Which two statements are true about route leaking in a Level 1-Level 2 network environment? (Choose two.)
A.
Level 1 internal routes can be leaked into Level 2, but require a policy.
B.
Level 1 external routes are leaked into Level 2 without a policy if the wide-metrics parameter is configured.
C.
Level 2 internal routes can be leaked into Level 1, but require a policy.
D.
Level 2 external routes are leaked into Level 1 without a policy if the wide-metrics parameter is configured.
Explanation:
B: Normally, IS-IS metrics can have values up to 63. The total cost to a destination is the sum of the metrics on
all outgoing interfaces along a particular path from the source to the destination. By default, the total path metric
is limited to 1023. This metric value is insufficient for large networks and provides too little granularity for traffic
engineering, especially with high-bandwidth links. A wider range of metrics is also required if route leaking is
used.
C: By default, IS-IS protocol leaks routing information from a Level 1 area to a Level 2 area. However, to leak
routing information from a Level 2 area to a Level 1 area, an export policy must be explicitly configured.
https://www.juniper.net/techpubs/en_US/junos16.1/topics/example/example-configuring-is-isroute-leaking-L2-to-L1.html
http://www.juniper.net/documentation/en_US/junos14.1/topics/topic-map/isis-wide-metrics.html
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Ans : B&C
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B/C
By default, IS-IS protocol leaks routing information from a Level 1 area to a Level 2 area. However, to leak routing information from a Level 2 area to a Level 1 area, an export policy must be explicitly configured.
the normal IS-IS operation is not to propagate IS-IS L1 external routes to L2 areas and other L1 areas. This is true but with the wide-metrics used in the network IS-IS has no way to determine whether the route is Internal or External as it was the case with standard metrics. Hence, the system considers all routes to be Internal, and thus, pass them from an area to another. If the wide metrics were not used, a route leaking policy from Level 1 to Level 2 would have been necessary on the R4 L1L2 router.
With the wide-metrics-only keyword, the TLV 2, 128 and 130 are no more attached in link state PDUs. The remaining TLVs 22 and 135 make no distinction between Internal and External.
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Options B & C, final answer.
An IS-IS Level 2 area is analogous to the OSPF backbone area (0), while a Level 1 area operates much like an OSPF totally stubby area,
in that a default route is normally used to reach both inter-level and AS external routes.
By default, IS-IS protocol leaks routing information from a Level 1 area to a Level 2 area.
However, to leak routing information from a Level 2 area to a Level 1 area, an export policy must be explicitly configured.
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