Avoiding Transient Black Holes - Juniper IGP - CONFIGURATION GUIDE V11.1.X Configuration Manual

Avoiding Transient Black Holes

When you start or reload a transit router that is running both IS-IS and BGP, the
router is temporarily unavailable to the routing domain. Other routers in that routing
domain must select alternative paths to destinations that used the transit router.
When the transit router becomes available again, the other routers soon select it
again as the optimal path to those destinations.
The other routers select the transit router again before it has loaded the complete
BGP routing table. Because the transit router does not yet have all the reachability
information that is needed to reach some external destinations, traffic to destinations
that were not learned by means of the IGP is dropped until the transit router has
complete external reachability information again. This condition is known as a
transient black hole.
You can use the overload bit to avoid these black holes. When the overload bit is set
in the LSP header, other routers in the domain do not include the transit router in
their SPF calculations and thus do not use that router for traffic forwarding.
When the transit router boots, it begins establishing adjacencies with its neighbors.
As soon as it establishes an adjacency, it creates (or updates) its LSP, sets the overload
Use to create aggregate addresses of routes that are redistributed from other
protocols in the routing table or distributed between level 1 and level 2 by a
summary address. This process is called route summarization.
A single summary address includes groups of addresses for a given level.
Use the summary-address command for IP routes. Use the summary-prefix
command for IPv6 routes.
The metric value is used when the router advertises the summary address. When
the metric value is not used, the value of the lowest cost route (the default) is
used.
This command reduces the size of the neighbor's routing table and improves
stability because a summary advertisement depends on many more specific
routes.
A disadvantage of summary addresses is that other routes might have less
information to calculate the optimal routing table for all individual destinations.
Use the optional tag keyword to specify a tag value for an IS-IS summary address.
The tag value must be a number in the range 1–4294967295.
Example 1 For IP routes
host1(config-router)#summary-address 10.2.0.82 255.255.0.0 level-1-2 tag 34
Example 2 For IPv6 routes
host1(config-router-af)#summary-prefix 2001:2000::0/8 level-1 metric 10 tag
100
Use the no version to restore the default, the value of the lowest-cost route.
See summary-prefix
Chapter 6: Configuring IS-IS
Configuring Global IS-IS Parameters
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