LAG with CISCO generating REALLY strange Problems

Hi all,

 
we have a really strange behaviour occuring in connections between systems in our networks:

The setup is a VLAN between the Sophos and the Cisco-Cluster (sophos has IP 10.10.0.1 and Cisco .2) each being the default router for the other concerning the networks 10.10.xy.zz
CISCO is Router for networks 10.10.10.0 (Server) and 10.10.20.0 (Client), communication between these networks is fine and has no problems. Sophos has "any"-Rules for testing, so nothing is blocked.

What now happens is:

ComputerA (172.16.1.211) <--> (172.16.1.254) Sophos (10.10.0.1) <--> (10.10.0.2) CISCO (10.10.10.1) <--> (10.10.10.11) ServerA

    no Ping, no TCP-Communication (e.g. RDP) nothing, traceroute "stops" at 10.10.0.2
    DNS, Firewall, default gateway: everything double-checked and OK when trying to connect from ComputerA to ServerA
    
    ServerA also can´t ping or connect to ComputerA
    
BUT:

ComputerB (172.16.1.123) <--> (172.16.1.254) Sophos (10.10.0.1) <--> (10.10.0.2) CISCO (10.10.10.1) <--> (10.10.10.11) ServerA

    all good ! communication works, RDP, ICMP, all fine, ServerA can ping back or establish TCP-communication
    
to get this all even more complicated/strange:

changing ComputerA´s IP to a different one (or activating DHCP) establishes communication instantly

This behaviour is not limited to a single computer but occurs randomly on any system.


Does anybody have an idea of whats happening here or who´s the culprit?

Kind regards !

Markus

Please ask for more details if needed !

Sophos LAG-Config:

================

Sophos UTM SG430 (v 9.605-1) 2 Devices in passive-Standby, each equipped with a 4x 10G module,
each UTM connected to X4500-Cluster with 2 Cables on eth9 & eth10 configured as LACP:

Ethernet Channel Bonding Driver: v3.7.1 (April 27, 2011)

Bonding Mode: IEEE 802.3ad Dynamic link aggregation
Transmit Hash Policy: layer2 (0)
MII Status: up
MII Polling Interval (ms): 100
Up Delay (ms): 0
Down Delay (ms): 0

802.3ad info
LACP rate: fast
Min links: 0
Aggregator selection policy (ad_select): stable
Active Aggregator Info:
        Aggregator ID: 1
        Number of ports: 2
        Actor Key: 33
        Partner Key: 12
        Partner Mac Address: 02:00:00:00:00:0a

Slave Interface: eth9
MII Status: up
Speed: 10000 Mbps
Duplex: full
Link Failure Count: 0
Permanent HW addr: 00:1a:8c:f0:e8:c9
Aggregator ID: 1
Slave queue ID: 0

Slave Interface: eth10
MII Status: up
Speed: 10000 Mbps
Duplex: full
Link Failure Count: 0
Permanent HW addr: 00:1a:8c:f0:e8:ca
Aggregator ID: 1
Slave queue ID: 0

here´s the Ciscos LACP-Config:

CISCO X4500: (2x in ClusterMode)
================================

interface Port-channel12
 switchport
 switchport mode trunk
 logging event link-status
 logging event trunk-status
!
interface TenGigabitEthernet1/1/2
 switchport mode trunk
 logging event link-status
 logging event trunk-status
 load-interval 30
 channel-protocol lacp
 channel-group 12 mode active
!
interface TenGigabitEthernet1/1/3
 switchport mode trunk
 logging event link-status
 logging event trunk-status
 load-interval 30
 channel-protocol lacp
 channel-group 12 mode active
!
interface TenGigabitEthernet2/1/2
 switchport mode trunk
 logging event link-status
 logging event trunk-status
 load-interval 30
 channel-protocol lacp
 channel-group 12 mode active
!
interface TenGigabitEthernet2/1/3
 switchport mode trunk
 logging event link-status
 logging event trunk-status
 load-interval 30
 channel-protocol lacp
 channel-group 12 mode active
!
interface Vlan200
 description LBAG_DMZ
 ip address 10.10.0.2 255.255.255.248

ip route 0.0.0.0 0.0.0.0 10.10.0.1

## where 10.10.9.1 is the Sophos´ UTM lag0-Interface