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EIGRP default route and summary

Dragon0x40
Dragon0x40 asked
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In the BSCI Certification Guide they have the following config.

interface s1/0
ip summary-address eigrp 100 172.xx.x.0 255.255.224.0
interface s1/1
ip summary-address eigrp 100 0.0.0.0 0.0.0.0

the ip summary-address is to summarize  the networks 172.xx.x.0/24 - 172.xx.xx.0/24 back towards the core of the network right?

The guide says "Thus the division is summarized to the larger network and receives back a simple default route."

What does that mean? I can understand the summary address being placed in the routing table and matching any destination in  172.xx.x.0 /19 but how does the default route get matched? Does 0.0.0.0 get installed in the local route table or advertised to other routers?

Sorry it does not have a diagram and it is just confusing me.

It also states: "Summarizing the default route can be a useful technique. However, if used in cases where there are multiple default paths, this will reduce routing options and prevent fail over to alternate paths when the primary path fails."

I don't understand that statement either. We don't have EIGRP at work and I am trying to learn it.
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Author

Commented:
Would the Larger Corporate network be connected to s1/1 and the smaller division be connected on s1/0?
Commented:
I think this statement "Thus the division is summarized to the larger network and receives back a simple default route." is refering to this configuration

interface s1/0
ip summary-address eigrp 100 172.xx.x.0 255.255.224.0
interface s1/1
ip summary-address eigrp 100 0.0.0.0 0.0.0.0

Default route is advertised out S1/1 while summary is sent out S1/0. When EIGRP creates a summary route it will inject the same summary route into its local routing table but this route will point to a Null interface. This is for loop avoidance.

> ip summary-address eigrp 100 172.xx.x.0 255.255.224.0

Yes, This statement summarizes the range 172.xx.x.0-172.xx.xx.255 as 172.xx.x.0/19 out the S1/0 interface. Without seeing the Network topology we really cant say much about what direction the summary is destined.


"Summarizing the default route can be a useful technique. However, if used in cases where there are multiple default paths, this will reduce routing options and prevent fail over to alternate paths when the primary path fails."

Can you provide more information?

Commented:
"Summarizing the default route can be a useful technique. However, if used in cases where there are multiple default paths, this will reduce routing options and prevent fail over to alternate paths when the primary path fails."

Can you provide more information? What book is this quoted from? chapter? page?
Top Expert 2004
Commented:
>Would the Larger Corporate network be connected to s1/1 and the smaller division be connected on s1/0?
   I suspect it practice it would be the other way around. Basically all routes are summarized to the default, and only the default would be sent out s1/1.

The second statement refers to the fact that if you have multiple paths to a route but you only know about the default, then how do you determine which path is best, or even valid, to a particular network? This is always the balancing act when you want to limit your routing tables but also provide for optimal decision-making.
Top Expert 2004
Commented:
I personally would never summarize to a default; if a default is what I wanted to send, I would use a distribute list.

the other thing about summarizing to a default is that as nazsky said, the router would create a default route to Null0. Since that shows as "connected" it would override any learned default route and so the router doing it would not have a true default. So you would never want to do this unless that core router was the center of the world and did not have (or need) internet access.

Author

Commented:
Summarization may be manually applied at any point in the network. You can configure
manual summarization on any router interface. Consider summarization for both
upstream and downstream neighbors. Upstream neighbors should receive a consolidated
route, and downstream neighbors can receive a default route.
There are two commands for summarization with EIGRP: no auto-summary and ip
summary-address eigrp autonomous-system-number address mask. The no autosummary
command disables automatic summarization. With no auto-summary
configured, all known subnets are advertised out of each interface, limited only by splithorizon.
The ip summary-address eigrp command is used to announce a specific range on a
particular interface. This allows bidirectional summarization and allows summarization
to be more widely utilized.
Manual summarization is configured at the interface level, as shown here:
Router(config)#interface S0
Router(config-if)#ip summary-address eigrp autonomous-system-number address mask
For example, imagine a router that connects to the corporate network and to a division
with IP subnets such as 172.xx.x.0/24, 172.xx.x.0/24...172.xx.xx.0/24 on serial 1/1. It also
has a default route on serial 1/0. Configuration of manual summarization is done at the
interface level. The corporation uses AS 100. This is shown in Example 4-3.
Example 4-3. EIGRP Summarization
m. Router(config)#interface S1/0
n. Router(config-if)#ip summary-address eigrp 100 172.xx.x.0 255.255.224.0
o. Router(config-if)#interface S1/1
Router(config-if)#ip summary-address eigrp 100 0.0.0.0 0.0.0.0
Thus the division is summarized to the larger network and receives back a simple default
route.
Caution
Summarizing the default route can be a useful technique. However, if used in cases
where there are multiple default paths, this will reduce routing options and prevent
failover to alternate paths when the primary path fails.
Page 78
CCNP BSCI Official Exam Certification Guide, Fourth Edition By Brent Stewart ISBN:
158720147X Publisher: Cisco Press