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Working through a lab for CCNP prep. Task is simple - create 4 loopback addresses on R3 - all 172.16.X.0/24 addresses, and share the routes to neighboring routers so that they appear as /22's instead...

I went to the eigrp process on R3 and typed in 'network 172.16.0.0 0.0.3.255' but no summary routes are shared with neighbors. What am I missing?

For the record several other routes are already shared through this particular process so there is no underlying issue with with the eigrp setup (afaik).

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  • could we see the output of sh ip int brief, show runn | b router eig and sh ip eigrp interfaces?
    – Mike Pennington
    Jul 31, 2013 at 16:27
  • As requested sir ! imgur.com/a/AY3cO#0
    – A L
    Jul 31, 2013 at 17:01

2 Answers 2

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The "network" command is not responsible for summarizing routes. In EIGRP, this command merely enables the EIGRP process on all matched interfaces. You should use the 

ip summary-address

command on all interfaces on which you wish to advertise a summary.

What the "network" command does is define a range of addresses that enables EIGRP on all interfaces with IP addresses that fall within that range. For example, if you define a network like:

network 192.168.0.1 0.0.0.0

For an IP address with the address 192.168.0.1 and a 24-bit mask, it would not advertise a 32-bit network, but rather the 24-bit network of the interface matched by THAT SPECIFIC IP address. This "pool" of addresses only contains that single IP address because of the all-zeros wildcard mask.

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  • It should be mentioned that this is common mistake with understanding of what 'network' command does. Mierdin has summarized it ok.
    – Milan Prpic
    Jul 31, 2013 at 17:39
  • This is interesting to me, I do understand the distinction you mentioned, but wouldn't the command I input activate the EIGRP process on the loopbacks and then share the routes? Doesn't the /22 encompass the /24 IP's on each Loopback interfaces - or does it only match on classful boundaries (even without auto-summary) unless you use the ip summary-address command like you and Ken posted?
    – A L
    Jul 31, 2013 at 18:52
  • The summary-address command is separate from the network command. The network command you listed will enable EIGRP on your loopback interfaces only, because it does encompass the /24 IP addresses on these loopbacks. That does not mean that those /24s will be automatically summarized. The "network" command in any IGP does not inherently advertise routes. It merely enables EIGRP on all matched interfaces, preserving their original prefix length by default. This is one of the most common sources of confusion regarding the network command, so you're not alone.
    – Mierdin
    Jul 31, 2013 at 18:58
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    You're very close. Check out this blog article (keepingitclassless.net/2013/07/…) which goes into a little more detail. (Full disclosure, this is my own technical blog, I figured it was overdue and a better way to convey the idea than in the comments section here.)
    – Mierdin
    Jul 31, 2013 at 19:59
  • Cool, my distinction for EIGRP was: does it advertise the route when you use the network command if it has the networks exact subnet mask, or it falls on classful boundaries? I ask because I don't have to do anything additional to share 192.168.1.0/24 when I input 'network 192.168.1.0' - Thanks so much for your clarifications - it seems simple but it's great to know the low level detail!
    – A L
    Jul 31, 2013 at 20:00
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ip summary-address eigrp 172.16.0.0 255.255.252.0

Configure this on the intefaces facing the neighbor(s) you want to advertise the summary route.

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  • Checked syntax you'd need the ASN # after the 'eigrp' piece of that command :)
    – A L
    Jul 31, 2013 at 19:52

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