Figure 5-11 illustrates the operation of local acknowledgment.
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Cisco CCIE Fundamentals: Network Design
SDLLC Configuration
Figure 5-11
Local acknowledgment operation.
LLC2
TCP/IP
SDLC
session
session
session
IBM
3278
IBM
3x74
Arbitrary
IBM
Token
WAN
3278
Ring 10
Router
backbone
Router
3745
Virtual ring 100
Virtual ring 200
Note Local acknowledgment requires that TCP sessions be maintained between the routers. It is not uncommon to see high router CPU utilization at idle traffic times and then decreased utilization as traffic increases. Polling overhead in the router may increase processor use.
Multidrop Access
There are two ways to configure multidrop operation for the SDLC link in an SDLLC environment.
The first way is to use a line-sharing device or a modem-sharing device (MSD) to connect multiple controllers at a single site to a single SDLC port on the router. The second way is to connect multiple controllers at different sites through a multidrop service provided by a telephone company. For more information about multidrop connections, refer to Appendix B, “IBM Serial Link Implementation
Notes.”
Consider line speed, link utilization, and the number of controllers that will share a single line when designing a multidrop environment. In addition, consider the number of attached LUs associated
with individual PUs, and determine if these LUs are being heavily used. If so, increase the bandwidth of the attached serial line. When implementing multidrop environments featuring large numbers of
PUs and LUs, contact your technical support representative for specific capabilities.
Router Configuration
To configure a router for SDLLC, you need certain virtual telecommunications access method
(VTAM) and NCP definition statements. Figure 5-12 illustrates the required configuration
information.
Designing SDLC, SDLLC, and QLLC Internetworks 5-17
SDLLC Implementation
Figure 5-12
Required end-to-end SDLLC information.
Virtual ring 100
Virtual ring 200
4000 0000 0100
3745
VTAM
FEP
A
B
Arbitrary
Token
WAN
Ring 2
Router
backbone
Router
IBM
3x74
Addr=C1
SWMAJ node:
sdlc address C1:
IDBLK=0E2, IDNUM=00001
sdllc xid C1 0E200001
PATH=10144000000001C1
sdllc traddr 4000 000 0100 200 1 100
TIC address:
4000 0000 0001
sdllc partner 4000 0000 0001 C1
Consider an example of two routers that implement the SDLLC functionality in an environment that
interconnects a remote site to a host channel attached to a 3174 Token Ring gateway, as shown in
Figure 5-13.
Figure 5-13
SDLLC implementation with 3174 Token Ring gateway.
SDLC
Leased
link
line
Router 1
Router 2
3174R
Cluster controller
Channel
Token
Ring
IBM host
3174L
Token Ring gateway
Note Routers also support SDLLC implementations in environments with a 3745 Token Ring
gateway.
The following conditions apply to the sample network illustrated in Figure 5-13:
• The SDLC address of the 3174R is C1.
• The device called 3174L is a 3174 Token Ring that is channel attached to an IBM mainframe.
The 3174R must be defined in the configuration of the 3174L using the virtual Token Ring MAC
address. This address is created in the router configuration; it includes the SDLC address as the last byte. This virtual MAC address is mapped to a host subchannel address. One host subchannel
address is assigned for each downstream physical unit at host system generation time. PU and LU
functions are defined to VTAM within the switched major node function. The following
configuration commands are required on Router 1:
• The sdllc traddr interface configuration command with a virtual ring address for the 3174R.
Note that the last byte must be 00 and that you must specify the appropriate SDLC address (in
this case, C1) for the same last byte during the 3174L customization.
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Cisco CCIE Fundamentals: Network Design
SDLLC Guidelines and Recommendations
• The sdllc partner interface configuration command with the MAC address of the 3174L gateway and the SDLC address of the 3174R.
• The following version of the sdllc xid interface configuration command:
sdllc xid c1 00000000
The sdllc xid interface configuration command is specified with all zeros in the IDBLK/IDNUM
field to establish the LLC session between Router 1 and the 3174L. All zeros in the node ID field of the XID command indicate that there is no unique node identifier in this field.
Encapsulation Overhead
Cisco routers provide several types of encapsulation solutions. Because encapsulation always incurs a certain amount of overhead, you need to assess the advantages and performance trade-offs of each encapsulation solution within the constraints of your environment.
TCP/IP encapsulation is recommended most frequently because it is very robust, provides a high
quality of service, and is media independent. If SDLLC local acknowledgment is required, TCP/IP
encapsulation is required. If SDLLC local acknowledgment is not required, Fast-Sequenced
Transport (FST) encapsulation is highly recommended because it is less CPU intensive.
Direct High-Level Data Link Control (HDLC) encapsulation can be used only in point-to-point
environments. FST and direct HDLC encapsulation are comparable in performance, but FST has