Juniper IGP - CONFIGURATION GUIDE V11.1.X, Configuration Manual

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–> 4 –> B. Because transit router 3 already had synchronized its BGP routing tables,
traffic forwarding continues without delay.
When transit router 2 reloads, it establishes adjacencies with routers 1 and 4, and
sends out its LSP advertising its neighbors. While router 2 begins to synchronize its
BGP routes, the network reconverges to the original path of A –> 1 –> 2 –> 4 –>
B. Traffic from A to B is forwarded to router 2. Typically, BGP has not converged by
then, so router 2 does not have the BGP route that it needs to forward the traffic,
and drops the packets, resulting in a black hole until the BGP convergence is complete.
You can avoid this black hole by configuring the overload bit for the transit router.
In this circumstance, router 2 sends out its LSP with the overload bit set in its header
as soon as it reloads, before it establishes all adjacencies. The bit set in the header
indicates to all the routers in the domain that router 2 is overloaded and not to use
it to carry transit traffic. The forwarding path continues to be the alternative path,
A –> 1 –> 3 –> 4 –> B, even after router 2 reloads.
When BGP convergence is complete at router 2, router 2 sends out a new LSP with
the overload bit cleared. The other routers then include router 2 in their SPF
calculations and revert to the original path, of A –> 1 –> 2 –> 4 –> B.
Suppression for IS-IS Graceful Restart
When graceful restart is configured on the transit router, the black hole avoidance
feature is suppressed.
Configuration
You can configure the transit router to set the overload bit when it reloads and to
then wait for a specified interval before it clears the bit and retransmits its LSP. More
commonly, and to avoid the transient black holes, you configure the transit router
to wait for BGP to converge, and specify an interval it waits after convergence before
it clears the bit and retransmits its LSP.
set-overload-bit
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Use to configure the router to set the overload bit in the header of its
nonpseudonode LSPs.
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While the overload bit is set, other routers in the domain do not include this
router in their shortest-path-first (SPF) calculations. Consequently, the other
routers do not detect any paths through this router and do not forward traffic
through this router. However, IP prefixes directly connected to this router are
still reachable. When the bit is cleared, the router is again included in SPF
calculations.
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You can set the overload bit for a number of reasons, including the following:
■
To prevent traffic through the router from disappearing into transient black
holes.
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To reduce routing table inaccuracies caused by router problems such as
memory shortage.
Configuring Global IS-IS Parameters
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Chapter 6: Configuring IS-IS