Setup:
We have couple of N3K connected back to back on a trunk link, dual home Internet feed terminating on each of them.
We have single multihomed L2 (transparent) circuit wiz capable of transporting multi-tag frame and L2 protocol between these N3K and Remote CPE switch 3850 (stack of 6).
N3K--------- N3K
| |
| |
L2 Crkt1 L2 Crkt2 (Crkt1 & 2 are from different carrier)
| |
| |
3850-Stack-of-6
Problem statement:
Observing STP loop if enable both the circuit. STP roots are elected differently for both the end of LAN segment.
STP counters for Interfaces connected between N3Ks and 3850, increment only for "sent bpdu" however the counter remains 0 for "receive bpdu".
Other interfaces of the switches doesn't show such kind of STP malfunctioning issue.
Diagnose:
Circuits are bidirectional as Udld aggressive works good.
Circuit transport L2 protocols, since cdp works fine.
Able to ping connected subnet, If enable any single circuit.
Packet captured at both the end of circuit which shows BPDU received on the wire however the interface seems not processing it and the counter still remains 0 for "receive bpdu" on both the side of the interfaces.
Default the interfaces and re configure the ports however no luck.
Logs:
N3K1 & N3K2 are identical
N3K2# sh spanning-tree interface Eth1/49 deta
Port 177 (Ethernet1/49) of VLAN0500 is designated forwarding
Port path cost 4, Port priority 128, Port Identifier 128.177
Designated root has priority 33268, address 707d.xxxx.737c
Designated bridge has priority 33268, address 707d.xxxx.7601
Designated port id is 128.177, designated path cost 4
Timers: message age 0, forward delay 0, hold 0
Number of transitions to forwarding state: 2
Link type is point-to-point by default
BPDU: sent 164140, received 0
!
interface Ethernet1/49
switchport mode trunk
switchport trunk allowed vlan 500
speed 1000
udld aggressive
========================================================
Output for tenGigabitEthernet 1/1/2 & tenGigabitEthernet 2/1/2 are identical
3850-Stack-of-6#show spanning-tree interface tenGigabitEthernet 1/1/2 det
Port 54 (TenGigabitEthernet1/1/2) of VLAN0500 is designated forwarding
Port path cost 4, Port priority 128, Port Identifier 128.54.
Designated root has priority 500, address 00e1.xxx.6580
Designated bridge has priority 500, address 00e1.xxxx.6580
Designated port id is 128.54, designated path cost 0
Timers: message age 0, forward delay 0, hold 0
Number of transitions to forwarding state: 1
Link type is point-to-point by default
Loop guard is enabled on the port
BPDU: sent 26, received 0
!
interface TenGigabitEthernet2/1/2
description xxxxxx
switchport trunk allowed vlan 500
switchport mode trunk
udld port
spanning-tree guard loop
end
