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Black-hole route and Route black hole

Latest reply: Dec 31, 2018 11:08:23 1699 4 6 0 0

Hello everyone,

Today I will share with you what is the black-hole route and route black hole.

Black-hole route

The null interface is a route to the null interface. A null interface is a virtual interface that cannot be configured with an IP address. All packets forwarded to the interface are discarded. Therefore, a blackhole route is added to all routes that may generate a route loop due to an interruption. For example, in an environment where a default route is configured, if a routing entry of the router is interrupted due to a fault, a routing loop may occur on the two routers.

Route black hole

Route black holes are usually generated when aggregation and return routes are summarized on the network border. Sometimes, some network segments do not exist on the internal network but are included in the summarized network segment, if the default route is configured on the aggregated boundary device, some problems may occur. In this case, if a data packet is sent to the router that does not belong to the network segment (but also included in the summary network segment) that does not exist in the internal network, the router does not find the corresponding route according to the longest matching principle, but can return to the original router according to the default route, as a result, a loop is formed, and the packet is discarded until the TTL value expires.


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yiyi0519
Created Dec 29, 2018 02:57:49

Configuring a routing black hole in a router is entirely for security reasons. A black hole route silently discards the packet without specifying the cause. A black hole router refers to a router that does not support PMTU and is configured not to send a "Destination Unreachable--Unreachable" response message.
It can be seen that if a router does not support PMTU and is configured not to send ICMP Destination Unreachable message packets, then the source host may send a large packet that is never routed. Because the router does not send a response message to the source host, the host cannot determine that the PMTU is the problem. However, if the source host has PMTU enabled, the source host automatically sets the PMTU to 576 bytes after retrying several large MTUs and if the router does not receive a response.
The PMTU enable setting under Windows 2000 is --- HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters
EnablePMTUBHDetect REG_DWORD 0 (disabled by default) or 1 (enabled)
You can also try it under Windows XP.
(NOTE: PMTU--Path Maximum Transfer Unit means that the size of a packet to be sent is the same as the minimum MTU value in the current path)
Different from black hole routing, this is the side effect that the router automatically summarizes after generating the summary route.
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chouhao
Created Dec 29, 2018 09:24:22

Route black holes are usually generated when aggregation and return routes are summarized on the network border. Sometimes, some network segments do not exist on the internal network but are included in the summarized network segment, if the default route is configured on the aggregated boundary device, some problems may occur. I agree with this.
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user_2915719
Created Dec 30, 2018 01:26:09

Very good introductions. This feature can be very useful to prevent messages looping in the network.
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Mysterious.color
Created Dec 31, 2018 11:08:23

very good
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