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.
distance vector and link-state, is found in Chapter 5. Specific routing protocols for TCP/IP and
IPX are listed in Chapter 6, "Routing."
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.
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.
(Consecutively
numbered values in this
field can be combined
into one group, called a
cable range.)
sizes
(DSP) (typically 56,
including NSAP)
(Bits)
Grouping Field
Local Address Field