Network Working Group J. Rekhter
Request for Comments: 1092 T. J. Watson Research Center
February 1989 EGP and Policy Based Routing in the New NSFNET Backbone
Status of this Memo
This memo discusses implementation decisions for routing issues in
the NSFNET, especially in the NSFNET Backbone. Of special concern is
the restriction of routing information to advertize the best route as
established by a policy decision. Distribution of this memo is
The NSFNET backbone routes packets between the Regionals Networks to
which it is connected, (i.e., the packets arriving at a backbone
entry node are routed to an exit node). How they travel through the
network is determined by two components:
the NSFNET backbone routing protocol/algorithm, and
additional information about the externally connected networks.
This paper is concerned with how reachability information between the
external networks and the NSFNET backbone is exchanged so that
packets can be routed to the correct destination by using a
EGP as reachability protocol
The EGP (Exterior Gateway Protocol) routing method will be used to
exchange reachability information between the NSFNET backbone and the
There are several problems with using EGP as a reachability protocol
for routing in a meshed environment. Some EGP components require
further definitions for the NSFNET backbone - regional network
interactions. It should be noted that the use of EGP is only viewed
as an interim measure until better inter autonomous system protocols
are defined and widely deployed for gateways used by regional
The following is a list of some EGP problems and issues:
The EGP model assumes an engineered spanning tree topology,
however, the NSFNET (due to the presence of backdoor routes) does
not fit into this model. In the NSFNET the same network may be
advertized as reachable by more than one regional network.
Besides the fact that the overall NSFNET does not fit into a
spanning tree model there are serious concerns with the concept
of the "core" (central to the EGP) and its obvious deficiencies.
While EGP is going to isolate intra-Regional routing from the
intra-NSFNET-Backbone routing, it does not address the issue of
false information which may be supplied by regional networks.
EGP by itself does not protect a particular network from unwanted
and unsolicited representation by some regional network. As an
example, if network N1 is reachable through regional network R1
as well as through regional network R2, EGP has no provisions to
specify one of these paths as a primary and one as a secondary,
since there is not generally accepted interpretation of EGP
metrics today. Also, there is nothing in EGP which can prevent one
or more regional networks from advertizing other networks (in
particular, networks which belong to other regional networks) as
reachable with zero distance. This could result in the creation
of a "black hole" or at least in suboptimal IP routing.
EGP by itself has no provisions to guarantee that routes through
the NSFNET Backbone will be preferred over routes through the
backdoor routers or vice versa.
Policy Based Routing
Looking at the problems listed above the appearance of the new
factors like autonomy and mutual trust becomes obvious. While trying
to achieve the routing functionality required for the new NSFNET
backbone we should realize that one of our primary concerns has to be
the accommodation of those new factors.
This means that some kind of a rudimentary Policy Based Routing
method becomes imperative. We would like to emphasize, however, that
we are not talking about complete Policy Based Routing, but that we
are rather concerned about supporting a minimum subset of a policy
functionality to be an initial solution to the above mentioned
problems. This requires support and cooperation between the
management of each of the networks connected to the NSFNET backbone.
We need to support the ability of a particular network N, which
belongs to one of the regional networks, to establish a bilateral
agreement with one or more regional networks of the type "network N
can be reached via one or more regional networks (RN1, RN2, ...
RNx)". This allows each network to select one or more
representatives at the regional network level. Once this agreement
is established the information will be available to:
The network which initiated the agreement.
The management of the regional network(s) with whom this
agreement has been established.
The NSFNET backbone Network Operation Center where it will be
entered into the Routing Policy Data Base which will be available
through the NSFNET information services.
Supporting multiple routes to the NSFNET core requires the guarantee
that for a certain network N, no regional network other than the
one(s) selected by N, will advertize N as reachable, which
necessitates that the NSFNET core will ignore unauthorized
advertisements for network N.
EGP and Rudimentary Policy Based Routing
Each network which belongs to the NSFNET will select a specific
regional network as its primary representative to the NSFNET core by
bilateral agreement with the management of same regional network as
well as the NSFNET backbone management. The same network can
furthermore select an arbitrary number of other regional networks as
their secondary, tertiary, etc., representative by establishing
bilateral agreements with the management of the corresponding
regional networks as well as the NSFNET backbone management.
Reachability information supplied by each regional network will be
distributed to all other NSS nodes of the NSFNET Backbone. We would
like to emphasize that we are not going to flood EGP packets
internally within the backbone, but to rather use the learned
information for the interior gateway protocol, which uses the ANSI
The implementation allows for a defined regional network to advertize
a particular leaf network in the EGP NR packets with a distance of
zero. Secondary representatives may advertize the same network with
distance one or higher. If the path through the primary regional
representative is available all secondary paths will be ignored. If
the path through the primary regional representative goes down (which
will be discovered via the EGP NR information), the next path with
the lowest available EGP metric will be used.
We will also be able to detect and report unsolicited
representations. This will be done by examining (on a periodic
basis) all reachability information obtained via EGP. The result
will be compared against the Routing Policy Data Base which will hold
information about all bilateral agreements between networks and their
regional representatives. Any mismatch will cause an alarm to the
Network Operations Center. For example, network N established a
bilateral agreement with the regional network R1 electing it as its
primary representative. The EGP NR record received from the regional
network R5 advertizes the network N as reachable with distance zero.
By comparing the Routing Policy Data Base entry for the network N
with the EGP NR record a mismatch will be detected and an alarm is
forwarded to the Network Operation Center.
Since the whole scheme is based on a combination of the network
number and the autonomous system number, to allow for further
verification, it is also important to insure the correctness of the
autonomous system numbers as advertized by the regionals networks to
the NSFNET core.
The autonomous system number validation for each regional network
will be performed at the NSS which connects the particular leaf
network to the NSFNET backbone. All discrepancies wil be reported to
the Network Operations Center.
The NSFNET backbone will be considered as a separate Autonomous
System with its own autonomous system number.
Backbone versus Backdoor Routes
There are instances where regional networks prefer paths through some
backdoor route over paths through the NSFNET backbone. Therefore,
the reachability information advertized by the NSFNET core to the
regional networks (via EGP NR records) will always use a fixed metric
of 128 for all routes. This may aid to encourage traffic to flow
through backdoors, if desired and available.
The regional networks can use a variety of techniques to determine
how they route traffic for any particular network at their own
What do we expect from the Regional Networks
Each regional network should get its own Autonomous System number.
The connection between regional networks to NSFNET backbone will be
done via EGP. It is the responsibility of the regional backbone to
provide an EGP functionality via the attachment to the E-PSP
dedicated to the regional network.
The EGP functionality may require a translation of network numbers in
and out of the regional network. In any case, the NSFNET backbone
expects individual network numbers of the leaf networks of the
regional network, as long as they should be advertised, and will
announce individual networks known to the NSFNET core to the regional
The EGP support should includes the ability to configure EGP metrics
from some statically definable configuration table. If the EGP
metrics cannot be defined or if they are not fixed the metric
determination will be done by the NSFNET backbone routers, as taken
from their databases, themselves. In that case, it is the
responsibility of the regional network to provide the NSFNET backbone
management with the metric data to allow for proper use of metrics.
We also expect each regional network to handle all bilateral
agreements with its leaf networks regarding Policy Based Routing and
supply a copy of those agreements to the NSFNET backbone management.
I would like to express my thanks to Barry Appelman (T.J. Watson
Research Center, IBM Corp.) and Hans-Werner Braun (Merit) for their
contributions to this document.
T.J. Watson Research Center
P.O. Box 218
Yorktown Heights, NY 10598
Phone: (914) 945-3896