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TCP/IP Exterior Gateway Protocol (EGP)
(Page 2 of 2)
Routing Information Exchange Process
The actual process of exchanging
routing information involves several steps to discover neighbors and
then set up and maintain communications. Briefly, the steps are:
- Neighbor Acquisition: Each router
attempts to establish a connection to each of its neighboring routers
by sending Neighbor Acquisition Request messages. A neighbor
hearing a request can respond with a Neighbor Acquisition Confirm
to say that it recognized the request and wishes to connect. It may
reject the acquisition by replying with a Neighbor Acquisition Refuse
message. For an EGP connection to be established between a pair of neighbors,
each must first successfully acquire the other with a Confirm
- Neighbor Reachability: After
acquiring a neighbor, a router checks to make sure the neighbor is reachable
and functioning properly on a regular basis. This is done by sending
an EGP Hello message to each neighbor for which a connection
has been established. The neighbor replies with an I Heard You (IHU)
message. These messages are somewhat analogous to the BGP
Keepalive message, but are used in matched
- Network Reachability Update:
A router sends Poll messages on a regular basis to each of its
neighbors. The neighbor responds with an Update message, which
contains details about the networks that it is able to reach. This information
is used to update the routing tables of the device that sent the Poll.
A neighbor can decide to terminate
a connection (called neighbor de-acquisition) by sending a Cease
message; the neighbor responds with a Cease-ack (acknowledge)
As I mentioned earlier, the primary
function in the early Internet was to connect peripheral routers or
groups of routers to the Internet core. It was therefore designed under
the assumption that the internetwork was connected as a hierarchical
tree, with the core as the root. EGP was not designed to handle
an arbitrary topology of autonomous systems like BGP, and cannot guarantee
the absence of routing loops if such loops exist in the interconnection
of neighboring routers. This is part of why BGP needed to be developed
as the Internet moved to a more arbitrary structure of autonomous system
connections; it has now entirely replaced EGP.
An Error message is also defined,
which is similar to the BGP
Notification message in role and structure.
It may be sent by a neighbor in response to receipt of an EGP message
either when the message itself has a problem (such as a bad message
length or unrecognized data in a field) or to indicate a problem in
how the message is being used (such as receipt of Hello or Poll
messages at a rate deemed excessive). Unlike the BGP Notification
message, an EGP router does not necessarily close the connection when
sending an Error message.
Key Concept: The Exterior Gateway Protocol (EGP) was the first TCP/IP exterior routing protocol and was used with GGP on the early Internet. It functions in a manner similar to BGP: an EGP router makes contact with neighboring routers and exchanges routing information with them. A mechanism is also provided to maintain a session and report errors. EGP is more limited than BGP in capability and is now considered a historical protocol.
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Version 3.0 - Version Date: September 20, 2005
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