So the first example we will look at is
dealing with the case where one link is
going to be used as the primary link and
the other link is the backup link and
this will be for two links to the same
ISP and it applies when the end site has
bought maybe a large primary one link to
the upstream provider and a small
secondary one link which the users
backup. For example the primary path
might be an E1 2 megabits per second
the backup might be 64 kilobits per
second or something similar obviously to
apply to your situation scale the
bandwidths appropriately if you look at
the diagram you'll see that a s 100 is
connected by two paths to a s 6 5534 the
link between router a and C is the
primary path and the link between router
B and D is the backup path as you can
see in the diagram we've used a s 6 5534
for the N sight that is multihoming this
is a private a s as we learned earlier
there's no need for a public air snubber
for this type of multihoming the end
site has only got two external
connections and only connecting to the
same upstream provider s 100 will remove
the private e s it hears from its
customer and of course it will remove
any customer sub prefixes from internet
announcements so let's have a look at
see how we configure this well to start
off we announce the slash 19 aggregate
on each link the primary link will
receive the slash 19 the backup link
will receive the slash 19 this means the
upstream provider will see the slash 19
on both paths and if either link fails
because we're announcing the 19 on the
alternative path as well it ensures
continued connectivity for the inside
for inbound well the upstream provider
simply announces a default route on both
paths so the insight will see the
default route on both links and again if
either link should fail the default
being heard on other paths ensures
continued connectivity but how do we
make one path the primary and the other
one the backup well as we learned when
we looked at the BGP attributes we can
use the metric and the local preference
to achieve this if we do all the traffic
engineering from the insight perspective
if the insight announced the slash 19 on
the backup path with an increased metric
then the upstream provider will see the
two paths one with metric zero which is
the default and the other one with a
larger metric larger non zero metric
which makes this the backup path for the
other way around the insight will
receive the default route on both links
one path will have the default local
preference of 100 and the other path the
end sight will reduce the local
preference say make it 80 this lower
local preference means the backup path
is less preferred hence making it the
backup so will this set up the backup
link having the slash 19 announced with
increased metric and the inbound default
route being tagged with a lower local
preference will ensure that the backup
link functions as designed and of course
if either link fails the other one
becomes the primary path ensuring
continued connectivity between the and
site and the operator let's look at the
configuration for router a we've
included Cisco IOS configuration showing
how this might be configured router a is
the primary path on the end site and you
see router is announcing the aggregate
and is accepting that
fault drought in from the upstream
provider router here of course is also
originating the slash 19 address space
if we look at router B we see the same
prefix list letting the aggregate out to
the upstream provider and we see another
prefix list which also accepts the
default route in from the upstream
provider so this is the same
configuration as on router a but we now
have to write Maps one route map which
I've called med tank out is applied on
the outbound announcement and the other
route map LP low in is applied on the
incoming announcements if we look at the
next slide we'll see what the write Maps
actually are doing right map med turn
out sets the metric on all prefixes
being sent out to the upstream provider
we're only announcing the slash 19 so
the slash 19 gets a med of 10 set on it
the route map LP low in matches all
prefixes and sets local preference 90
inbound and then the N site will see the
two paths one with local preference 100
the default on router a and this one on
router B with local preference 90 and we
achieve the setup that we were aiming
for if we look at the router C
configuration on the upstream provider
or the upstream provider does is
originated default route sets up a
prefix list to let that default outbound
to the customer and creates another
prefix list to allow the customer slash
19 address block in that it's a very
simple configuration and indeed it's
exactly the same as for router D again a
default originated allowing the customer
prefix in and allowing the default route
outbound the upstream provider has a
very simple configuration which they
don't need to adjust if the customer
chooses to swap around which link
becomes Brown
which were think becomes backup if we
look at rotary router II was the
upstream providers connection to the
rest of the internet and I want to point
out to the remove private es command the
upstream provider there is stripping out
the private a s six five five three four
in the announcement of the customer
prefix out to the Internet as we saw
earlier missus best practice private a s
number should not be seen on the public
internet so the result of this is router
II will announce the slash nineteen
prefix learned from the customer end site
out to the internet
as though it was originated from AS100.
© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.
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