ip classless and auto-summary

how are you everyone ?

I'm just a little bit confused about these two commands: ip classless and auto-summary.

It sounds like both commands have the same function.. Please tell me what the differnece is..

I have read a lot about this commands, but I couldn't find a point -.-;

when auto-summary is used ? if i use no auto-summarry, do i still have to concern about ip classess ?

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"ip classless" should always be enabled. Back in the old days, the first octet of the IP address determined its "class" 1 through 127 were class A,

128 through 191, were class B, 192 to 223 were Class C, 224 to 239 were Class D, and 240 to 255 were Class E. The class determined the network mask. Class A networks had mask, class B,, class C, class D is reserved for multicast, and class E was reserved This idea went out a long time ago, and now class does not have any meaning except for Class D, which is reserved for multicast, and Class E, which is still reserved.

auto-summary is a similar concept, but is used for routing, where the routing protocol would "auto-summarize" the routes using the above obsolete mechanism. You should also enable "no auto-summary" as well.

"classfull" addresses and "auto-summary" were the original IOS defaults way back when we had a "classfull" Internet. Today that is not the case, but Cisco very, very, very rarely ever changes IOS default settings.


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For an explanation of ip classless see Cisco doc:

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"ip classless" is used when the router is searching its routing table to determine how to forward a packet. Without this option, if it has a route for any subnet of a classic network, and it's searching for a route within that network, it will not use any aggregates outside the network. E.g. if you have routes:

and you're trying to send to, it will report "No route", even though matches. In effect, the route causes it to create a virtual null route for the 172.15 class B network.

Since a default route is just the ultimate aggregate, the most common cause of leaving out "ip classless" is that packets for unknown subnets are dropped rather than being sent to the default gateway.

"auto-summary" is use by routing protocols. It causes the router to

*advertise* this virtual route when it has routes for the subnets.

If you have both auto-summary and ip classless, it's easy to create routing loops. If the router advertises the class B to its default gateway, the gateway will send packets for any subnet to this router. But if the router doesn't have a specific route for that subnet it will send the packets back to the default gateway, and they'll go back and forth.

What I recommend instead of auto-summary is that you create an explicit Null0 route for the class B if you want to advertise it.

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Barry Margolin

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