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Cisco Defense Orchestrator

How Forwarding Decisions are Made

Forwarding decisions are made in this order:

  • Use NAT translations (xlates) and rules to determine the egress interface. If the NAT rules do not determine the egress interface, the system uses the routing table to determine the path for a packet.
  • If the destination does not match an entry in the routing table, the packet is forwarded through the interface specified for the default route. If a default route has not been configured, the packet is discarded.
  • If the destination matches a single entry in the routing table, the packet is forwarded through the interface associated with that route.
  • If the destination matches more than one entry in the routing table, then the packet is forwarded out of the interface associated with the route that has the longer network prefix length. For example, a packet destined for 192.168.32.1 arrives on an interface with the following routes in the routing table:
  • 192.168.32.0/24 gateway 10.1.1.2
  • 192.168.32.0/19 gateway 10.1.1.3

In this case, a packet destined to 192.168.32.1 is directed toward 10.1.1.2, because 192.168.32.1 falls within the 192.168.32.0/24 network. It also falls within the other route in the routing table, but 192.168.32.0/24 has the longer prefix within the routing table (24 bits verses 19 bits). Longer prefixes are always preferred over shorter ones when forwarding a packet.

Note: Existing connections continue to use their established interfaces even if a new similar connection would result in different behavior due to a change in routes.

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