We're now going to look at partial routes
and what strategy we would adopt here.
What we will do is ask one upstream
provider for a default route because a
default route says I know how to get to
the will Internet and it's easy to
originate a default route towards a bgp
neighbor we've seen the one line
configuration earlier in this series we
still ask the other upstream provider
for a full routing table and then we
filter it based on the neighboring AS
or other policies that we want to
implement for example we want traffic to
their neighbors to go over the link to
that AAS and in fact what will happen is
most of what the upstream sends is
thrown away it's much easier than asking
the upstream provider to set up custom
BGP filters for you let's look at the
router C configuration it bears a lot of
similarities to the previous fool
routing table example we still have an
inbound prefix filter allowing all
prefixes apart from the RFC 6890 blocks
I'd Bank we still sin or aggregate but
now the AOS pass filter we used earlier
which allowed a s130
and a s 130s immediately adjacent asses
the es pass filter is applied directly
to the bgp neighbor it's no longer used
within a write map we also have a route
map tag default law which looks for the
default route and tags it with a low
local preference the route map looks for
the default sets local preference 280
the second line of the rat map leaves
all other prefixes alone and you can see
the a s path access list in the example
here looks exactly the same as it did
previously
the router D configuration is greatly
simplified we now accept the default
route in and we send our address block
out the summary of this the router C
configuration except the full BGP table
from a s 130 oh we could get them to
send less but it's very few operators
these days will actually permit this
it's usually foo BGP table or defaults
we filter the a SNS we get from s 130
such that only s 130 and a s1 30s
neighboring asses are accepted we allow
the default route and set it to local
preference 80 traffic to those a asses
we'll go over a s 130 link traffic to
the other a SS we'll go over the link to
s 140 if the s 140 link fails we get
back up here s 130 and vice-versa
well let's take the table with full
route entries as we saw earlier and add
in the partial rights from es 140 we get
on prefix just the default and we leave
it local preference 100 from es 130 well
we still get the 30,000 prefix as we had
in the previous example originated by a
s 130 and the neighbors but now we just
leave them local prep 100 we also get
the default route from a s 130 and we
set it to local preference 80 so in this
example a grand total of 30,000 and two
paths compare that with a 1.3 million
that we had before so the outcome of
this is such is such that if the link to
s 140 fails with partial routes it's
just a default route that was is
withdrawn from the local BGP table the
whole network will fail over to the s
130 link very very quickly in a matter
of seconds and same goes the other way
around if the s 130 link fails the 30
thousand and one prefixes learned from
es 132 be quickly removed
and the default right from PS 140 will
carry the traffic compare this with the
full routes what if the s 140 link goes
down we have 650,000 prefixes that have
to be withdrawn from the local BGP table
this will not happen in a few seconds
even with the best will of the vendors
and the best software and the fastest
CPUs there'll be a significant amount of
time required to remove all 650,000
prefixes learn from s 140 across the
entire tournament system customers will
see this as an artic or worst case funny
routing or maybe looping routing or
traffic and so forth customers will
notice this outage the partial route
example it's unlikely any user will
notice it unless they're doing some
really delay sensitive or time-sensitive
traffic or content analysis so the power
out solution is much preferred and
indeed in the Operations community
several operators have talked about the
benefits to their network when they have
moved from carrying full routes for
multihoming in an example like this to
simply using partial routes.
© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.
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