January 2002
Beginner
238 pages
6h 20m
English
Content preview from IP Routing
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RIP-2 Packet Format
The additions in the RIP-2 update occupy the “must be zero” or unused fields in the RIP-1 update. This careful selection of fields allows older (pre-RIP-2) implementations of RIP to interpret a RIP-2 update by just ignoring the new fields. Let’s look closely at the fields in the RIP-2 update shown in Figure 5-2.
Figure 5-2. Format of RIP-2 update packet
RIP-2 updates are encapsulated in UDP port 520, like RIP-1 updates.
However, the destination IP address for a RIP update can be the
all-ones broadcast address of 255.255.255.255 or
the reserved multicast address of 224.0.0.9. The
use of the reserved multicast address frees devices not listening to
RIP-2 from the task of unwrapping RIP-2 updates.
The fields AFI, IP address, and metric have the same semantics as in a RIP-1 update packet. See Chapter 2 for details on these fields. The version field in RIP-2 updates is 2.
The route tag field is not used by RIP but can be used to carry an attribute assigned to the route, such as the AS number of the EGP (for example, BGP) from which the route was imported. The use of route tags is discussed further in Chapter 8.
The subnet mask field in each route entry classifies RIP-2 as a classless routing protocol and permits the use of VLSM and the support of discontiguous networks.
The next hop IP address is usually identical to the IP address of the source of the RIP update. For example, ...
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