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    #1

    Default EIGRP summary routes and administrative distance

    EIGRP summary routes have an AD of 5... Is this to make these routes take precedence over more specific EIGRP routes?
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  3. Console champ of TE Mrock4's Avatar
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    #2
    The AD of 5 will only be local, not propagated to other nodes. After looking all over the net, the best thing I can come up with is to protect against the router receiving the same summary from a different node..in that case, it will still select the local summary route with an AD of 5. Maybe someone else can chime in.
    Last edited by Mrock4; 03-16-2009 at 06:31 PM.
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    #3
    Quote Originally Posted by liven View Post
    EIGRP summary routes have an AD of 5... Is this to make these routes take precedence over more specific EIGRP routes?
    As far as I remember, doesnt the new route get an AD of 5, and pointing to null0 ?

    If thats the case, then the AD of 5 is a loop prevention system, making sure the summary route gets installed in the local routing table, and if a packet comes in with a destination of a non-known route, it gets dropped.
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    #4
    Quote Originally Posted by liven View Post
    EIGRP summary routes have an AD of 5... Is this to make these routes take precedence over more specific EIGRP routes?
    liven,

    It depends. What is the AD for the more specific EIGRP route?
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    #5
    More specific routes always take precedence over less specific routes, regardless of AD.
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    #6
    Quote Originally Posted by kalebksp View Post
    More specific routes always take precedence over less specific routes, regardless of AD.
    kalebksp,

    That is not always true. What if the specific route's AD value has been manually configured? Case in point, "floating static route."
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  8. Senior Member gojericho0's Avatar
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    #7
    Quote Originally Posted by tech-airman View Post
    kalebksp,

    That is not always true. What if the specific route's AD value has been manually configured? Case in point, "floating static route."
    if there is a route with a more specific prefix it will still take precedence
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    #8
    Quote Originally Posted by tech-airman View Post
    kalebksp,

    That is not always true. What if the specific route's AD value has been manually configured? Case in point, "floating static route."
    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.
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    #9
    Quote Originally Posted by gojericho0 View Post
    if there is a route with a more specific prefix it will still take precedence
    gojericho0,

    Let's say you've got an EIGRP route to 192.168.1.0/24 with default internal EIGRP AD of 90. Let's say you've got a static route to 192.168.1.0/24 with default static route AD of 1. The routing process compares AD 90 with AD 1. The AD 1 wins, so the static route gets placed into the routing table and NOT the EIGRP route to 192.168.1.0/24. So how "more specific" than the IDENTICAL subnetwork can you get to determine which one gets installed into the routing table?

    Source:
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    #10
    Quote Originally Posted by kalebksp View Post
    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.
    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:
    1. Route Selection in Cisco Routers - Cisco Systems
    2. What Is Administrative Distance? - Cisco Systems
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  12. Resident Underachiever EdTheLad's Avatar
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    #11
    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
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    #12
    Quote Originally Posted by tech-airman View Post
    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:
    1. Route Selection in Cisco Routers - Cisco Systems
    2. What Is Administrative Distance? - Cisco Systems
    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.
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    #13
    Quote Originally Posted by EdTheLad View Post
    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
    EdTheLad,

    You're looking from the wrong side of the routing table. You're talking about the forwarding process, not the routing process side of the routing table. The forwarding process takes the longest prefix and then consults the routing table. The forwarding process doesn't care about administrative distance. The forwarding process only cares if a route exists for the sub/network of the destination IP address.

    Since the title of this post is "EIGRP summary routes and administrative distance" and this is the CCNP board, it is more relevant to look at how the routing process decides to place routes into the routing table for the forwarding process to use. The OP asked "Is this to make these routes take precedence over more specific EIGRP routes?" So "...more specific EIGRP routes" in relation to "summary routes" would be the individual subnets that make up the summary route. 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."

    Penalty EdTheLad, -1 point for speaking from the wrong side of the routing table.
    Ed = 0, TA = 1
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  15. Resident Underachiever EdTheLad's Avatar
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    #14
    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.
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    #15
    Quote Originally Posted by kalebksp View Post
    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.
    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 ...
    Quote Originally Posted by Route Selection in Cisco Routers - Building the Routing Table section
    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
    You can modify the administrative distance for EIGRP by using the following command...
    Code:
    Router(config-router)#distance eigrp internal-distance external-distance
    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.
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  17. Resident Underachiever EdTheLad's Avatar
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    #16
    Quote Originally Posted by tech-airman View Post

    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.
    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.
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  18. CCIE Bound kryolla's Avatar
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    #17
    AD only comes into play when you have the SAME prefex advertised by different routing protocols then it will take the best AD and the prefix will get installed in the routing table.
    The longest prefix will ALWAYS have precedence over shorter.


    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."

    These 2 are not the same prefex so even though you change the ad they both will get installed in the RIB of the router generating the summary.
    192.168.2.0/23
    192.168.2.0/24
    Last edited by kryolla; 03-18-2009 at 02:06 PM.
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    #18
    Quote Originally Posted by tech-airman View Post
    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...
    Code:
    Router(config-router)#distance eigrp internal-distance external-distance
    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.
    From the Building the Routing Table section:
    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.
    To echo what kryolla and EdTheLad said, AD only comes into play with routes with the same network and prefix portions. You would be correct if summary routes and the more specific routes encompassed in that summary had the same network and prefix portions, which is by very definition not the case.
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    #19

    Smile My thoughts

    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.
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  21. Resident Underachiever EdTheLad's Avatar
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    #20
    Quote Originally Posted by cowood2676 View Post
    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.
    They're not your thoughts,you've taken that straight from a book.
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    #21
    Quote Originally Posted by EdTheLad View Post
    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.
    Hi ED,

    I tried doing what you said
    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
    ....but this does not create a static to null with ad 5 in my routing table. The command is in the running-config but nowhere to be seen in the routing table. Maybe this cant be done since nothing is being summarize because it is the same address with the same mask be are trying to summarize?

    Please confirm... by the way I'm using GNS3
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    #22
    Quote Originally Posted by EdTheLad View Post
    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.

    hey guys!
    could you explain more about this :"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" and " locally" with an example?
    I don't understand mean of summary and locally and I don't know that this route with AD 5 can be loop prevention system?
    please explain with an example
    tnx
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    #23
    Hadi maybe I can shed some light on whats going on here.... First off a summary *route* is configure on an interface that points to a neighbor running the same routing protocol as the originating router.... It is used to minimize the amount of routes being advertised and processed, for example normally if you have an eigrp router (R1) advertising 10.0.0.0/24, 10.0.1.0/24, 10.0.2.0/24, 10.0.3.0/24 and 10.0.4.0/24 to R2, R1 will send to R2 five advertisements, one for each network. This will make 5 entry's in the routing table. In this scenario route summarization will be of benefit to you by minimizing routing traffic and cpu\memory utilization by advertising a summary route to R2. This route would reduce the size of your routing tables and in turn reduce the time, cpu, and memory to do route lookups to know where to forward a packet to one of the 10.0 networks. So when you enter the route summary command on the interface you would like to specify a longer prefix to the 10.0 networks probably 10.0.0.0/21. In return on R2 it will recieve one route advertisement for the 10.0.0.0/21 networks which covers all existing 10.0 networks. Now that R2 is sending all 10.0.0.0/21 traffic to R1 there might accidently be a packet injected into the network addressed to 10.0.5.22/24. So what is going to happen to this packet if there is no null0 interface on R1 and R2 is the last resort is it will be looped forever indefinitely between R1 and R2 because they will both have a valid exiting route that matches that packet. So what R1 does to avoid this is to make a route that matches anything that doesnt match one of its 10 networks and forward it to Null0(Bit Bucket- Trash can). This prevents those kind of loops while minimizing update traffic and resource utilization. I am pretty sure that the Administrative Distance of 5 is also an added loop prevention mechanism to make the bit bucket more preferred for non matching routes than the default next hop.
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  25. Resident Underachiever EdTheLad's Avatar
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    #24
    Lol, that thread is 4.5 years old, last post was more than 1.5 years ago, i hope Hadi hasn't been waiting for a reply!
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    What's going on here?
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