Aggregation means announcing the address
blocks received from the regional
internet registry to the other ASes
connected to your network.
Sub prefixes of this aggregate may be
also announced. They'll be used internally
in the ISP network for ISP infrastructure
or distributed for customer use.
All these sub prefixes may also be
announced to other autonomous systems
to aid with multihoming as we'll see later on
in this series; however, it seems to be that
too many operators are still thinking
about class C's classfull routing and
Class A, Class B, Class C addresses were
obsoleted in 1994 and what we're seeing
as a proliferation of /24s
in the internet routing table. When this
recording was made in October 2019 we
see four hundred forty eight thousand
/24s in IPv4 table. This
table has seven hundred seventy eight
thousand prefixes in it. Interesting
enough the same is happening for IPv6.
We see thirty six thousand slash 48 in a
recent table of 75,000 prefixes so how
do you configure an aggregate we will be
providing examples based on using the
Cisco IOS imagine an isp has been
allocated 166 0.0 /spaceafter 10 network
statement under the address family ipv4
of the BGP configuration as shown on the
screen he would then have to add an IP
route statement of the prefix with an X
table of non zero this static route is
called a pull-up route
connectivity to ISPs customers can be
achieved by using more specific prefixes
remember that the longest match is
always preferred how would you want to
announce this aggregate by using a
filter on the ebgp peer a configuration
example of how to properly announce an
aggregate using filters on the ebgp
pairs
shown on the screen as you can see the
router bgp statement denotes that the
ISP has been assigned an ASN of 64 511
the aggregate is announced using the
Network statement network 166 that 0.0
mask 255 255 - 2 4 0 remember that a
pull-up route must be configured to
announce the aggregate or the route must
be present in the routing table an
outbound filter called out - filter is
applied to its neighbor 100 at 67 that
10.1 when an ASN of 101 which only
permits its prefix of 100 at 6600 /19
to be announced and denies everything
else and inbound fields are called
default is applied to the same neighbor
permitting only the default route from
this neighbor.
© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.
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