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ACE assumptions and their impact on teaming network drivers


As part of the basic design ACE does not send traffic to IP's, but to encapsulation "targets".

You can see this with the "show ip fib" command:

cdn-ace-1/pevaneyn# show ip fib

FIB for Context pevaneyn (RouteId 16)

   Codes: H - host,   I - interface

          S - static,      N - nat

          A - need arp resolve,      E - ecmp

Destination         Interface         EncapId  Flags


...     vlan499               69   H [0x3]

This EncapId (here 69) is then used to send commands to the server, but also to classify traffic coming from that IP. We can check this by:

cdn-ace-1/pevaneyn# show arp | i    00.19.aa.1a.36.1b  vlan499   LEARNED    69     4834 sec     up


cdn-ace-1/pevaneyn# show np 1 adjacency 69 69

   id   S:Ver flag  imph 1  imph 2           DstMac           SrcMac  MTU ifid


   69 1:0       1     1f3       0 0019:aa1a:361b 000b:fcfe:1b7e   1500    76

As you can see the adjacency uses the MAC address. If you want to send traffic from a different MAC you need to first send a gratuitous ARP.

All of this means that if you send traffic from the correct IP but with the wrong MAC the ACE will just drop the packet. So if you are using a teaming server which sends traffic with a different MAC on failover or when the load is high enough this will cause problems.

See for example documentation bugs CSCtt33758 or CSCth66757.

If at all posible configure and use a port-channel with a unique, non-changing, MAC address.

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