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).
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