background image
110 Chapter 3: OSI Reference Model & Layered Communication
If a protocol is not routable, then bridging must be enabled to support those protocols. (Bridging
concepts are covered in Chapter 4.) To support nonroutable protocols over WAN links, some
other protocol must be used, such as encapsulated transparent bridging and data link switching
(a form of remote bridging for SNA and NetBIOS).
The details of how to support nonroutable protocols is beyond the scope of CCNA. What is
reasonably expected to be in the scope of CCNA is to know the most popular nonroutable
protocols. Consider Table 3-11, which lists protocols that some people consider to be
nonroutable:
DEC LAT and NetBIOS (sometimes referred to as NetBEUI, for NetBIOS End User Interface)
are definitely nonroutable. IBM's SNA has two general categories: Subarea SNA is the
traditional Mainframe DataCenter SNA, and Advanced Peer-to-Peer Networking (APPN) is a
newer, more easily routable variation. Both are routable, have Layer 3 addressing, and can be
routed by products you can purchase today. However, be careful--Cisco folklore has it that
SNA is not routable. If CCNA exam questions touch on this topic, focus on the context and be
sure to remember that LAT and NetBIOS are truly nonroutable.
This section, however, presents an anecdote that may help you remember the difference
between the terms routing, routed protocols, and routing protocols.
NOTE
This somewhat silly story is the result of the Cisco World Wide Training division's proctors for
the instructor certification process, who emphasize that the instructors should be creative in the
use of tools to help students remember important details. After I tried this story during
certification, it was propagated by other instructors. I am curious--if you have heard this story
or a variation, please let me know when you heard it and from whom (wendell@lacidar.com).
Table 3-11
Purported Nonroutable Protocols
Protocol
Do Protocol Specifications
Allow Routing?
Does IOS Support Routing
This Protocol?
DEC Local Area Transport
(LAT)
No
No
NetBIOS
No
No
SNA (Traditional Subarea SNA)
Yes; routed by IBM products
running VTAM and NCP
No
SNA (APPN)
Yes
Yes
ch03.fm Page 110 Monday, March 20, 2000 4:58 PM