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The Exterior Gateway Protocol


The Exterior Gateway Protocol (EGP) is an exterior routing protocol used for exchanging routing information with gateways in other autonomous systems. Unlike interior protocols, EGP propagates only reachability indications, not true metrics. EGP updates contain metrics, called distances which range from 0 to 255. GateD will only compare EGP distances learned from the same AS.

Before EGP sends routing information to a remote router, it must establish an adjacency with that router. This is accomplished by an exchange of Hello (not to be confused with the HELLO protocol, or OSPF HELLO messages) and I Heard You (I-H-U) messages with that router. Computers communicating via EGP are called EGP neighbors, and the exchange of HELLO and I-H-U messages is referred to as acquiring a neighbor. Once the neighbor is acquired, the system polls the neighbor for routing information. The neighbor responds by sending an update containing routing information. If the system receives a poll from its neighbor, it responds with its own update packet. When the system receives an update, it includes routes from the update into its routing database. If the neighbor fails to respond to three consecutive polls, GateD assumes that the neighbor is down and removes the neighbor's routes from its database.

The EGP Statement

egp yes | no | on | off { preference preference ; defaultmetric metric ; packetsize number ; traceoptions trace_options ; group [ peeras autonomous_system ] [ localas autonomous_system ] [ maxup number ] { neighbor host [ metricout metric ] [ preference preference ] [ preference2 preference ] [ ttl ttl ] [ nogendefault ] [ importdefault ] [ exportdefault ] [ gateway gateway ] [ lcladdr local_address ] [ sourcenet network ] [ minhello | p1 time ] [ minpoll | p2 time ] [ traceoptions trace_options ] ; } ; } ] ;

Tracing options

The state and policy options work with EGP. Packet tracing options (which may be modified with detail, send and recv):

All EGP packets

EGP HELLO/I-HEARD-U packets which are used to determine neighbor reachability.

EGP ACQUIRE/CEASE packets which are used to initiate and terminate EGP sessions.

EGP POLL/UPDATE packets which are used to request and receive reachability updates.

next up previous contents index
Next: BGP Up: Configuration Guide Previous: PIM

Laurent Joncheray
Wed Jun 12 15:35:22 EDT 1996