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JunosE 11.3.x IP, IPv6, and IGP Configuration Guide
360
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 bit
in the LSP header, and transmits the LSP with the current neighbor information. By sending
the updated LSP with the overload bit set immediately after forming the first adjacency,
IS-IS reduces the convergence time across the network.
If IS-IS waits for all adjacencies to be up before it sends the updated LSP with the overload
bit set, the other routers in the domain still have the transit router's old LSP and continue
to forward transit traffic to the transit router until all adjacencies are formed. That traffic
is lost.
Waiting for BGP Convergence
When BGP converges, the transit router again has the reachability information it needs
to forward traffic to destinations that are not directly connected. Typically, you then want
the transit router to clear the overload bit in its LSP and retransmit the LSP to inform the
other routers in the domain that they can use it as a transit router.
BGP is assumed to have converged when all of the following conditions have been met:
90 percent of BGP peers have reached an established state,
The transit router has received an end-of-rib marker from all IBGP peers that advertise
the graceful-restart capability.
The average rate of learning new routes has dropped to a low level.
Example Topology
Figure 21 on page 361 shows a sample topology where source end system A is
communicating with destination end system B through routers 1, 2, 3, and 4.
Copyright © 2010, Juniper Networks, Inc.
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