EIGRP also enables you to manually configure a prefix to
use as a summary address. Manual summary routes are configured on a
per-interface basis, so you must first select the interface that will
propagate the route summary. Then you can define the summary address
with the ip
summary-address eigrp
command, which has the following syntax:
router(config-if)#ip
summary-address eigrp autonomous-system-number ip-address mask
administrative-distance
EIGRP summary routes have an administrative distance
of 5 by default. Optionally, they can be configured for a value between
1 and 255.
In the figure, RTC can be configured using the commands shown:
RTC(config)#router eigrp 2446
RTC(config-router)#no auto-summary
RTC(config-router)#exit
RTC(config)#interface serial0
RTC(config-if)#ip summary-address eigrp 2446 2.1.0.0 255.255.0.0
Thus, RTC will add a route to its table, as follows:
D 2.1.0.0/16 is a
summary, 00:00:22, Null0
Notice that the summary route is sourced from Null0,
and not an actual interface. That's because this route is used for
advertisement purposes and does not represent a path that RTC can take
to reach that network. On RTC, this route has an administrative distance
of 5.
In the figure,
RTD is oblivious to the summarization but accepts the route, and it
assigns the route the administrative distance of a "normal"
EIGRP route (which is 90, by default). In the configuration for RTC,
automatic summarization is turned off, with no
auto-summary
command. If it wasn't, RTD would receive two routes, the
manual summary address (2.1.0.0 /16) and the automatic, classful summary
address (2.0.0.0 /8).
In most cases, when you manually summarize, you should
also issue the no
auto-summary
command.
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