L2 loop on my network

[gm45]

Good day

kindly new to this platform and still a newbie on networks especially switching..

got a traditional network design i.e 2 cores, 4 distributions and access SWs, with redundant links between, between cores GLBP..and also running eigrp internally. all this how i found the setup.

past 2 months ive joined the company network randomly goes down, happened twice now. 1st time it recovered without intervention..2nd time i could see one of the distribution SW was flapping on eigrp neighbors and rebooted that switch to restore connectivity.

some strange thing is on 1 of the distribution sw - "sh cdp nei" there's 1 port showing multiple devices including itself and can confirm that other devices are not plugged in...cross over cable goes from it straight to the CORE. it basically showing me whats also neighbors of the CORE, is this an issue?

[balaji.bandi]

Looks like spanning-tree loop somewhere as per the information., still this require more information to understand the issue.

1. Do you have a high-level network diagram?

2. Core are VSS ? or L3 ? 

3. Distribution again VSS or Core? (how they connected to Core ?)

4. Access switches connected to distribution over L2 or L3 with IGP.

post configuration also to understand better. along with show spanning information.

[gm45]

hi,

Core and distributions are L3

access connects to distributions via L2..

will share diagram and STP info..

saw the below loggs on 1 of the access switches used for connecting servers, server/hosts end they doing NIC Teaming just asked them to change load balancing on their to Active/Passive instead of active/active not sure if that would assist on my issue though...just something i saw on the internet. saw again saying i should also enable loop guard and BPDU skew detection but not sure on which switches.. 

Logs:

Log Buffer (4096 bytes):
43f6.0544 in vlan 42 is flapping between port Gi0/11 and port Gi0/12
8359617: Jun 20 07:02:21.600: %SW_MATM-4-MACFLAP_NOTIF: Host 08bd.43f6.0544 in vlan 42 is flapping between port Gi0/11 and port Gi0/12
8359618: Jun 20 07:02:40.222: %SW_MATM-4-MACFLAP_NOTIF: Host 08bd.43f6.0544 in vlan 42 is flapping between port Gi0/12 and port Gi0/11
8359619: Jun 20 07:02:55.473: %SW_MATM-4-MACFLAP_NOTIF: Host 08bd.43f6.0544 in vlan 42 is flapping between port Gi0/12 and port Gi0/11
8359620: Jun 20 07:03:08.374: %SW_MATM-4-MACFLAP_NOTIF: Host 08bd.43f6.0544 in vlan 42 is flapping between port Gi0/12 and port Gi0/11
8359621: Jun 20 07:03:13.173: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/11, changed state to down
8359622: Jun 20 07:03:13.257: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/12, changed state to down
8359623: Jun 20 07:03:14.179: %LINK-3-UPDOWN: Interface GigabitEthernet0/11, changed state to down
8359624: Jun 20 07:03:14.263: %LINK-3-UPDOWN: Interface GigabitEthernet0/12, changed state to down
8359625: Jun 20 07:03:30.604: %LINK-3-UPDOWN: Interface GigabitEthernet0/11, changed state to up
8359626: Jun 20 07:03:30.973: %LINK-3-UPDOWN: Interface GigabitEthernet0/12, changed state to up

IEM-EMB-SVRFARM-SW03#sh mac address-table int Gi0/11
Mac Address Table
-------------------------------------------

Vlan Mac Address Type Ports
---- ----------- -------- -----
42 08bd.43f6.0544 DYNAMIC Gi0/11
Total Mac Addresses for this criterion: 1
IEM-EMB-SVRFARM-SW03#
IEM-EMB-SVRFARM-SW03#sh mac address-table int Gi0/12
Mac Address Table
-------------------------------------------

Vlan Mac Address Type Ports
---- ----------- -------- -----
IEM-EMB-SVRFARM-SW03#
IEM-EMB-SVRFARM-SW03#
IEM-EMB-SVRFARM-SW03#
IEM-EMB-SVRFARM-SW03#sh run int Gi0/11
Building configuration...

Current configuration : 112 bytes
!
interface GigabitEthernet0/11
switchport access vlan 42
switchport mode access
spanning-tree portfast
end

IEM-EMB-SVRFARM-SW03#

UPLINK TO DISTRIBUTION SW

IEM-EMB-SVRFARM-SW03#sh run int Gig 0/48
Building configuration...

Current configuration : 166 bytes
!
interface GigabitEthernet0/48
description EMBANKMENT-DS1_G0/21
switchport trunk encapsulation dot1q
switchport trunk native vlan 100
switchport mode trunk
end

IEM-EMB-SVRFARM-SW03#

[balaji.bandi]

is your Core Switches are part of VSS?

here is a good document where to apply Loopgaurd.

https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/10596-84.html

is this network outage for the short term you mentioned the whole network or only to servers?

[gm45]

VSS no. models WS-C3750X-12, thanks for loopguard info.
outage affect whole network

CORE-FDS1#sh run int vlan 100
Building configuration...

Current configuration : 160 bytes
!
interface Vlan100
description Management IP
ip address 172.20.100.254 255.255.255.0
glbp 100 ip 172.20.100.1
glbp 100 priority 110
glbp 100 preempt
end