Most service provider networks use IS-IS as the IGP in large single-area
Level 1-only or Level 2-only domains. For those with Level 1-only backbones,
the capability to redistribute into Level 1 provides flexibility to import
external routes into the IS-IS domain. Even though this behavior is not
standardized, it should not pose interoperability issues with other vendors’
routers because both existing IS-IS standards, ISO 10589 and RFC 1195, require
IS-IS implementations to ignore unsupported or unknown optional TLVs
encountered while parsing IS-IS-packets.
The IOS router-level command
redistribute enables redistribution. This command takes on
other options, such as metric value, metric type, route map, and so on. In the
Cisco implementation of IS-IS, CLNS static routes are automatically distributed
into IS-IS. However, IP static routes are redistributed only by manual
configuration.
When static IP routes need to be redistributed, the
redistribute command requires the keyword
ip to go with it, in addition to the other arguments
previously mentioned. The metric type for external routes can be either
internal or external. Internal metrics are comparable to metrics used for
internal routes. External metrics require the I/E bit (bit 7) of the metric
field to be set in addition to the actual metric, resulting in higher metric
values. In current Cisco IOS Software releases, when using narrow metrics, bit
8 of the default metric field is set for external metrics, resulting in an
increase of the metric value by 128.
By default, the internal metric
type is assigned if nothing is specified in the configuration. Also, the
external routes are added into Level 2 unless Level 1 is explicitly stated in
the configuration. Figure
illustrates
basic examples of redistribution in IS-IS. In Figure
, only
the ip keyword is used with the
redistribute command.
Note that below show
running-config, the internal metric type has been assigned by
default and the metric applied is 0. The output of show isis
database on RT1 shows that the external static route has been added
to only the Level 2 LSP.

The metric type is explicitly set to external in this configuration, but no
metric value is applied.
As explained
previously, the I/E bit needs to then be set for the external metric type,
effectively increasing the metric value by 64. However, Cisco IOS Software set
bit 8 of the narrow metric instead of bit 7, consequently adding 128 instead to
the original value of 0. The Level 2 LSP displayed shows 128 as the metric
value for the external route, 172.16.1.0/24.
The IP routing table output
from RT2 shows the external route, 172.16.1.0/24, which was redistributed from
a static source into IS-IS on router RT1.
The metric
entered for this route, 138, is the total of the metric on the outgoing
interface from RT2 to RT1 (10) plus the metric of 128 advertised by RT1. Other
routes received from RT1 (10.0.0.1/32 and 10.1.1.0/24) are registered with a
metric 20 (10 advertised by RT1 and additional 10 for the metric from RT2 to
RT1).
The route-map option of the
redistribute command provides more flexibility for
configuring redistribution, such as selective importation of external routes
into the IS-IS environment, applying special tags, and even setting the metric
of redistributed routes. When used for selective importation of routes into
IS-IS, route maps provide a filtering effect by controlling which elements from
an external source are allowed or denied into IS-IS.

In Figure
, static routes
are redistributed into IS-IS while filtering through the route map TEST. Route
map TEST matches the static routes against access list 1, which permits only
172.16.2.0/24 into the IS-IS environment. RT1 LSP is shown from RT2. Also shown
is the routing table of RT2.
In Figure
, the route map
approach is used to set the metric for routes imported into IS-IS.