The final EIGRP module. Deep dive into route summarization with Null0 discard protection, EIGRP stub routing options and query confinement, distributed route filtering with prefix-lists and route-maps, and hardened redistribution practices for enterprise deployments.
Author: Hrishi Pradhan
Level: Basic to Advanced
Sections: 4 Theory
Diagrams: 4 Mermaid
Recommended: Complete Module C first
1. Route Summarization & Null0 Loop Protection
Condensing routing tables while preventing blackholes
Section 1
What, Where, How & Why
Route summarization condenses multiple specific subnets into a single aggregate advertisement. EIGRP automatically installs a Null0 discard route with AD 5 to prevent loops and blackholes for unassigned subnets within the summary range. Use it at boundary routers to shrink routing tables and confine query scope.
Configuration: Per-interface with ip summary-address eigrp
Null0 Route: Automatically installed with AD 5 for loop protection
Summary Metric: Based on the best underlying component metric
Multiple Interfaces: Must configure on EVERY interface to prevent suboptimal routing
Why Null0 Matters
Without the Null0 route, traffic to unassigned subnets within the summary would be routed to the summary's next-hop — creating a blackhole or loop. The Null0 route catches these packets and drops them, alerting you to the misconfiguration.
Scenario
Without Null0
With Null0
Specific subnet 10.1.15.0/24 down
Traffic loops or blackholes silently
Traffic is discarded at Null0 with ICMP unreachable
Summary route still advertised
Remote routers install bogus path
Remote routers know summary is valid but reachable only for existing subnets
Troubleshooting
Difficult — no clear signal
Clear signal — Null0 hit counters increment
2. Stub Routing & Query Confinement
Terminating queries at the network edge
Section 2
What, Where, How & Why
Stub routing tells EIGRP neighbors that a router is at the edge of the query domain. The router advertises only specified route types and never forwards queries to other stub routers. Use it on branch, access, and edge routers to prevent cascading SIA events in large EIGRP networks.
Query Confinement: Stub routers do not forward queries to other stub routers
Route Advertisement: Limited to configured route types
Best Practice: Every edge/branch router should be a stub
Result: Query domain stays small — no cascading SIA events
Common Options:connected summary for branch routers
Stub Configuration Options
Keyword
Advertises
Best For
connected
Directly connected networks
Branch routers with local subnets
static
Static routes
Routers with static routes to advertise
summary
Summary routes
Routers that summarize their subnets
redistributed
Redistributed routes
Rare on stubs (usually external connections)
receive-only
Nothing
Pure consumers with no local subnets
Production Example
router eigrp 1234
eigrp stub connected summary
! Advertises only connected + summary routes
! Never forwards queries to other stub routers
! Verify stub status
show ip protocols | section Stub
show ip eigrp neighbors detail
3. Route Filtering with Prefix-Lists & Route-Maps
Precision route control at the EIGRP boundary
Section 3
What, Where, How & Why
Route filtering allows engineers to control exactly which EIGRP routes are accepted from or advertised to neighbors. Prefix-lists match both network and prefix length, while route-maps add conditional logic for complex policies. Use them to prevent route leaks, enforce security boundaries, and control traffic flow.
Direction: Inbound filtering is safer than outbound filtering
Prefix-List vs Standard ACL
Feature
Standard ACL
Prefix-List
Matches network
Yes
Yes
Matches prefix length
No
Yes
Range operators
None
ge, le, eq
Readability
Basic
Clear and structured
Recommended for routing
No
Yes
Prefix-List Examples
! Permit only 10.1.10.0/24
ip prefix-list EXACT_24 permit 10.1.10.0/24
! Permit any subnet within 10.1.0.0/16 with prefix length 20-28
ip prefix-list RANGE permit 10.1.0.0/16 ge 20 le 28
! Permit all /24s in 10.1.0.0/16
ip prefix-list ALL_24 permit 10.1.0.0/16 le 24
! Deny default route (safety)
ip prefix-list NO_DEFAULT deny 0.0.0.0/0
! Apply with distribute-list
router eigrp 1234
distribute-list prefix RANGE in GigabitEthernet0/1
Production Warning
Always apply filters in the inbound direction when possible. Outbound filtering can break downstream routers that depend on the filtered routes. If outbound filtering is required, verify with show ip eigrp topology on the receiving router before deploying.
4. Redistribution Best Practices & Route Tagging
Safe introduction of external routes into EIGRP
Section 4
What, Where, How & Why
Redistribution injects routes from other protocols (OSPF, BGP, static) into EIGRP. Without careful configuration, redistribution can introduce loops, suboptimal routing, and unmanaged traffic patterns. Use route-maps with tags and metrics to control exactly what gets redistributed and how.
External AD: Redistributed routes get AD 170 (vs. internal 90)
Route Tagging: Prevents redistribution loops
Metric Setting: Always specify seed metric during redistribution
Route-Map Control: Filter and manipulate what gets redistributed
Passive Interfaces: Prevent EIGRP from running on external-facing interfaces
Redistribution Safety Checklist
Practice
Why It Matters
Use route-maps for all redistribution
Prevents accidental redistribution of unintended routes
Set explicit seed metric
Prevents infinite metric causing route rejection
Apply route tags
Enables loop detection across redistribution boundaries
Filter by prefix-list
Restricts which prefixes enter EIGRP
Use passive-interface default
Prevents EIGRP neighborships on external links
Document the redistribution policy
Future engineers understand the intent
Production Redistribution Example
! Step 1: Define prefix-list of routes to redistribute
ip prefix-list REDISTRIBUTE_OSPF permit 10.16.0.0/16 le 24
ip prefix-list REDISTRIBUTE_OSPF deny 0.0.0.0/0 le 32
! Step 2: Create route-map with tag and metric
route-map OSPF_TO_EIGRP permit 10
match ip address prefix-list REDISTRIBUTE_OSPF
set tag 100
set metric 100000 100 255 1 1500
! Step 3: Apply in EIGRP process
router eigrp 1234
redistribute ospf 1 route-map OSPF_TO_EIGRP
passive-interface default
no passive-interface GigabitEthernet0/0
! Step 4: Verify
show ip route eigrp | include EX
show ip eigrp topology | section External
The Golden Rule of Redistribution
Never redistribute "all routes" from one protocol into another without filtering. Use route-maps with prefix-lists to control exactly what enters EIGRP. Tag the redistributed routes so that if they are re-redistributed back into the original protocol, loops can be detected and prevented.