What does auto-summary do in EIGRP, and is it on by default?
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Say this first
Auto-summary summarizes to the classful network boundary when a route is sent out an interface in a
different major network, for example 10.20.1.0/24 sent as 10.0.0.0/8 across a 172.16.0.0 link. It
breaks discontiguous networks. It was on by default in older IOS; from IOS 15.0(1)M and in IOS-XE it is
off. In classic mode I still write no auto-summary so the intent is visible.
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The discontiguous problem
Site A has 10.1.0.0/16 and site B has 10.2.0.0/16, joined through a core addressed from 172.16.0.0/16. With auto-summary, both edge routers advertise 10.0.0.0/8 into the core. The core sees two equal routes to 10.0.0.0/8 and sends half the traffic for site A to site B, where it is dropped.
Turning it off
router eigrp 100
no auto-summaryIn named mode it lives under topology base and is off by default.
The same issue in RIP
RIPv2 has auto-summary too and has historically had it on by default, so I configure no auto-summary
there as well. RIPv1 is classful and always summarizes, which is one reason it is obsolete.
They will ask next
How do you spot auto-summary in a routing table?
A classful route such as 10.0.0.0/8 pointing to Null0 on the summarizing router, and the same classful prefix arriving from two directions on the routers in the middle.
Traps
- Assuming the default is the same on every IOS version. Check the running config and be explicit.