EXTERNAL GATEWAY PROTOCOL
Definition
Exterior Gateway Protocol. A protocol that collects and shares routing information among
the routers connecting autonomous systems. The term gateway is historical; router is
currently the preferred term.
The Origins of EGP
In the early 1980s, the routers (gateways) that made up the ARPANET (predecessor
of the modern Internet) ran a distance vector routing protocol known as the
Gateway-to-Gateway Protocol (GGP). Every gateway knew a route to every
reachable network, at a distance measured in gateway hops. As the ARPANET grew,
its architects foresaw the same problem that administrators of many growing
internetworks encounter today: Their routing protocol did not scale well
Operation of EGP
Version 1 of EGP was proposed in RFC 827. Version 2, slightly modified from version 1,
[2]
was proposed in RFC 888 , and the formal specification of EGPv2 is given in RFC
[3]
904 .
EGP Topology Issues
EGP messages are exchanged between EGP neighbors, or peers. If the neighbors are
in the same AS, they are interior neighbors. If they are in different autonomous
systems, they are exterior neighbors. EGP has no function that automatically
discovers its neighbors; the addresses of the neighbors are manually configured, and
the messages they exchange are unicast to the configured addresses.
Shortcomings of EGP
The fundamental problem with EGP is its inability to detect routing loops.
Because there is an upper boundary on the distance EGP uses (255), you
might be tempted to say that counting to infinity is at least a rudimentary
loop-detection mechanism. It is, but the high limit combined with the typical
Poll interval makes counting to infinity useless. Given a default Poll interval of
180 seconds, EGP peers could take almost 13 hours to count to infinity.
As a result, EGP must be run on an engineered loop-free topology. Although
that was not a problem in 1983, when EGP was intended merely to connect
stub gateways to the ARPANET backbone, the creators of EGP already foresaw
that such a limited topology would soon become inadequate. The autonomous
systems making up the Internet would need to evolve into a less structured
mesh, in which many autonomous systems could serve as transit systems for
many other autonomous systems.
Configuring EGP
You can configure EGP on a router in four basic steps:
Step 1. Specify the router's AS with the command autonomous-system.
Step 2. Start the EGP process and specify the neighbor's AS with the command
router egp.
Step 3. Specify the EGP neighbors with the neighbor command.
Step 4. Specify what networks are to be advertised by EGP.
Troubleshooting EGP
The earlier section "Shortcomings of EGP" discussed several reasons why EGP cannot
be used in complex inter-AS topologies. An unexpected benefit is that by forcing a
simple topology, EGP is easy to troubleshoot.
As with any routing protocol, the first step in troubleshooting EGP is examining the
routing tables. If a required route is missing or an unwanted route is present, the
routing tables should lead you to the source of the problem. Because the EGP metrics
have very little meaning, using the routing tables for troubleshooting is greatly
simplified in comparison with other routing protocols.
The Exterior Gateway Protocol
(egp) Group (1.3.6.1.2.1.8)
The EGP protocol is used by routers to exchange routing
information between different Autonomous System .
Indeed, internet routing is organised in areas called
Autonomous System. A common protocol, the Internal
Gateway Protocol (IGP) , such as RIP or OSPF is used for
routing inside an Autonomous System. However, when
neighbouring routers want to exchange information
between two Autonomous System, they use an External
Gateway Protocol (EGP).
The egp subtree is composed of variables that records EGP
traffic, the EGP Neighbour Table (egpNeighTable) and the
EGP Autonomous System variable (egpAs).
The EGP Neighbour Table
This table provides information about the neighbouring of
the system. This information are the IP Address of this
neighbour, its Autonomous System number, some counts of
incoming and outgoing EGP messages and others variables
such as timers and a control variable.
The EGP Autonomous System Variable
This variable contains the Autonomous System number for
the EGP router. The Autonomous System Number of the
sender is included in the header of each EGP message that
it transmits.