Routing - fully specified route

A fully specified route refers to a static route entry in the routing table in which both the next-hop IP address and the exit interface are specified.

When configuring static routes on a Cisco router, there are a couple of ways to specify the forwarding or next-hop information:

Next-Hop IP Address Only:
- Here, you specify only the IP address of the next-hop router or device. In the following example, only a next hop IP of 192.168.1.1 has been specified. No exit interface is indicated. The exit interface must be determined using recursive routing.
```
Router(config)# ip route 10.10.10.0 255.255.255.0 192.168.1.1
```
Exit Interface Only:
- Here, you specify only the exit interface through which the packet should be sent. No next hop IP address is indicated.
```
Router(config)# ip route 10.10.10.0 255.255.255.0 GigabitEthernet 0/1
```
Fully Specified (Both Next-Hop IP and Exit Interface):
- Here, you specify both the IP address of the next-hop router/device and the exit interface.
```
Router(config)# ip route 10.10.10.0 255.255.255.0 192.168.1.1 GigabitEthernet 0/1
```

A fully specified route is often used in point-to-point connections, such as serial links, where there's only one possible destination over an interface. By specifying both the next-hop IP address and the exit interface, the router has precise information about where to forward the packet.

It's worth noting that while fully specifying a route can make routing determinate in certain scenarios, it can add to configuration complexity, so it's essential to use this method judiciously and understand the design implications.

A fully specified route is one that is typically employed for static routing in which both the next-hop IP address and the exit interface are defined manually. It's primarily used in static routing because dynamic routing protocols (like OSPF, EIGRP, BGP, etc.) automatically determine the next-hop IP and use that information to populate the routing table, without requiring explicit definition of exit interfaces.

That said, in some routing scenarios, especially with Policy-Based Routing (PBR) or with certain implementations of redistribution, you might manually define next-hop IPs and interfaces. However, in these scenarios, it's not commonly referred to as a "fully specified" route.

Links

https://networklessons.com/cisco/ccna-200-301//ip-routing-explained

https://networklessons.com/cisco/ccna-200-301//how-to-configure-static-route-on-cisco-ios-router