ISPs who don't and won't aggregate
are generally held in poor regard by the
internet operations community. The
regional internet registries publish the
minimum allocation sizes. At the time of
making this recording for all registries
the minimum allocation for IPv4 was a
slash 24 and for IPv6 it was a slash 4 to 8
for an assignment.
Basically going to an end user or slash
32 for an allocation which is what goes
to a network operator under 2010 there
really wasn't any reason to see anything
longer than a slash twenty two ipv4
prefix and Internet but now it's quite
likely that the ipv4 run out is having a
serious impact on the internet routing
table and is determining what is PS are
announcing to the global Internet so
what happens if aggregation is not
properly done on the Internet we will
now show you an example of what happens
if aggregates are not properly announced
as you can see on the screen a customer
has been assigned a slash twenty tree
network from a s 100 slash 19 address
block you see that only a s 100
announces the customers individual
networks to the internet and not its
aggregate what happens when the customer
link goes down their slash 22 your
network becomes unreachable and is
withdrawn from the a s 100 cybg P since
their ISP doesn't aggregate its slash 19
network block the slash twenty tree
network withdrawal is announced to its
peers and this starts rippling through
the internet
this causes added load on all Internet
backbone routers as the network is
removed from routing tables when the
customer link returns the ax / 23
network is now visible to their ISP and
is riad ver ties to peers this starts
rippling through the internet again and
adds the load on internet backbone
routers as network is reinserted into
the routing table some ISPs suppress the
flaps an internet may take 10 to 20
minutes or longer for this network to be
visible
this means that quality of service is
not guaranteed for this network in the
good example for example imagine the
same example where the customer has a
slash 23 net worth assigned from the
same AS 100 slash 19 block in this
scenario a s 100 announces it slash 19
aggregate to the internet if the
customer link goes down there slash 23
network becomes unreachable and it's we
drawn from s 100 cybg P the slash 19
aggregate is still announced the
Internet
so there are no BGP hold on problems or
propagation delays as well as no damping
by other ISPs when the customer link
returns their slash 23 network is
visible again and re-injected into a s
100 ibgp the whole internet becomes
visible immediately and the customer has
quality of service perception in
conclusion the good example is what
everybody should do as it adds the
Internet stability as well as reduce the
size of the routing table this reduces
routing churn thereby improving Internet
QoS for everyone. That example is what
many ISPs do due to either lack of
knowledge or laziness.
This must be avoided.
© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.
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