OSI Network Layer Functions 109
For more information about IP and IPX addresses, refer to Chapter 5.
Routing Protocols
Conveniently, the routing tables in the example based on Figure 3-18 all had the correct routing
information already in their routing tables. In most cases, these entries are built dynamically by
use of a routing protocol. Routing protocols define message formats and procedures, just like
any other protocol. With routing protocols, however, the goal is not to help with end-user data
delivery--the end goal is to fill the routing table with all known destination groups and with the
best route to reach each group.
A technical description of the logic behind two underlying routing protocol algorithms,
distance vector and link-state, is found in Chapter 5. Specific routing protocols for TCP/IP and
IPX are listed in Chapter 6, "Routing."
Nonroutable Protocols
In the early and mid-1990s, one of the reasons that Cisco sold a lot of routers is that the IOS
could route more Layer 3 protocols than most--if not all--competitors. However, some
protocols are not routable. To support those, Cisco supported and evolved variations of bridging
to support nonroutable protocols.
What makes a protocol nonroutable? Basically, a protocol stack that does not define an OSI
Layer 3 equivalent, including a logical Layer 3 address structure, cannot be routed. To be fair,
because the answer to the question "Is a protocol routable?" for any particular protocol is more
of a geek-party discussion, there are no hard and fast rules that govern what has to be true for a
protocol to be considered routable. As this chapter shows, however, forwarding packets
(L3PDUs) based on a destination Layer 3 equivalent address involves routing; a protocol stack
with no Layer 3 is considered nonroutable.
AppleTalk
24
Network (16)
(Consecutively
numbered values in this
field can be combined
into one group, called a
cable range.)
Node (8)
OSI
Variable
Many formats, many
sizes
Domain Specific Part
(DSP) (typically 56,
including NSAP)
Table 3-10
Layer 3 Address Structures (Continued)
Protocol
Size of Address
(Bits)
Name and Size of
Grouping Field
Name and Size of
Local Address Field
ch03.fm Page 109 Monday, March 20, 2000 4:58 PM