Brocade Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide (Supporting R05.6.00) - Manual (Page 622)

598 Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide
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IS-IS shortcuts
5
The announce metric and relative metric are described in detail in the following sections. Use the
show isis shortcuts command to display the metric used to determine the cost.
The announce metric
When IS-IS shortcuts are enabled on an LSP tunnel, the MPLS router does not announce
(advertise) the IS-IS shortcuts unless specifically configured to do so. When announce is enabled,
the user can optionally specify an announce metric, which is used to compute the LSP cost of the
IS-IS shortcut. When an announce metric is not explicitly configured, IS-IS uses a default metric
value of 10.
To configure an announce metric for IS-IS shortcuts, refer to “Configuring the announce metric” on
page 601.
The relative metric
When announce is not enabled and an LSP metric is not explicitly configured under the LSP
configuration mode of the CLI, the relative metric is used to compute the LSP cost, which is the
native IGP cost, plus or minus the relative metric.
The relative metric is optionally specified when IS-IS shortcuts are enabled, and is used to make an
LSP tunnel less or more preferred over other paths. The default relative metric value is zero (0), but
can be configured to be a positive or negative number. A positive number disables an LSP tunnel
from participating in the SPF calculation. A negative number ensures that the LSP tunnel is
preferred over native IGP paths in the SPF calculation.
Figure 68 shows an example of this
configuration.
FIGURE 68 SPF calculation adjustment using the relative metric
Router A Router B Router D Router E Router C
10 10 10 10
LSP tunnel X
In this example, when there are no IS-IS shortcuts, Router A adds routes in the routing table for
routers C, D, and E with the metrics 20, 30, and 40, respectively. When an IS-IS shortcut is
configured on LSP tunnel X and the relative metric is –5 (minus 5), Router A installs the same
routes in the routing table with the metrics 15, 25, and 35, respectively, over the LSP tunnel X.
To configure the device to use a relative metric value other than 0, refer to “Configuring the relative
metric” on page 601.
Why LSP tunnels may be excluded from the SPF calculation
LSP tunnels may be excluded in SPF calculations in the following cases:
• The system did not find mapping between the LSP tunnel destination (the To address) and the
IS-IS system ID.
• There is no IS-IS native route to the LSP tunnel destination.