Bridging two serial ports inside same router

[lasthouseontheright]

Hello all, 

     Probably first time posting here and looking for assistance.  

Scenario:   Single 8300 router, running 17.12.3.  Trying to have two old school fireberd 8000s talk to one another on Serial 2/0/1 and 2/0/3 but cannot seem to get the ports to bridge together.  Fireberds are DTE obviously, connected to 2/0/1 and 2/0/3 via DCE cables.  I can get a single FB to talk to the serial port when it is in loop.  Tried assigning them (the ports) to same bridge domain but run into invalid commands at different locations of syntax or incomplete when just using the serial interface.  

Ex1. : HOSTNAME (config-domain) member serial 2/0/1 (incomplete)

When I do the ? I receive <0-0> Serial interface number (asking for sub interface?)

HOSTNAME (config-domain)member serial 2/0/1:0

                                                                     ^ (invalid at slash between 2 and 0)

Relatively new with serial on Cisco and grasping at straws at this point so any help would be appreciated and feel free to sanity check me if I am in the incorrect forum

Thanks in advance!

[liviu.gheorghe]

Hello @lasthouseontheright ,

The router, which has connected the DCE serial cable, must provide the clock rate of the serial line:

Router#conf t
Router(config)#
Router(config)#interface serial 2/0/1
Router(config-if)#clock rate 64000
Router(config)#interface serial 2/0/3
Router(config-if)#clock rate 64000

The configuration for bridging the two serial interfaces:

interface Serial2/0/1
no ip address
negotiation auto
service instance 100 ethernet
encapsulation untagged
bridge-domain 100
!
interface Serial2/0/3
no ip address
negotiation auto
service instance 100 ethernet
encapsulation untagged
bridge-domain 100

Now the two serial interfaces should be UP and bridging functional between the two serial interfaces.

HTH

[lasthouseontheright]

Apologies for taking so long.  Our lab only runs certain hours.  I seem to be running into some issues with the syntax on the interfaces...

interface Serial2/0/1
no ip address
negotiation auto (negotiation doesn't appear to be valid)
service instance 100 ethernet (invalid. Also and I may not have clarified, we are using DB25-Cisco smart serial cabling.)   
encapsulation untagged (untagged doesn't appear to be a valid option)
bridge-domain 100 (leads me into the bridge domain submenu.  Is there something else that belongs here?)
!

2/0/3 gave me the same indications but I didnt want to repeat.

[balaji.bandi]

Do you want to configure this configuration for Cat 8300?