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I have given the following network diagram and been told that,
Since EIGRP has a better (lower) administrative distance than OSPF, the router on the far left will use the EIGRP path and publish only the EIGRP path to the destination network in its routing table.
Question: Why does EIGRP have a better (lower) administrative distance than OSPF?
Image courtesy: Cisco Systems, Inc
That is because Cisco determines the AD used by its devices, and EIGRP is a Cisco protocol, so Cisco decided to make EIGRP have a lower AD than other protocols.
Each vendor determines the relative reliability of protocols for its equipment; there is no outside standard for this. Some vendors may not even have an AD equivalent.
According to Cisco default distance value table,
| Route Source | Default Distance Values |
|--------------------- |------------------------- |
| Connected interface | 0 |
| Static route | 1 |
| EIGRP summary route | 5 |
| eBGP | 20 |
| Internal EIGRP** | 90 |
| IGRP | 100 |
| OSPF** | 110 |
| ............. | ... |
cisco administrative distance, and you will get things like, What Is Administrative Distance?. As I said, there is no standard. Cisco created Administrative Distance. Other vendors may call it something else (assuming they have something similar). AD is local to a single device, so there is no interoperability requirement.
Administrative distance is defined for a protocols by protocols designer . Cisco has developed EIGRP and designed in such a way AD value is 90 .
It's purely depends upon protocol developer .
The AD is a vendor concept (from the vendor who in this case is Cisco), independent of the routing protocols. It is just Cisco's way of prioritizing between routes from different routing protocols, in cases where multiple routing protocols are running at the same time.
Since, furthermore, EIGRP was created by Cisco (unlike OSPF), it is not surprising that Cisco may slightly prefer EIGRP over OSPF and therefore assign it a better/smaller AD.
