Thu Mar 11 03:54:51 UTC 1993
Silent Nets, CIDR, and Conservation of Routing Table Space
March 10, 1993
Merit has started a project to analyze the NSFNET Policy Routing
Database (PRDB) to look for networks that are not being announced to
the backbone. Enke Chen of Merit has been sending each regional or
midlevel (AS administrator actually) a list of the "silent nets" for
their autonomous system. Our goal is to prune out of the database any
networks not being used and not expected to be used in the near
future. This is important for a couple of reasons: 1) we are
approaching a hard limit in the table size in the ANS backbone routers;
and 2) storing information for the non-announced networks requires
human resources and some router space resources since all of the
configured nets are stored in the configuration file on each router.
As of March 5, there are 10057 networks configured in the database.
The number of silent nets for February was 1955, ie. 1955 networks
which were in the database as of February 1 were never announced in the
entire month. So far we have received requests to delete about 200 of
the silent nets. The maximum number of nets that were announced to the
backbone as of 2/28 was 7037. Historical growth in network
announcements is summarized in the following table.
MONTH MAX RATE(%)
===== ==== ======
08/92 4866 5.9
09/92 5070 4.2
10/92 5432 7.1
11/92 5772 6.3
12/92 6239 8.1
01/93 6654 6.7
02/93 7037 5.8
(Avg monthly growth rate: 6.3%)
The interface forwarding tables on the ANSnet routers are currently
configured to support 10000 destinations. In the near term, microcode
changes will be deployed to support improved address compression in
the forwarding tables which will support 12000 destinations. Later
the gated software when deployed will support a number of enhancements
that increase the forwarding table capacity.
When Classless Inter-Domain Routing (CIDR) and Supernetting is
available (target is June, 1993), regional and midlevel peer routers
will begin announcing aggregated routing information to the backbone.
The backbone will be configured to receive and redistribute aggregated
routes to other networks that support the BGP-4 protocol. This will be
possible since regionals/midlevels have been allocating network numbers
to members and customers from contiguous blocks of network numbers
allocated by the NIC. It is expected that this plan will significantly
curtail the growth of routing table size for networks carrying full
In the interim before aggregation can occur, we as regionals, midlevels
and backbone operators must cooperate to conserve routing table size.
We propose the following steps to help manage the problem,
and would appreciate your feedback on them.
1) Reply to the "silent nets" lists from Enke Chen and indicate
your intent to delete any nets that are not planned to be
announced in the next 30 days.
2) Send in delete requests to nsfnet-admin at merit.edu using the
3) Make sure you are using a CIDR block of class C net numbers
obtained from the NIC for allocation to your member networks.
Only send in requests for addition to the NSFNET database
for those nets which will actually be used within 30 days.
1) Merit will announce our intent to temporarily delete
network numbers from the configuration files that have not been
announced for 90 days. We will retain the nets in the database but
will mark them as "inactive" and will not include them in the router
configuration files. We will re-activate them at such time as the
backbone routing table size limitation is resolved. We will also
re-activate individual nets upon request.
2) If it would be convenient, Merit could set up a holding
database so that network numbers can be submitted which
would not actually be included in the router configuration
files. The holding database could include all of the
regular administrative and routing information associated
with the network. Additional requests could be sent to Merit
to have a network moved from holding status to active.
More information about the NANOG