Administrative Distance for CCNA students

Hello Marit

Administrative distances are indeed assigned by the vendor, and the values you see in the lesson are values assigned by Cisco. This does make sense when you see that EIGRP is given a better AD that OSPF. Although they are assigned by vendors, they are not entirely based on preference of the proprietary over the open architecture protocols. The default ADs are specifically chosen to ensure an optimum functionality under the most common circumstances.

Other vendors use AD values as well, but the interoperation of routers from various vendors are not that much of an issue because the AD is local to the router and is not something that is shared in any routing updates. However, it is of utmost importance that the ADs on routers within the same routing domain be the same. Otherwise, routing loops and black holes may arise.

Like in many disciplines, professionals become attached to specific protocols and methodologies because they have experience with them. I too am not immune to this. :stuck_out_tongue:. To be fair, both EIGRP and OSPF are exceptional protocols and I have seen both be implemented successfully in many large networks. The truth is that each has its strengths and weaknesses for specific situations. I stated that EIGRP was better than OSPF, at least as far as convergence goes, because it has the feasible successor built in which provides exceptional convergence speed. OSPF on the other hand is better in scalability due to the fact that you can separate a network into somewhat autonomous areas. So it really depends on what your requirements are.

Now Cisco has chosen to place EIGRP above OSPF, which is a design choice, which may indeed be influenced by the fact that it is proprietary, but it is based on valid technological research as well.

(But to go as far as to say that EIGRP is obsolete, in my opinion, simply shows contempt rather than rational thought.)

Just for argument’s sake, EIGRP has now been opened to be freely used by other vendors as well, and vendors such as HP for example, are indeed using it in their equipment. It is described in the IETF RFC 7868.

I hope this has been helpful!

Laz

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Very helpful Laz, thanks a lot! :slight_smile:

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Hi.

I have a question, which raised during lab practice.

I have two routers and there are multiple Loopback interfaces for testing. I implemented different routing protocols, specifically, OSPF, RIPv2, and Static routes.

I made the OSPF and RIP have the same Administrative Distance (90) for the same networks.

However, the routing tables of both routers only show OSPF routes not RIP or both (Since they have the same administrative distance). I also change the AD of the static route to have (90), which is the same advertised route in OSPF with the same AD (90). Nevertheless, the router table only shows the static route only not both static and OSPF.

My question is, since the administrative distance of OSPF, RIP, and Static is the same, then why the routing table is not showing both routes? Is there another factor that makes a router prefer one route over another?

The only answer that came to my mind is that when the AD is same, then the router would look which one has close IP match based on the Subnet mask. But I’m not sure about this.

Thank you.

Hello Ameen

I haven’t been able to find any Cisco documentation that describes what happens when you configure the AD to be the same for multiple routing protocols. However, I too did some experimentation in the lab and I found the following:

  1. If eBGP, EIGRP, RIP, OSPF, and a static route are all advertising the same prefix with the same AD, then they are installed in the routing table in the same order as their original AD values. in other words, they are installed in the following order:
  • static
  • eBGP
  • EIGRP
  • OSPF
  • RIP
  1. Load balancing will never take place between two paths that are advertised from different sources, even if the AD is the same. For example, a route to 192.168.55.0/24 learned from both EIGRP and OSFP with the same AD will never have two entries in the routing table, and will never be load balanced. EIGRP will take precedence over OSPF, and thus only the EIGRP learned route is installed.

  2. The above are true only for identical prefixes. A prefix is the same only if the network address and the subnet mask are the same. For example, if EIGRP advertises a route to 172.16.0.0/23, and RIP advertises a route to 172.16.0.0/24, both routes will appear in the routing table, regardless of the AD. This is because they are considered different prefixes.

I hope this has been helpful!

Laz

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Thank you for the informative answer :+1:.

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