Suppression For Is-Is Graceful Restart; Configuration; Figure 21: Transit Router Topology - Juniper JUNOSE SOFTWARE FOR E SERIES 11.3.X - IP-IPV6-IGP CONFIGURATION GUIDE 2010-10-31 Configuration Manual

Chapter 6: Configuring IS-IS

Figure 21: Transit Router Topology

The transit routers, 2 and 3, learn the route to B from BGP. In a steady state environment,
the BGP routing tables are synchronized on all the transit routers.
Suppose the traffic forwarding path is currently A –> 1 –> 2 –> 4 –> B. If transit router 2
goes down, the network converges to the alternative path, A –> 1 –> 3 –> 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
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