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

• Use to configure the router to set the overload bit in the header of its nonpseudonode LSPs.
• 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.
• 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.
  • To reduce routing table inaccuracies caused by router problems such as memory shortage.
  • To prevent real traffic from flowing through a router to an IS-IS network, such as might be the case for a test router connected to a production network.
• Use the on-startup keyword to set the overload bit when the router reboots and to specify a period in seconds that IS-IS waits after the reboot before it clears the overload bit.
• Use the on-startup wait-for-bgp keywords to instruct IS-IS to set the overload bit when the router reboots and then wait until BGP has completed convergence after the reload before IS-IS clears the overload bit. You can specify a maximum interval that IS-IS waits for BGP notification. When that interval passes, IS-IS clears the overload bit. If you do not specify an interval, IS-IS waits a default 600 seconds and then clears the overload bit.
• If you issue the on-startup keyword but do not issue the wait-for-bgp keyword, then you must specify the number of seconds that IS-IS waits after a reload before clearing the overload bit.
• If you issue both the on-startup keyword and the wait-for-bgp keyword, you cannot specify a time interval for on-startup but can optionally do so for wait-for-bgp.
• By default, the overload bit is not set.
• Example 1

  host1(config-router)#set-overload-bit

• Example 2

  host1(config-router)#set-overload-bit on-startup 900

• Example 3

  host1(config-router)#set-overload-bit on-startup wait-for-bgp 450

• Use the no version to disable the setting.
• See set-overload-bit

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