5

I am trying to setup a network that includes an EtherChannel link between two Cisco Layer 3 switches (Catalyst 3750). I am using a collapsed core hierarchical network.

collapsed core topology

The picture above is my topology. On my layer 2 access switches (S1, S2, S3) I have several hosts connected right now, on several VLANs. My issue tho is that a host on for example S2, can't communicate with L3-1 through the EtherChannel between L3-1 and L3-2. I've tried making it a Layer 3 EtherChannel with IP's, and I can ping across, but it's like it wont propagate my tagged VLAN traffic.

Example:

I connect HOST-A to L3-1 on interface GigabitEthernet2/0/1.

I make GigabitEthernet2/0/1 switchport access on VLAN 20.

HOST-B is connected to a switchport access VLAN 20 port on S2.

HOST-B tries to ping L3-2. Succesful.

HOST-B tries to ping across to L3-1 / HOST-A. Not succesful.

PS. I also have HSRP setup on the Layer 3 switches, for all my VLANs. Also I have a Main Windows Server 2016 Domain Controller connected to L3-1 and a Backup connected to L3-2.

Let me know if you need the IP scope.

I appreciate all help! I just want to know what I have to think of/be sure of is working, to make this network communicate. Any help that can guide me to what i can do to get traffic all the way to the WAN, and so that my hosts on S1, S2 and S3 can connect to the internet. Much appreciated.

CONFIGURATION:

L3-1

Port-Channel:

!
interface Port-channel1
 no switchport
 ip address 172.16.0.1 255.255.255.252
!
!
interface GigabitEthernet2/0/19
 no switchport
 no ip address
 channel-group 1 mode desirable
!
interface GigabitEthernet2/0/20
 no switchport
 no ip address
 channel-group 1 mode desirable
!

L3-2 has same configuration on same ports and channel-group. It's IP address is 172.16.0.2 255.255.255.252 instead.

VLAN-config: show vlan brief in cisco terminal

All links between layer-3 and layer-2 switches are trunks. A snippet from L3-2: interface overview from layer 3 switch

S1

VLAN-config: vlan overview on switch

S2

VLAN-config: vlan overview on switch

S3

VLAN-config: vlan overview on switch

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6
  • you need to add the configuration of the devices. Also the result of the show vlancommand on S1, S2 & L3-1 along with the name of the interfaces that connect those switches together would be useful.
    – JFL
    Commented Jan 25, 2018 at 8:40
  • Definitely need configurations to give any help. Remember to sanitize the configuration files and remove all passwords etc.
    – user36472
    Commented Jan 25, 2018 at 9:25
  • @Cown I have updated the thread which should highlight your requests. I didn't put in the full configurations for the sake of trying to keep an overview. Do let me know if it's not satisfying enough, tho.
    – Roy T.
    Commented Jan 25, 2018 at 9:41
  • @RoyT. On your links between L3-1 and L3-2, how come you don't use a normal trunked etherchannel? Does the 172.16.0.1 255.255.255.252 have any purpose?
    – user36472
    Commented Jan 25, 2018 at 9:58
  • It will be a lot easier if you would be able to show all the config from the L3-1 and L3-2 plus config from each Access Switch
    – bluehatgeek
    Commented Jan 25, 2018 at 10:00

3 Answers 3

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4

@RoyT. On your links between L3-1 and L3-2, how come you don't use a normal trunked etherchannel? Does the 172.16.0.1 255.255.255.252 have any purpose?

For instance, if you put this configuration on the interlink between L3-1 and L3-2:

interface Port-channel1
 switchport
 switchport trunk encapsulation dot1q
 switchport mode trunk
!
interface GigabitEthernet2/0/19
 switchport
 switchport trunk encapsulation dot1q
 switchport mode trunk
 channel-group 1 mode desirable
!
interface GigabitEthernet2/0/20
 switchport
 switchport trunk encapsulation dot1q
 switchport mode trunk
 channel-group 1 mode desirable
!

Then everything will be able to communicate.

In order to do it:

(config)# int range gi2/0/19-20
(config-if)# no channel-group
(config-if)# switchport
(config-if)# switchport trunk encapsulation dot1q
(config-if)# switchport mode trunk
(config-if)# int po1
(config-if)# no ip address
(config-if)# switchport
(config-if)# switchport trunk encapsulation dot1q
(config-if)# switchport mode trunk
(config-if)# int range gi2/0/19-20
(config-if)# channel-group 1 mode desirable
(config-if)# end
# write memory
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4
  • Hi @Cown - I think I already tried this, without success.. I will try again tho, right now I get the error that an interface with encapsulation set to "auto" can't be configured to trunk mode. I will try and set encapsulation 802.1q and try again.
    – Roy T.
    Commented Jan 25, 2018 at 10:44
  • @RoyT. Yeah i used the example on Cisco 6500 series, so no encapsulation needed.
    – user36472
    Commented Jan 25, 2018 at 10:46
  • @RoyT. edited to include encapsulation.
    – user36472
    Commented Jan 25, 2018 at 10:49
  • @RoyT. no problem :-)
    – user36472
    Commented Jan 25, 2018 at 11:52
3

L3 EtherChannel and VLAN/Trunk concepts DO NOT work together.

In this scenario, you should use L2 EtherChannel between L3-1 and L3-2 switches.

Ensure that you create all L3 Int VLANs / L2 VLANs, and trunk them on this L2 EtherChannel between L3-1 and L3-2 switches.

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0

Possibly spanning tree is blocking the ports to communicate with each other...

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1
  • Spanning tree is a layer-2 protocol. It does not cross a layer-3 boundary. You cannot use layer-2 concepts like VLANs or spanning tree on a layer-3 link. The accepted answer was to change the link to a layer-2 link.
    – Ron Maupin ♦
    Commented Oct 11, 2019 at 2:12

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