              Set hold queue depth

  ip                      Interface Internet Protocol config commands

  ipv6                    IPv6 interface subcommands

  isis                    IS-IS commands

  iso-igrp                ISO-IGRP interface subcommands

  keepalive               Enable keepalive

  l2protocol-tunnel       Tunnel Layer2 protocols

  link                    Configure Link

  load-interval           Specify interval for load calculation for an interface

  logging                 Configure logging for interface

  macro                   Command macro

  max-reserved-bandwidth  Maximum Reservable Bandwidth on an Interface

  mvr                     MVR per port configuration

  neighbor                interface neighbor configuration mode commands

  network-policy          Network Policy

  nmsp                    NMSP interface configuration

  no                      Negate a command or set its defaults

  service-policy          Configure QoS Service Policy

  shutdown                Shutdown the selected interface

  snmp                    Modify SNMP interface parameters

  source                  Get config from another source

  spanning-tree           Spanning Tree Subsystem

  speed                   Configure speed operation.

  storm-control           storm configuration

  switchport              Set switching mode characteristics

  timeout                 Define timeout values for this interface

  transmit-interface      Assign a transmit interface to a receive-only interface

  tx-ring-limit           Configure PA level transmit ring limit

  vtp                     Enable VTP on this interface


3750switchstackA(config-if)#



Thanks to all,


Roberto Taccon

Correct Answer
Lei Tian Fri, 02/19/2010 - 04:42
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  • Cisco Employee,

Hi Roberto,


LACP is an L2 control protocol, there is no encapsulation done by LACP. The LACP peers send out the special PDU to remote peer to verify if the peer is alive.

You can find out whether SP allow LACP frame by turn on debug lacp on your switch. Configure LACP mode active on both ends, and turn on debug LACP. If your provider allows the LACP control frame, you should be able to see "LACP packet received" and "Send LACP-PDU packet" from the debug output.


HTH,


Lei Tian

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