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Network Advertisement and Summarization

Posted on 2014-07-19
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Last Modified: 2014-07-20
I have seen in some configuration where they Advertise the Network with a prefix different than the Interface IP mask.
They also summarize the network different than the interface IP mask.

Example:

Let's say I have R1 connected to R2 through serial interfaces on each router.
R1 serial interface has Ip address 10.10.10.1/24 and R2 serial interface has ip address 10.10.10.2/24
when Network is advertised  in EIGRP for instance, they use the command:
EIGRP 90
Network 10.0.0.0
** I believe The Network command above will by default add a mask of /8 , it is like typing 255.0.0.0
which is different than /24  mask of the serial interface.
So what would be the impact in that case?

another scenario is Summarization,example :
Interface serial 1/0
ip summary-address EIGRP 90 10.10.0.0 255.255.0.0

I see the summarization is based on /16 , so how this would impact the /24 configured on the serial interfaces ?


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Question by:jskfan
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by:Don Johnston
Don Johnston earned 500 total points
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when Network is advertised  in EIGRP for instance, they use the command:
EIGRP 90
Network 10.0.0.0

The network statement is EIGRP (and OSPF) does not define how a network is advertised.  It simply defines which interfaces are classified as EIGRP member interfaces. Which means that EIGRP will attempt to create adjacencies through these interfaces.  For example if this router had interfaces with IP addresses of 10.1.1.1/24, 10.2.2.2/24 and 10.3.3.3/24, all of those interfaces would have EIGRP hellos sent out.

another scenario is Summarization,example :
Interface serial 1/0
ip summary-address EIGRP 90 10.10.0.0 255.255.0.0

I see the summarization is based on /16 , so how this would impact the /24 configured on the serial interfaces ?
Now this actually does affect how an advertisement is sent.  If there was a 10.10.1.0/24 network in the topology table, a summary route of 10.10.0.0/16 would be sent instead.
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by:jskfan
ID: 40207166
<<The network statement is EIGRP (and OSPF) does not define how a network is advertised>>
Sometimes they add the Wild card mask and sometimes not.
if interface is configured with 10.10.10.1/24 and the Network is advertised without mask, then there is no Impact if I understand your comment

EIGRP 90
Network 10.0.0.0
(This implicitly will look just for the first octet to be matched, the rest of the octets do not matter)


****for the Summary , let's say we have a bunch of Loopback interfaces :
2.2.2.1/24
2.2.2.2/24
2.2.2.3/24
2.2.2.4/24

if I am not wrong ,The IP summary has to match the first 3 octets otherwise , the router receiving the summary routes will not be able to reach the loopbacks… Correct ?

it has to be:
ip summary-address EIGRP 90 2.0.0.0 255.255.255.0

and Not:
ip summary-address EIGRP 90 2.0.0.0 255.255.0.0
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Don Johnston earned 500 total points
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Like I said before, the network statement has no bearing on how the network is advertised.

For example, You have a router with four interfaces:
E0: 10.0.0.1/24
E1: 10.1.1.1/24
E2: 10.1.2.1/24
E3: 10.3.3.1/24
And an EIGRP network statement of: network 10.0.0.0

The router will attempt to form an adjacency on every single interface.  Once those adjacencies are formed, the four networks will be advertised (10.0.0.0/24,  10.1.1.0/24, 10.1.2.0/24 and 10.3.3.0/24).

If you don't want E1 and E2 to play EIGRP, then you can use:
network 10.0.0.0 0.0.0.255
network 10.3.3.0 0.0.0.255

The router will only attempt to form adjacencies on E0 and E3. Once those adjacencies are formed, only 10.0.0.0/24 and 10.3.3.0/24 will be advertised since E1 and E2 are not participating in EIGRP.

There's a bit more (passive interfaces and auto summary) but we're just talking about network statements, right? ;-)
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by:jskfan
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