DLSw over WAN to Ethernet to Token Ring

Question

Currently, a 3745 in Portland is token ring attached and SNA traffic travels that ring to an IBM 2210 router.
That IBM router connects to a second token ring, which a Cisco 2613 is also connected to. From there
the traffic travels DLSw over the WAN (3 T1s, Multilink PPP bundle) to a 7507 in Cincinnati where a 3745
is connected via serial.
3745-TIC<->TR<->2210<->TR<->2613(DLSw)<->WAN<->(DLSw)7507<->LIC-3745
The 7507 serial port is configured as follows:
interface Serial4/3
description 3745 LIC connection for Fred Meyer DLSW
mtu 4400
no ip address
encapsulation sdlc
no ip mroute-cache
no keepalive
serial restart-delay 0
clockrate 125000
sdlc vmac 4000.3745.0000
sdlc N1 35200
sdlc address 29 seconly
sdlc partner 4000.0910.0008 29
sdlc dlsw 29
The 2613 has an ethernet. The folks in Portland want to move off of the TR and on to Ethernet.
They will replace the IBM router with one that has both TR and ENET.
If we move to the ethernet, the result will be:
3745-TIC<->TR<->2210<->ENET<->2613(DLSw)<->WAN<->(DLSw)7507<->LIC-3745
Should I change the MTU size on the serial port to 1500 (Ethernet)?
Are there any changes that should occur on the 3745/Mainframe side?
Do you see any other issues?

Answer 1

You cannot increase the MTU for ethernet larger than 1500 bytes because 802.3 (the standard for ethernet) does not allow that.
You may see issues. The problem is that both 3745 does not know the existance of the ethernet. If either 3745 sends a packet larger than 1500 bytes, the 2613 will drop the packet. (I am not an expert on 2210, I do not know if the 2210 is smart enough to segment the SNA packet into 2. I doubt it does, but please confirm it with IBM.) This causes the remote FEP detects a gap in SDLC, LLC2, or TH sequence number. No matter which layer detects the problem. The end result is still the same. The DLSw circuit will be disconnected.
There are several suggestions to your problem:
1. Make sure the both FEP will not send any PIU larger than 1500 bytes. This can be done either on the NCP gen or modtab. Checking the modtab is tough because your system programmers have to know all application running over the link. Then, they have to check the corresponding modtab entry for the applications.
2. run DLSw between 7507 to IBM 2210
3. directly connect the 2613 to the TIC (3745)