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Update linkstate.rst #195

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2 changes: 1 addition & 1 deletion book-2nd/principles/linkstate.rst
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Expand Up @@ -22,7 +22,7 @@ Other variants are possible. Some networks use optimisation algorithms to find t

.. index:: Hello message

When a link-state router boots, it first needs to discover to which routers it is directly connected. For this, each router sends a HELLO message every `N` seconds on all of its interfaces. This message contains the router's address. Each router has a unique address. As its neighbouring routers also send HELLO messages, the router automatically discovers to which neighbours it is connected. These HELLO messages are only sent to neighbours who are directly connected to a router, and a router never forwards the HELLO messages that they receive. HELLO messages are also used to detect link and router failures. A link is considered to have failed if no HELLO message has been received from the neighbouring router for a period of :math:`k \times N` seconds.
When a link-state router boots, it first needs to discover to which routers it is directly connected. For this, each router sends a HELLO message every `N` seconds on all of its interfaces. This message contains the router's address. Each router has a unique address. As its neighbouring routers also send HELLO messages, the router automatically discovers to which neighbours it is connected. These HELLO messages are only sent to neighbours who are directly connected to a router, and a router never forwards the HELLO messages that it receives. HELLO messages are also used to detect link and router failures. A link is considered to have failed if no HELLO message has been received from the neighbouring router for a period of :math:`k \times N` seconds.

.. figure:: ../../book/network/svg/ls-hello.png
:align: center
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