Importance of specifiying MAC add of next hop L3 device in FWSM config

Question

Hi,
With refrence of Cisco Secure Firewall Services Module (FWSM) of Cisco Press book it's mentioned that
"While configuring the transparent mode in FWSM, it is important to specify the MAC address and the CAM entries on the Layer 3 next hop device of FWSM."
This part of configuration is not very much clear to me please let me know the logic of this things
The following are two examples:
Layer 3 Device A (PFC) at the Outside Security Domain
! IP address of the next hop for the outside security domain
interface Vlan20
mac-address 0000.0000.0001
ip address 10.10.1.1 255.255.255.0
! Specify the IP address and MAC address at the first hop layer 3 interface
! of the inside security domain
arp 10.10.1.21 0000.0000.0001 ARPA
Layer 3 Device B at the Inside Security Domain
! IP address of the next hop for the inside security domain
interface Vlan21
mac-address 0000.0000.0021
ip address 10.10.1.21 255.255.255.0
! Specify the IP address and MAC address defined at the first hop interface
! of the outside security domain
arp 10.10.1.21 0000.0000.0002 ARPA
Regards
Ambivert Skill

Answer 1

Hello Mikis,
Fair enough, Just remember beggining on 8.3 how the ASA handles the packets it's different from 8.2 and older versions.
As you said now the ASA is going to check the proper Nat rules first and then the Acl's that is why when we want to allow traffic from outside to an inside server  we need to poing the ACL to the private or un-nated Ip as the nat rule was taken in place first
Good post by the way,
Remember to rate all the community answers, for us that is more important than a thanks
Julio