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Many network administrators will choose to filter dynamically learned
default routes to avoid situations where traffic ends up where it is not
supposed to be, and set their own internal default route statically.
Statically set defaults typically provide more control over routing
within an AS.
An operator can statically set the default route 0/0 to point to the
following:
- The IP address of the next-hop gateway
- A specific router interface
- A network number
Figure illustrates the first two possibilities. On the left, a router is
statically pointing its own 0/0 default toward the IP address 1.1.1.1.
On the right, the same router is pointing its default toward an Ethernet
interface. In the latter approach, further processing is needed to
figure out to whom on the segment the traffic should be sent. Such
processing usually involves sending Address Resolution Protocol (ARP)
packets to identify the physical address of the next-hop router.
It's very dangerous to point a static route out a broadcast
interface, since the router will ARP for every address within the
default address range, expecting the next hop to answer the ARP. This
can lead to extremely large ARP table, and even result in the router
crashing, or failing to route packets altogether.
A system can also set its default based on some network number it
learns from another system. In Figure ,
AS1 is dynamically learning route 192.213.0.0/16 from AS2. If AS1 points
its default to 192.213.0.0/16, that network will automatically become
the gateway of last resort. This approach uses recursive route lookup to
find the IP address of the next-hop gateway. In this example, the
recursive lookup will determine that 192.213.0.0/16 was learned via the
next-hop 1.1.1.1, and traffic would be directed accordingly.
It is important for defaults to disappear dynamically if what they
point to disappears. Cisco enables a statically defined default to
follow the existence of the entity to which it is pointing. If the
default, for example, is pointing to a network number and that network
is no longer reachable (does not show in the IP routing table), the
default will also disappear from the IP routing table. This behavior is
needed in situations where multiple defaults exist. One default can be
used as primary and others as a backup in case the primary default is no
longer valid.
Default networks should be selected as
far upstream (as close to the Internet) as possible so that they are
more representative of the as much of the path to the Internet backbone
as possible. This is important if the AS you are connected to has a
single connection to the Internet backbone. AS1 can set the default
toward its provider AS2 by pointing to prefix 128.213.11.0/24 or the
supernet 192.213.0.0/16. Pointing the default to 128.213.11.0/24 makes
it dependent on the stability of a portion of the link (AS1 to AS2) and
not the whole link (AS1 to AS3) toward the NAP. If the link between AS2
and AS3 goes down, AS1 would be still sending traffic toward AS2 rather
than directing it to some other default (assuming that AS1 has other
providers). A better default choice would be the supernet
192.213.0.0/16, because its existence is more representative of the
whole link toward the Internet backbone..
Multiple static defaults can be used at
the same time. One way to set multiple static defaults is to point to a
network learned through multiple connections and establish a degree of
preference by using the local preference attribute. This would apply to
a single router connected to the provider via multiple connections, or
multiple routers running IBGP inside the AS. Both scenarios are
illustrated in Figure .
These are similar to the scenarios you saw earlier, the only difference
being that the customer is setting its own default rather than relying
on the provider to send the 0/0 default route. In this example, the
customer will choose 128.213.0.0/16 with the local preference of 100 via
the upper link. The lower link will be used as backup in case of failure
in the primary link.
Another way of setting defaults
statically involves using the Cisco administrative distance parameter to
establish a degree of preference. This would work only in the case of
one router connected to multiple connections because the distance
parameter is not exchanged between routers.
If two static default entries are defined
with different distances, the default with the lowest distance wins. If
the better default goes away, the second default becomes available. If
both defaults have the same distance, then traffic will be balanced
between the two defaults.
Figure
illustrates the use of the distance parameter in setting multiple
defaults. AS1 is connected to AS2 via two links and is setting its own
defaults toward AS2. AS1 uses one link as primary by giving the static
default a distance of 50, lower than the distance of 60 given to the
backup link. If the primary link fails, traffic will shift toward the
backup.
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