16.1.1. The next hop calculation
Connected: An Internet Encyclopedia
16.1.1. The next hop calculation
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RFC 1583
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16. Calculation Of The Routing Table
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16.1. Calculating the shortestpath tree for an area
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16.1.1. The next hop calculation
16.1.1. The next hop calculation
This section explains how to calculate the current set of
next hops to use for a destination. Each next hop consists
of the outgoing interface to use in forwarding packets to
the destination together with the next hop router (if any).
The next hop calculation is invoked each time a shorter path
to the destination is discovered. This can happen in either
stage of the shortestpath tree calculation (see Section
16.1). In stage 1 of the shortestpath tree calculation a
shorter path is found as the destination is added to the
candidate list, or when the destination's entry on the
candidate list is modified (Step 2d of Stage 1). In stage 2
a shorter path is discovered each time the destination's
routing table entry is modified (Step 2 of Stage 2).
The set of next hops to use for the destination may be
recalculated several times during the shortestpath tree
calculation, as shorter and shorter paths are discovered.
In the end, the destination's routing table entry will
always reflect the next hops resulting from the absolute
shortest path(s).
Input to the next hop calculation is a) the destination and
b) its parent in the current shortest path between the root
(the calculating router) and the destination. The parent is
always a transit vertex (i.e., always a router or a transit
network).
If there is at least one intervening router in the current
shortest path between the destination and the root, the
destination simply inherits the set of next hops from the
parent. Otherwise, there are two cases. In the first case,
the parent vertex is the root (the calculating router
itself). This means that the destination is either a
directly connected network or directly connected router.
The next hop in this case is simply the OSPF interface
connecting to the network/router; no next hop router is
required. If the connecting OSPF interface in this case is a
virtual link, the setting of the next hop should be deferred
until the calculation in Section 16.3.
In the second case, the parent vertex is a network that
directly connects the calculating router to the destination
router. The list of next hops is then determined by
examining the destination's router links advertisement. For
each link in the advertisement that points back to the
parent network, the link's Link Data field provides the IP
address of a next hop router. The outgoing interface to use
can then be derived from the next hop IP address (or it can
be inherited from the parent network).
Next: 16.2. Calculating the interarea routes
Connected: An Internet Encyclopedia
16.1.1. The next hop calculation
