in this section we are going to look at
how you can scale BGP in the
ISP backbone so inside iBGP.
So using communities for
iBGP the ISPs are going to tag prefixes
learned from their BGP and their static
customers with different communities to
do different things so for example they
would like to identify services that the
customer may have purchased or you want
to identify prefixes which are part of
the highest peace provider assigned
space PA space or to identify the
customers PII addresses provider
independent addresses that they might
have got from the original internet
registry or you want to control prefix
distribution into ibgp or you want to
control the prefix announcements to
customers and up streams and several
other reasons so for service
identification what the ISP is going to
provide is transit very abstruse
connectivity to major ixps and
connectivity to private peers or to
customers now the customer can choose to
buy all or any of the above access
options and each option would be
identified with a unique community then
the ISP identifies where the address
space from the customer comes from their
provider assigned block or is their
customers own provider independent space
and you'll have one community for the
provider signed block and one community
for the provider independent space so
this slide shows the typical county
definitions and we assuming that we
ain't as 100 so 1000 would be the
aggregates out of s one hundred one
thousand one would be the sub prefixes
that come out of the a s 100 aggregate
then one thousand five for example could
be the static customer P I space
providing independent space
mm could be customers who get transit
2100 could be customers who get access
to the ixp 2200 could be customers who
get bgp customer access this is fruits
which are announced by other bgp
customers then all routes land from the
IXP could be tagged with 3000 and this
is how you could implement it in Cisco
IOS you have a community list 10 for too
much the aggregates which is 100 : 1000
11 for 1,000 112 for 1000 five which is
a customer static PA space all the way
down to 16 which is for the 3000
community which is routes land from the
ixv inside s 100 you have sample just a
little bit of a BGP session and you see
you have Ruta BGP 100 on a slide and
you're redistributing static routes
using a particular route map static -
BGP not that this is an advanced
configuration and typically we would not
redistribute statics into BGP however
for this use case because you have lots
of customers who have static routes that
you would like to announce then with
control redistribution it makes it ok so
you first need a prefix block that is
going to match which statics are
permitted to reduce beauty into BGP and
so the first thing that you do is have
the aggregate and then say I'm going to
have an IP prefix list s 100 - block
permit 164 1000 it's a slash 19 all the
way to a slash 32 so any prefix out of
this block will be redistributed then
for your s 100 prefixes you set the
community to 1000 which remember is the
aggregate community set
and now for the static - BGP route map
that we talked about just earlier you
first have a match statement to match
the prefix list is 100 - block for the
very reason that when you're
redistributing statics you need to be
very careful about which prefixes you
distribute and once you matched anything
out of that block you set the community
to 1001 which means these are static
prefixes which come out of the aggregate
block so for permit 20 which you don't
have a match command you set the
community to one thousand five which
means these are prefixes from your
customers out of their P I space so the
prefixes s 100 - block would need to
have all the address space that is 100
uses in this case it's just a single
slash 19 but if s 100 had add address
space you need to add it as well - this
prefix list.
© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.
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