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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Internet Layer (OSI Network Layer) Protocols
           9  TCP/IP Routing Protocols (Gateway Protocols)
                9  TCP/IP Interior Routing Protocols (RIP, OSPF, GGP, HELLO, IGRP, EIGRP)
                     9  Other Interior Routing Protocols

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The HELLO Protocol (HELLO)
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TCP/IP Gateway-to-Gateway Protocol (GGP)
(Page 2 of 2)

Propagation of Routing Table Information

One interesting difference between GGP and RIP is that in GGP, networks and costs aren't sent in pairs. Instead, a GGP router sends its routing table in groups. If it has three networks it can reach at a cost of 1 it sends those in a group with distance value 1, then if it has a few at a cost of 2 it sends those in a group with distance value 2, and so on. Another difference is that GGP Routing Update messages are acknowledged. Each Routing Update message is sent with a sequence number, which is used to ensure that out-of-date information is not propagated. If the Routing Update is received and has a new sequence number (indicating that it is recent information), the router processing it sends back a GGP Acknowledgment message back to the originator. If the sequence number indicates the message is stale, a Negative Acknowledgment is sent instead and the message discarded.

As a distance-vector algorithm using hop count as a metric, GGP shared most of the same pros and cons as RIP. It had simplicity on its side, but had various problems such as slow convergence and issues such as the “counting to infinity” problem. GGP was a much more rudimentary protocol than RIP, however, and did not include many of the features included in RIP to handle such issues, such as split horizon. GGP was also limited to unsubnetted classful networks, due to its age.

Current Role in TCP/IP

When Internet architecture moved to the use of autonomous systems, GGP was obsoleted. While it was an important part of TCP/IP history, it is today not formally considered a part of the TCP/IP protocol suite.

Key Concept: The Gateway-to-Gateway Protocol (GGP) was used to communicate route information between core routers on the early Internet. It is a distance-vector protocol that operates in a manner very similar to RIP. Each router periodically sends out its routing table to neighboring routers, so each router can learn the cost, in hops, to reach every network in the autonomous system. GGP is now considered a historical protocol and is no longer part of TCP/IP.



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Other Interior Routing Protocols
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Next Page
The HELLO Protocol (HELLO)
Next Topic/Section

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