liven wrote: » EIGRP summary routes have an AD of 5... Is this to make these routes take precedence over more specific EIGRP routes?
kalebksp wrote: » More specific routes always take precedence over less specific routes, regardless of AD.
tech-airman wrote: » kalebksp, That is not always true. What if the specific route's AD value has been manually configured? Case in point, "floating static route."
gojericho0 wrote: » if there is a route with a more specific prefix it will still take precedence
kalebksp wrote: » A floating static route is a static route with a modified AD. The point of modifying the AD is to cause this route to be used only when the dynamic routing protocol has lost it's route to the same network. I don't see what it has to do with the specificity of the routes.
tech-airman wrote: » kalebksp, A static route has a default administrative distance of 1. An internal EIGRP route has a default administrative distance of 90. Let's say both routes are to 192.168.1.128/25. The routing process looks at the AD for the static route then compares it with the AD for the internal EIGRP route and decides to place the static route to 192.168.1.128/25 into the routing table instead of the internal EIGRP route to 192.168.1.128/25. So the purpose of creating a "floating static route" is to intentionally make the static route "less desirable" so some other routing protocol's route to a subnetwork is placed into the routing table instead of the static route, for example intentionally configure the floating static route's administrative distance to 91. That should result in the internal EIGRP route to 192.168.1.128/25 to be placed into the routing table instead of the newly manually configured administrative distance for the static route. So I think my comment is valid. Source: Route Selection in Cisco Routers - Cisco Systems What Is Administrative Distance? - Cisco Systems
EdTheLad wrote: » As quoted above: "More specific routes always take precedence over less specific routes, regardless of AD." This is the statement you disagreed with tech-airman. More specific = longer mask , longest mask gets preference regardless of AD. Ed = 1 , TA = 0
kalebksp wrote: » I'm going to assume you misunderstood what we meant by "more specific route", in which case, read EdTheLad's post above. There is also some explanation on the "Route Selection in Cisco Routers" page you posted, under the "Making Forwarding Decisions" section.
Route Selection in Cisco Routers - Building the Routing Table section wrote: The main considerations while building the routing table are: Administrative distance - This is the measure of trustworthiness of the source of the route. If a router learns about a destination from more than one routing protocol, administrative distance is compared and the preference is given to the routes with lower administrative distance. In other words, it is the believability of the source of the route. <snip> The router decides whether or not to install the routes presented by the routing processes based on the administrative distance of the route in question. If this path has the lowest administrative distance to this destination (when compared to the other routes in the table), it's installed in the routing table. If this route isn't the route with the best administrative distance, then the route is rejected. To understand this better, let's look at an example. Assume a router has four routing processes running: EIGRP, OSPF, RIP, and IGRP. Now, all four of these processes have learned of various routes to the 192.168.24.0/24 network, and each has chosen its best path to that network through its internal metrics and processes. Each of these four processes attempts to install their route toward 192.168.24.0/24 into the routing table. The routing processes are each assigned an administrative distance, which is used to decide which route to install. Default Administrative Distances Connected 0 Static 1 eBGP 20 EIGRP (internal) 90 IGRP 100 OSPF 110 IS-IS 115 RIP 120 EIGRP (external) 170 iBGP 200 EIGRP summary route 5
Router(config-router)#distance eigrp [i]internal-distance[/i] [i]external-distance[/i]
tech-airman wrote: » The abovementioned command does not modify the EIGRP summary route AD. So it is possible to make the internal EIGRP route "more attractive" than the EIGRP summary route. So I did not misunderstand the OP's question.
Therefore, a summary route of 192.168.2.0/23 includes the "more specific" routes of 192.168.2.0/24 and 192.168.3.0/24. What if you manually configured the internal EIGRP route to 192.168.3.0/24 to be lower than AD 5, say AD 4? So the routing process would prefer to install 192.168.3.0/24 with AD 4 into it's routing table instead of the summary route with AD 5. That's why I said "It depends."
tech-airman wrote: » kalebksp, No, I did not misunderstand the term "more specific route." You are looking on the wrong side of the routing table just like EdTheLad's post above. You're looking at the "Making Forwarding Decisions" section but I read the "Building the Routing Table" section that states ... You can modify the administrative distance for EIGRP by using the following command... Router(config-router)#distance eigrp [i]internal-distance[/i] [i]external-distance[/i] The abovementioned command does not modify the EIGRP summary route AD. So it is possible to make the internal EIGRP route "more attractive" than the EIGRP summary route. So I did not misunderstand the OP's question.
Prefix Lengths Let's look at another scenario to see how the router handles another common situation: varying prefix lengths. Assume, again, that a router has four routing processes running on it, and each process has received these routes: EIGRP (internal): 192.168.32.0/26 RIP: 192.168.32.0/24 OSPF: 192.168.32.0/19 Which of these routes will be installed in the routing table? Since EIGRP internal routes have the best administrative distance, it's tempting to assume the first one will be installed. However, since each of these routes has a different prefix length (subnet mask), they're considered different destinations, and they will all be installed in the routing table.
cowood2676 wrote: » I am fairly new to this forum, but decided to put my toughts in the mix. The administrative distance (AD) is used to rate the trustworthiness of routing information received on a router from a neighbor router. An administrative distance is an integer from 0 to 255, where 0 is the most trusted and 255 means no traffic will be passed via this route. Here is how the Router makes routing decisions: 1) The Route with longest prefix length will be selected. 2) If there are multiple routes with the same prefix length, the route with the lowest AD will be used. 3) If there are multiple routes with the same prefix length & same AD, the route with the lowest metric will be preferred. 4) Finally, if the preceding 3 values are all equal, equal-cost sharing load sharing will be put into action.
EdTheLad wrote: » In the situation both summary and internal route were the same. If you received an internal route 10.1.0.0/16 from an eigrp neighbor, you config a summary on another interface 10.1.0.0 255.255.0.0, this will create a static to null with ad 5. This static will take precedence over the internal route, you could however modify the internal eigrp distance to 4 so the internal route will take precedence over the summary.
If you received an internal route 10.1.0.0/16 from an eigrp neighbor, you config a summary on another interface 10.1.0.0 255.255.0.0
EdTheLad wrote: » I'm afraid you will have to remove that penalty. The summary gets an AD of 5 and a route null0 to protect from receiving the advertised summary back via another interface and creating a loop.Even if a summary is created locally, the networks falling within that summary will be present in the local rib, these routes will not be advertised due to a summary configured on the interface. So what i said before still holds true, your ideas are a little screwed.
