EIGRP -- no auto-summary

Ola_CISCO

Hello,

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


Thanks in advance

binargs

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

found.



Let's explore this reasoning a bit more: Say you have a 172.16.1.0 /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 172.16.0.0 /16 on one line and the next

line will show 172.16.1.0. 172.16.1.0 subnet is a child subnet to the

172.16.0.0 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 172.16.0.0 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 172.16.2.0

subnet, and the router only have the 172.16.1.0 subnet listed under the

172.16.0.0 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.