EIGRP -- no auto-summary

Ola_CISCOOla_CISCO Member Posts: 73 ■■□□□□□□□□

Could someone please explain why the "no auto-summary" is needed when when configuring EIGRP?

Thanks in advance


  • binargsbinargs Member Posts: 55 ■■□□□□□□□□
    eigrp does both class and classless
    it default to classful
    when you do no auto summ it will go into classless mode and start sending the subnet mask alone with its updates

    i hope you know the difference between class and classless routing.

    here is an good paragraph i found today:

    Short answer: Yes, the packet drops if no subnet of a classful network is


    Let's explore this reasoning a bit more: Say you have a /24

    network. When the router discovers this network (either through connected,

    static or dynamic routing processes) it adds it into the routing table. When

    you do a 'show ip route' you will see /16 on one line and the next

    line will show subnet is a child subnet to the parent network. The parent network was added automatically because

    all routers still structure their routing table in a classful way (even if

    the lookup process is classless). When the router does it's look up, it will

    find that (using binary) that is a match BUT there are child

    subnets with better matches that specify more specific subnets so it

    continues. The router continues to lookup the child subnets listed. If a

    match is found, the packet will be ultimately be forwarded to a destination

    specified by the child route. If no child subnet exist, the packet will be

    dropped. For example, if a packet comes through looking for a

    subnet, and the router only have the subnet listed under the network, the packet will be dropped.

    So why not use a default route if it is available? Why just stop and drop

    packet if there is a gateway of last resort available? The answer is a

    historical one. When routers were all classful, having an organization own a

    parent network meant that the organization owned all the child subnets and

    managed them. So if the packet came through with destination network address

    of a child subnet that the router couldn't find, the router would drop it

    because the network just simply didn't exist. There was no way that the child

    subnet could belong to a different organization (also known as discontiguous

    networks). It wasn't until classless routing behavior became possible that

    subnets could be in different places.
  • fsanyeefsanyee Member Posts: 171
    binargs wrote: »
    when you do no auto summ it will go into classless mode and start sending the subnet mask alone with its updates

    This is not true. EIGRP send the subnetmask in the updates if auto-summary is turned on or off.
    if auto summary is in effect, EIGRP automatically summarize at classfull boundaries if its needed.
  • rowelldioniciorowelldionicio Registered Users Posts: 2 ■□□□□□□□□□
    You don't have to configure no auto-summary but if you are going to have discontiguous networks then you should issue the no auto-summary command
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