So just a note before we conclude the
multihoming examples:
if we're distributing the default route
within the AS whether we hear the default route
by BGP getting the full routing table or
simply in the partial routing example
we should distribute the default route
using our chosen IGP whether that is
OSPF or IS-IS. The example on the
slide shows how to originate the default
route on the two border routers with
different metrics.
Remember in OSPF the lowest metric will
win. So we had traffic would default out
to router D unless we had more specific
paths out through router C.
So on router C we originated the default route
in OSPF with a high metric 30 in the example.
On router D we originated the default
route with a low metric 10 in the example.
Which means the primary
default route is via router D
with backup via router C.
We'll have more specific paths to AS130
in the neighbors
learned through iBGP on router C anyway.
Partial routes from upstream is not
expensive. It only carries the routes
necessary for load sharing. You need to
know how to filter on AS paths as in
examples we've shown you and the
previous example really is
only an example. It shows you the steps
and the process needed.
Real life will need improved fine tuning.
The previous example doesn't consider inbound
traffic but we've covered those before.
And just a note: when we're distributing
internal default by iBGP or OSPF or IS-IS
make sure that routers
connected to private peers
or to internet exchange points
do not carry the default route.
If it carries a default
route or have the full BGP table,
it means your peers could point
a default route to you
and unintentionally transit your
backbone. The simple fix for private peer
or exchange point based routers is to
put a static default route to the
null interface. An example shows how to
do it for IPv4 and for IPv6.
© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.
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