Dell S4820T, Configuration Manual

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• The Layer 3 status of an interface goes DOWN when its Layer 2 status goes down or the IP address is
removed from the routing table.
Track IPv4 and IPv6 Routes
You can create an object that tracks an IPv4 or IPv6 route entry in the routing table.
Specify a tracked route by its IPv4/IPv6 address and prefix-length, and optionally, by a virtual routing and
forwarding (VRF) instance name if the route to be tracked is part of a VRF. The next-hop address is not
part of the definition of the tracked object.
A tracked route matches a route in the routing table only if the exact address and prefix length match an
entry in the routing table. For example, when configured as a tracked route, 10.0.0.0/24 does not match
the routing table entry 10.0.0.0/8. If no route-table entry has the exact address and prefix length, the
tracked route is considered to be DOWN.
In addition to the entry of a route in the routing table, you can configure how the status of a route is
tracked in either the following ways:
• By the reachability of the route's next-hop router.
• By comparing the UP or DOWN threshold for a route’s metric with current entries in the route table.
Track Route Reachability
If you configure the reachability of an IP route entry as a tracked object, the UP/DOWN state of the route
is determined by the entry of the next-hop address in the ARP cache.
A tracked route is considered to be reachable if there is an address resolution protocol (ARP) cache entry
for the route's next-hop address. If the next-hop address in the ARP cache ages out for a route tracked
for its reachability, an attempt is made to regenerate the ARP cache entry to see if the next-hop address
appears before considering the route DOWN.
Track a Metric Threshold
If you configure a metric threshold to track a route, the UP/DOWN state of the tracked route is
determined by the current metric for the route entered in the routing table.
To provide a common tracking interface for different clients, route metrics are scaled in the range from 0
to 255, where 0 is connected and 255 is inaccessible. The scaled metric value communicated to a client
always considers a lower value to have priority over a higher value. The resulting scaled value is
compared against the threshold values to determine the state of a tracked route as follows:
• If the scaled metric for a route entry is less than or equal to the UP threshold, the state of a route is
UP.
• If the scaled metric for a route is greater than or equal to the DOWN threshold or the route is not
entered in the routing table, the state of a route is DOWN.
The UP and DOWN thresholds are user-configurable for each tracked route. The default UP threshold is
254 ; the default DOWN threshold is 255 . The notification of a change in the state of a tracked object is
sent when a metric value crosses a configured threshold.
Object Tracking
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