Which three statements are true about EIGRP route summarization? (Choose three.)
A.
Manual route summarization is configured in router configuration mode when the router
is configured for EIGRP routing.
B.
Manual route summarization is configured on the interface.
C.
When manual summarization is configured, the summary route will use the metric of the
largest specific metric of the summary routes.
D.
The ip summary-address eigrp command generates a default route with an
administrative distance of 90.
E.
The ip summary-address eigrp command generates a default route with an
administrative distance of 5.
F.
When manual summarization is configured, the routerimmediately creates a route that
points to null0 interface.
Explanation:
Theip summary-address eigrp{AS number} {address mask} command is used to
configure a summary aggregate address for a specified interface. For example with the
topology below:
R2 has 5 loopback interfaces but instead of advertising all these interfaces we can only
advertise its summarized subnet. In this case the best summarized subnet should be
1.1.1.0/29 which includes all these 5 loopback interfaces.
R2(config)#interface fa0/0
R2(config-if)#ip summary-address eigrp 1 1.1.1.0 255.255.255.248
This configuration causes EIGRP to summarize network 1.1.1.0 and sends out Fa0/0
interface
After configuring manual EIGRP summary, the routingtable of the local router will have a
route to Null0:
So why is this route inserted in the routing table when doing summarization? Well, you may
notice that although our summarized subnet is 1.1.1.0/29 but we don’t have all IP addresses
in this subnet. Assignable IP addresses of subnet 1.1.1.0/29 are from 1.1.1.1 to 1.1.1.6.
Imagine what happens if R1 sends a packet to 1.1.1.6. Because R1 do believe R2 is
connected with this IP so it will send this packet to R2. But R2 does not have this IP so if R2
has a default-route to R1 (for example R1 is connected to the Internet and R2 routes all
unknown destination IP packets to R1) then a loop will occur.
To solve this problem, some routing protocols automatically add a route to Null0. A packet is
sent to “Null0″means that packet is dropped. Suppose that R1 sends a packet to 1.1.1.6
through R2, even R2 does not have a specific route for that IP, it does have a general route
pointing to Null0 which the packet sent to 1.1.1.6 can be matched -> That packet is dropped
at R2 without causing a routing loop!
By default, EIGRP summary routes are given an administrative distance value of 5. Notice
that this value is only shown on the local router doing the summarization. On other routers
we can still see an administrative distance of 90 in their routing table.
