Visara CNA-8000 Planning Guide - Manual (Page 18)

Chapter 2. Support of PU 2 Platforms
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should result in a successful connection unless another node has already an established
connection to it. Once a connection is established VTAM will commence to activate the
SNA PU (PU type 2.0) and associated LUs. If the PU type is 2.1, then a peer session
activation occurs.
The CNA-8000 maintains a list of failed XID attempts. This list can be used to help
identify misconfigured nodes on the network and to identify failed attempts to hack the
network.
If multiple XCA gateways are defined to the same VTAM, the downstream PU can
connect through any of them to establish its session with VTAM. VTAM maintains a
connection through only one of the XCAs to a specific PU at a time. All communication
between VTAM and the LUs supported by that PU travel over the same established path.
If connection is lost, the downstream PU can establish a new connection through a
different XCA. The CNA-8000 supports XCA traffic between the FICON or ESCON
channel and Ethernet (LLC), Token Ring (LLC), and SDLC.
The CNA-8000 can also direct XCA traffic to its internal DLSw feature. When routed
through the DLSw feature, the SNA packets are encapsulated into IP and presented to the
configured Ethernet interface where it can be routed through the IP network to its
destination, another DLSw capable platform.
Using Multiple XCA Paths
One of the big advantages of using XCA technology is the ability for a node to be able to
access VTAM through multiple paths. In order for a downstream PU to establish a host
connection through multiple XCA gateways, one of two things must happen. Either the
downstream PU must have the ability to communicate with multiple upstream network
addresses, or there must be a way for the network address to exist in two different
locations. Token Ring provides an environment that allows for the same network address
to exist in two different places when source route bridging is used. Refer to the diagram
below.
When source route bridging is used, a downstream node will initially attempt to contact a
session partner using a Test frame, first with Source Route Bridging turned off, and if
unsuccessful in making contact another Test frame is sent using source route bridging.
Nodes are allowed to have identical MAC addresses as long as they are separated by a
Source Routing Bridge. Once a response is received from the target node a session is
established and the same route is used for the duration of the session.