When you configure a summary, the router installs a route for it pointing to Null0. Why, and what happens without it?
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The summarizing router advertises the whole block, so it attracts traffic for every address in it, including addresses that do not exist. Without the discard route, a packet for an unused part of the block matches the router's default route and goes back toward the core, which sends it straight back: a loop until TTL expires. The Null0 route drops it at the router that knows the block is not fully there. Real subnets are longer prefixes, so they still win.
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Worked example
DSW1 has 10.20.10.0/24 and 10.20.20.0/24, advertises 10.20.0.0/16 to CORE1, and uses a default route toward CORE1.
DSW1# show ip route | include Null0|0.0.0.0/0
S* 0.0.0.0/0 [1/0] via 10.0.1.1
D 10.20.0.0/16 is a summary, 00:31:12, Null0A packet for 10.20.99.5 arrives at DSW1.
- With the discard route: the best match is the /16 to Null0. Dropped once, done.
- Without it: the best match is 0.0.0.0/0, so DSW1 sends it to CORE1. CORE1's best match is the /16 via DSW1, so it comes back. The packet bounces until its TTL reaches 0, using bandwidth both ways, and a traceroute shows CORE1 and DSW1 alternating.
Who installs discard routes
| Source | Discard route |
|---|---|
| EIGRP summary on an interface | D ... is a summary, Null0, AD 5 |
OSPF area range and summary-address | installed by default on IOS (discard-route controls it) |
BGP aggregate-address | B ... Null0, AD 200 |
| Your own static summary | add ip route <summary> <mask> Null0 yourself |
The trap with AD 5
A hub learns its internet default from BGP or OSPF and sends spokes an EIGRP summary of 0.0.0.0/0. The discard route for that summary is 0.0.0.0/0 to Null0 at AD 5, which beats the real default (AD 20 or 110), and the hub black-holes internet traffic. Give that summary an AD of 255 so it is advertised but its discard route is never installed:
interface Tunnel0
ip summary-address eigrp 100 0.0.0.0 0.0.0.0 255They will ask next
Why AD 5 for the EIGRP summary and not 90?
So the discard route reliably beats copies of the same prefix learned from elsewhere, which would otherwise pull the summarized traffic away or create the loop it is meant to prevent.
Traps
- Deleting the Null0 summary route because "Null0 is dropping my traffic". The traffic it drops has nowhere real to go; the actual fault is a missing specific route.