Data Link Switch (DLSw) - A look at Cisco's implementation of data link switch

I have spent the last few weeks writing about topics that appear on the

CCIE qualification (written) exam that I do not
have much exposure to. The last three weeks I wrote about Token Ring and
Source-Route Bridging (SRB). This week I would like to write about how Cisco
handles transporting SNA traffic over the WAN, or their implementation of data
link switch (DLSw).


DLSw was developed in the early 1990s as a method to transport IBM Systems
Network Architecture (SNA) and NetBIOS traffic (RFC 1434, 1993). Version 1 of
DLSw was introduced in 1995 with the release of RFC 1795 (superceding RFC 1434).
SNA operates at the data link layer of the OSI model, or the SNA data link layer
of the SNA architecture (conveniently layer 2 in both models). Traditional
routing protocols operate at the OSI network layer, so DLSw is said to switch
the SNA traffic. DLSw uses the Switch-to-Switch protocol (SSP) to forward
traffic between DLSw endpoints, or peers. To route traffic over WAN links, or
the Internet, DLSw encapsulates the SNA or NetBIOS traffic in IP.


Initializing a DLSw connection involves a three-stage process. First, a peer
connection must be established. DLSw switches (routers in DLSw can be referred
to as switches--this can be confusing) set up a TCP connection. Once the peer
connection is established, the DLSw switches exchange their capabilities.
Typically exchanged are DLSw version number, initial pacing window size (receive
window size), NetBIOS support, list of supported link SAPs (LSAPs), number of
TCP sessions supported. Optionally, MAC address lists and NetBIOS name lists can
be exchanged to limit broadcasts and type of search frames supported can be
passed to limit the amount of discovery traffic. The final stage of DLSw
communication is to establish the connection circuit. Circuit establishment is
dependant on the type of traffic, but involves locating the target (destination)
end station and opening a data link connection between each endpoints and its
DLSw switch.


Cisco's implementation of DLSw is known as DLSw+. DLSw+ adds many enhancements
to DLSw. It is important to note that DLSw+ is fully compliant with RFC 1795;
only new functionality is added. Features added to standard DLSw by DLSw+ are:


- Peer groups and border peers

- Backup peers

- Promiscuous and on-demand peers

- Explorer firewalls and location learning

- NetBIOS dial-on-demand routing feature support

- UDP unicast support

- Load balancing

- Support for LLC1 circuits

- Support for multiple bridge groups

- Support for RIF Passthru

- SNA type of service feature support

- Local acknowledgment for Ethernet-attached devices and media conversion for
SNA PU 2.1 andPU 2.0 devices

- Conversion between LLC2 to SDLC between PU 4 devices

- Local or remote media conversion between LANs and either the SDLC Protocol or
QLLC

It is important to note that both connection peers must support DLSw+ (Cisco
devices only). A DLSw+ device can automatically detect if the remote peer
supports DLSw+ and can change to standards compliance mode if it is not.

DLSw switches can be grouped into peer groups where all the DLSw switches peer
with each other. Each peer group has one or more border peers that communicate
with each other. This eliminates the need for every DLSw switch to peer with
every other DLSw switch on a network and reduce explorer traffic and broadcasts.

DLSw+ allows SNA and NetBIOS traffic to be encapsulated in the standard TCP/IP
frames. DLSw+ will also transport SNA and NetBIOS traffic point-to-point WAN
links using HDLC or Frame Relay.

If you want to learn more about Cisco's implementation of DLSw, the links below
are an excellent start. You should also take the time to become familiar with
IBM's SNA. There is still quite a bit of SNA out there, so the information might
just help you in the real world (and not just on the test).

The DLSw RFC 1795

DLSw
documentation from Cisco

Cisco DLSw+ design guide