In the early days of the Internet very little examination of the
contents of the Routing Table had been carried out. While there
were many discussions about the size of the table, only a one or
two efforts at recording the size and contents of the table had
With the introduction of CIDR and the transition from classful to
classless addressing, the CIDR Report
was instigated by Tony Bates as an effort to encourage ISPs to
drop classful prefixes in favour of classless aggregates. The CIDR
Report looked at the Internet Routing Table on a global scale and
listed the top 30 ISPs who were contributing to the size of the
The CIDR Report was all that was really available to the general
public, and examination of the table on a global scale ignored
regional variations. The Routing Report was born out of this need.
The current Routing Report takes a BGP feed from host organisations
who volunteer an EBGP Multihop feed with the collector hosted by
NSRC at the University of Oregon.
When the report was originally established in 1999, the Routing
Report accessed APNIC's router at
DIX-IE in Japan. This router has a view
of the full Internet Routing Table as received from APNIC's transit
providers at DIX-IE. This report still operates and can be accessed
Initially the analysis was targeted at the Asia Pacific Region,
APNIC's service region, giving statistics on the Routing Table size
and the APNIC region ISP's contribution to the Routing Table.
However, other regions showed interest in the details delivered
by the report, and very soon it was extended to include other RIR
regions, most recently with AfriNIC joining the RIR system. The
report is now sent to the major operations lists around the world,
on a request basis.
The report is produced on a weekly basis, with the mail shot based on
data captured at 4am Australian Eastern Standard Time (+10GMT) every
Saturday. The report is actually run on a daily basis, with
a specialist mailing list
as the destination for interested subscribers.
The weekly mailshot is currently sent to the
RIPE Routing WG, and
AfNOG lists amongst others.
Explaining the Report
This section explains the contents of the report based on the headings
contained in it.
- BGP Routing Table Entries Examined: Lists the number of unique routing
entries in the Internet Routing Table.
- Prefixes after Maximum Aggregation: Lists how large the Internet
Routing Table would be after aggregating as much as possible. No attention
is paid to origin AS or AS paths, so no allowance is made for deaggregation
requirements to aid traffic engineering for multihoming.
For example, if 172.16.0.0/24, 172.16.8.0/21 and 172.16.0.0/16 are
all present in the BGP table, "maximum aggregation" will remove
the 172.16.0.0/24 and the 172.16.8.0/21 announcements, leaving
just 172.16.0.0/16. The origin AS and AS PATH of each of these
announcements is ignored in making this calculation.
- Deaggregation Factor: Lists the ratio of "Total BGP Table" and
"Maximum Aggregation". A perfectly aggregated BGP table would give
a Deaggregation Factor value of 1.0.
- Unique Aggregates Announced to Internet: Lists how large the
Internet Routing Table would be after aggregating the prefixes present
in the Internet Routing Table by origin AS.
For example, if 172.16.8.0/21 and 172.16.0.0/16 is originated
by AS100, we'd have just one unique aggregate, 172.16.0.0/16.
However, if 172.16.8.0/21 is originated by AS200 instead, then
there would be two unique aggregates. Also, if AS100 originates
172.16.0.0/16 and 172.17.0.0/16, then we'd have just one unique
- Total ASes present in the Internet Routing Table: Lists the
total number of 16-bit ASNs visible in the Internet Routing
- Prefixes per ASN: Lists the average number of prefixes announced
per 16-bit ASN.
- Origin-only ASes present in the Internet Routing Table: Lists
the ASNs that only originate prefixes - they do not provide transit
to any other ASN in this view.
- Origin ASes announcing only one prefix: Lists the origin ASNs
which announce just one prefix to the Internet.
- Transit ASes present in the Internet Routing Table: Lists those
ASes which provide transit to other ASNs in the Internet.
- Transit-only ASes present in the Internet Routing Table:
Lists those ASes which are only providing transit to other ASNs -
they do not originate any prefixes at all.
- Average AS path length visible in the Internet Routing Table: Lists
the average AS depth of the Internet (including prepends).
- Max AS path length visible: Lists the longest AS path (including
prepends) visible in the Internet Routing Table.
- Max AS path prepend: Lists the largest AS path prepend seen
in the Internet Routing Table and which ASN is prepended that often.
Intended to highlight silly prepends.
- Prefixes from unregistered ASNs in the Routing Table: Lists prefixes
being originated by either private, reserved, or unallocated ASNs.
- Unregistered ASNs in the Routing Table: Counts all the private,
reserved or unallocated ASNs, including prepends.
- Number of 32-bit ASNs allocated by the RIRs: Counts the number
of 32-bit ASNs which have been distributed by the RIRs.
- Prefixes from 32-bit ASNs in the Routing Table: Counts the number
of prefixes being originated from 32-bit ASNs. Router does not yet
support 32-bit ASNs, so this simply counts the prefixes being originated
- Special use prefixes present in the Routing Table: Lists any private
or special use address space (as defined in RFC3330)
announced to the Internet Routing Table. At this stage, the analysis
lists 0/8, 10/8, 127/8, 169.254/16, 172.16/12, 192.0.2/24, 192.168/16,
198.18/15 and 224/3.
- Prefixes being announced from unallocated address space: Lists
prefixes being announced from address space which is still in
the IANA reserved pool. This is basically all the /8s listed as
as well as address blocks from the former B-space (128/2) and the first
block of C-space (192/8) for which no registration information can be found.
- Number of addresses announced to Internet: Lists the amount of address
space being announced to the Internet. As well as a 32-bit integer, it is
also displayed in more human readable terms using natural address mask sizes.
Should the default route be announced, this is ignored in the calculation
of addresses announced, as that announcement is an error, and it isn't
desirable for the tally to be skewed by this error.
- Percentage of available address space announced: Lists how much of the
total usable IPv4 address space is announced to the Internet. Usable
address space is everything apart from most of the special use address
space described in RFC3330)
and specifically documented above.
- Percentage of allocated address space announced: Lists how much of the
address space which has been assigned to endsites and allocated to the RIRs
is announced to the Internet.
- Percentage of available address space allocated: Lists how much of the
available address space has been assigned to endsites and allocated to RIRs
- this shows how close to we are to the exhaustion of the IPv4 address pool.
Registry Region Analysis Summary
- Prefixes being announced by RIR Region ASes:
Lists the number of unique routing entries being announced by
ASNs which are listed as being located in the service region
covered by the specified RIR.
- Total RIR Prefixes after Maximum Aggregation:
Lists the contribution to the Internet Routing Table after
aggregating by RIR region origin ASN - no attention is paid to
different paths, so no allowance is made for deaggregation for
traffic engineeing to aid multihoming.
- Prefixes being announced by RIR address blocks:
Lists the number of unique routing entries being announced from
address blocks which have been allocated to the RIR.
- Unique Aggregates Announced from the RIR address blocks:
Lists the contribution to the Internet Routing Table after
aggregating just the prefixes from the address blocks allocated
to the RIR - it ignores the originating ASN.
- RIR Region origin ASes present in the Internet Routing Table:
Lists the number of ASNs from the RIR region visible in the
Internet Routing Table.
- RIR Origin ASes announcing only one prefix:
Lists the origin ASNs from the RIR region which announce just
one prefix to the Internet.
- RIR Transit ASes present in the Internet Routing Table:
Lists those ASes from the RIR region which provide transit to
other ASNs in the Internet.
- Average RIR Region AS path length visible in the Internet Routing Table:
Lists the average AS depth of the Internet in the RIR region
- Max RIR AS path length visible:
Lists the longest AS path (including pre-pends) visible in the
Internet Routing Table in the RIR region only
- Unregistered ASNs in the Routing Table:
Counts all the private, reserved or unallocated ASNs, including
- Number of RIR region addresses announced to Internet:
Lists the amount of address space being announced to the Internet
from the RIR region. As well as a 32-bit integer, it is also
displayed in more human readable terms using natural address
- Percentage of available RIR address space announced:
Lists how much of the total usable IPv4 address space allocated
to the RIR is announced to the Internet. Usable address space
is everything apart from most of the special use address space
described in RFC3330
- RIR AS Blocks:
Lists all the AS blocks which were originally allocated to the
RIR. Since the completion of the recent ERX transfer project,
there are now substantial holes in these blocks - documentation
of these holes can be found on the RIR websites.
- RIR Address Blocks:
Lists all the address blocks which have been allocated to the
RIR Region Per AS Prefix Count Summary
This section lists the top 20 ASNs per RIR service region and
their contribution to the Internet Routing Table. This is not
the same as listing the largest ISPs, but those who are contributing
most prefixes to the Internet. The columns list the ASN and the
registered name of the ASN holder, the number of prefixes being
announced, and the equivalent size in units of /20 blocks, and
the number of prefixes they would announce if the ASN operator
attempted to aggregate their announcements better.
Global Per AS Maximum Aggregation Summary
This section lists the top 20 ASNs who would contribute most to
reducing the size the Internet Routing Table if they would perform
better aggregation of their BGP announcements.
List of Unregistered Origin ASNs (Global)
This section lists any unallocated ASNs in use on the Internet.
Mostly this captures private ASNs, but can also capture unallocated
or ASNs still in the IANA reserved pool.
Advertised Unallocated Addresses
This section lists any prefixes being announced from unallocated
Number of prefixes announced per prefix length (Global)
This section lists how many prefixes are being announced per
prefix length in the Internet Routing Table
Advertised prefixes smaller than registry allocations
This section lists the top 20 ASNs which are announcing prefixes
smaller than the minimum allocations made by the RIRs. It indicates
the top deaggregators in the Internet.
Number of /24s announced per /8 block (Global)
This section lists how many /24s are being announced out of each
of the /8 address blocks. It is designed to show where all the
/24s in the Internet Routing Table are coming from.
The analysis software generally assumes it is going to receive a file
which is the snapshot of BGP feed from a Cisco IOS router. Generally a
show ip bgp summary" is sufficient.
The analysis software will take the BGP table from the router, and
process the best paths only, ignoring alternative paths. The idea is
that it makes most sense to use the paths that the router considers
best, rather than anything else.
If the feed provided to the router has no best path in it at all (for
example, it might be the BGP feed received from a peer only), the
software will process that in its stead. Where multiple paths to the
destination are available, with no best path chosen by the router, the
software will take the first path in the list (basically the oldest
path as the router sees it).
It is hoped that the data that the Routing Report supplies is useful
to service providers and Internet operations observers. While the
original aim was to encourage aggregation on a regional basis, it now
has a much larger role of providing info on various Internet
I really do hope that the analysis is bug free. It wouldn't
surprise me if there are errors though. If you find any, please let me
Some of the older reports had some small errors in them - while I've
fixed errors in the software, I haven't gone back and re-run the
I have no plans as yet to look at the IPv6 Routing Table. If you
really need to see what it is doing, please look at the
IPv6 version of the CIDR
Acknowledgements go to the many people who have reported bugs in
the analysis software, to Tony Bates for original inspiration, and
to Geoff Huston and Ronald F. Guilmette for many constructive suggestions
Also, appreciative thanks to APNIC for permitting access to their
Japanese node router, and for providing the server to run the
analysis software and store the copious quantity of data it
And finally thanks to NSRC for this version of the Routing Report which is
focusing on the global Research and Education network infrastructure.
Philip Smith - March 2021
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