Route Selection and AD

Lab Objectives

• Understand how routers choose the best route using Administrative Distance (AD), longest-prefix match, and metrics (EIGRP composite metric).
• Configure EIGRP 111 across the base topology and create loopbacks to represent LANs; use manual summarization to observe longest-match behavior.
• Observe how a static route (lower AD) overrides an EIGRP-learned route; verify with show commands.

Topology (use this exact base topology)

                [Internet]
               203.0.113.1
                    |
               R1 (Gateway)
              Gi0/0: 10.10.10.1
              Gi0/1: 10.10.20.1
              Gi0/2: 10.10.30.1
              /     |     \
           R2      R3      R4

Gi0/0: 10.10.10.2 Gi0/0: 10.10.20.2 Gi0/0: 10.10.30.2 Gi0/1: 10.10.40.1 | | / \ | | S1 S2 | | / \ | / \ / \

IP scheme (used in this lab)

• 10.10.10.0/24 — R1-R2
• 10.10.20.0/24 — R1-R3
• 10.10.30.0/24 — R1-R4
• 10.10.40.0/24 — R2-S1
• 192.168.1.0/24 — VLAN 10 (Sales) — represented on R2
• 192.168.2.0/24 — VLAN 20 (Engineering) — represented on R2
• 192.168.3.0/24 — VLAN 30 (Management) — represented on R3
• 203.0.113.0/24 — Public/Internet simulation

Lab Tasks (Try It Yourself First!)

Complete these tasks WITHOUT looking at the solution below. Use ? and show commands to figure it out.

Task 1: Enable EIGRP and add LAN loopbacks

On R1–R4: enable EIGRP 111 and advertise the connected 10.10.x.0 links. On R2 add loopbacks for 192.168.1.0/24 and 192.168.2.0/24. On R3 add a loopback for 192.168.3.0/24. Disable auto-summary.

Task 2: Configure a manual summary on R2

On R2 Gi0/0 (towards R1) summarize 192.168.1.0/24 and 192.168.2.0/24 into a single 192.168.0.0/22 summary (so R1 receives the summary instead of specifics).

Task 3: Demonstrate Administrative Distance

On R1 create a static route to 192.168.1.0/24 that points to the Internet next hop (203.0.113.1). Observe how the static route (AD 1) affects route selection; then remove it to return control to EIGRP (AD 90).

Think About It: If R1 knows a summarized route 192.168.0.0/22 via R2 and also learns a specific /24 for 192.168.3.0/24 via R3, which route will be used for an address 192.168.3.5? Why?

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Lab Solution

Task 1 Solution: Enable EIGRP and add LAN loopbacks

What we are doing: We will configure EIGRP 111 on each router so they exchange routes. Creating loopback interfaces models the internal LAN subnets so they are advertised into EIGRP. Disabling auto-summary ensures classful auto-summarization does not hide subnets across discontiguous networks.

! On R1
router eigrp 111
 network 10.10.10.0
 network 10.10.20.0
 network 10.10.30.0
 no auto-summary

! On R2
interface Loopback1
 ip address 192.168.1.1 255.255.255.0
interface Loopback2
 ip address 192.168.2.1 255.255.255.0
router eigrp 111
 network 10.10.10.0
 network 10.10.40.0
 network 192.168.0.0
 no auto-summary

! On R3
interface Loopback3
 ip address 192.168.3.1 255.255.255.0
router eigrp 111
 network 10.10.20.0
 network 192.168.3.0
 no auto-summary

! On R4
router eigrp 111
 network 10.10.30.0
 no auto-summary

What just happened:

• interface LoopbackX / ip address ... — creates loopback interfaces that simulate LAN subnets. Loopbacks are stable and always up, making them good for lab networks.
• router eigrp 111 — starts EIGRP process with AS number 111.
• network <network> — tells EIGRP to advertise interfaces in those networks.
• no auto-summary — prevents classful summarization at classful boundaries, ensuring specific prefixes are advertised as-is.

Verify:

! On R1: Check EIGRP routes learned
show ip route | include 192.168|D
D    192.168.1.0/24 [90/???] via 10.10.10.2, 00:00:12, GigabitEthernet0/0
D    192.168.2.0/24 [90/???] via 10.10.10.2, 00:00:12, GigabitEthernet0/0
D    192.168.3.0/24 [90/???] via 10.10.20.2, 00:00:12, GigabitEthernet0/1

Tip: The D code shows the route was learned via EIGRP. The metric shown ([90/metric]) displays the administrative distance (90) and the EIGRP composite metric.

Task 2 Solution: Configure a manual summary on R2

What we are doing: Configure ip summary-address eigrp on R2's interface toward R1 so R2 advertises a single summarized route (192.168.0.0/22) instead of separate /24s to R1. This demonstrates the effect of summaries on what remote neighbors learn and highlights the longest-prefix match behavior.

! On R2, apply summary on the interface facing R1 (Gi0/0)
interface GigabitEthernet0/0
 ip summary-address eigrp 111 192.168.0.0 255.255.252.0

What just happened:

• ip summary-address eigrp 111 192.168.0.0 255.255.252.0 — causes R2 to send the summarized route 192.168.0.0/22 to neighbors on Gi0/0. R2 will advertise the summary toward R1, which reduces routing table entries on R1 but removes information about the exact /24s in that direction.

Why this matters: Summarization reduces routing table size and update churn in large networks, but it affects route selection because remote routers may only see the summary. However, when a router has both a less-specific route and a more-specific route, the more-specific (longest-prefix) will be chosen.

Verify:

! On R1: Check what R1 learned from R2
show ip route 192.168.1.0
S? (no static here) 
! Example of what to expect in the routing table after the summary:
D    192.168.0.0/22 [90/2169856] via 10.10.10.2, 00:00:30, GigabitEthernet0/0

! On R1: Also check the /24 for 192.168.3.0 that comes from R3
show ip route 192.168.3.0
D    192.168.3.0/24 [90/2169856] via 10.10.20.2, 00:00:30, GigabitEthernet0/1

Interpretation:

• R1 now has a summarized route 192.168.0.0/22 via R2. If R1 does not know any more specific /24 for a destination like 192.168.1.10, it will use this summary route to reach R2.
• For 192.168.3.0/24, R1 has a more-specific /24 learned via R3 — the router will choose the /24 route for addresses in 192.168.3.0/24 (longest-prefix match).

Task 3 Solution: Demonstrate Administrative Distance

What we are doing: Add a static route on R1 for 192.168.1.0/24 via the Internet next hop. Static routes have AD=1, which is preferred over EIGRP (AD=90). This shows how AD controls which source is trusted when multiple sources advertise the same prefix.

! On R1 add a static route to 192.168.1.0 via the Internet gateway
ip route 192.168.1.0 255.255.255.0 203.0.113.1

! Verify the route selection now
show ip route 192.168.1.0

What just happened:

• ip route — creates a static route. Static routes default to AD 1, which is more trusted than an EIGRP-learned route (AD 90). The static will therefore appear in the routing table and be used for forwarding, even if EIGRP also provides the same prefix.

Verify:

! Expected output on R1 after adding static
show ip route 192.168.1.0
S    192.168.1.0/24 [1/0] via 203.0.113.1
D    192.168.1.0/24 [90/2169856] via 10.10.10.2, 00:00:30, GigabitEthernet0/0

! Note: The routing table will show the static (S) entry as the best route due to lower AD.

To return to EIGRP-controlled forwarding, remove the static:

no ip route 192.168.1.0 255.255.255.0 203.0.113.1

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Troubleshooting Scenario

Scenario: R1 forwards traffic for 192.168.1.0/24 out to the Internet (203.0.113.1) but that next-hop is unreachable.

Symptom: Ping from R1 to 192.168.1.10 times out; show ip route 192.168.1.0 shows a static route via 203.0.113.1. Your task: Find and fix the issue so R1 forwards toward the local LAN (via R2) instead of the unreachable Internet hop. Hint: Check administrative distance and look for static routes. Solution:

• On R1, remove the problematic static route so EIGRP-learned route to 192.168.1.0 is used:

no ip route 192.168.1.0 255.255.255.0 203.0.113.1

• Verify R1 now uses EIGRP route:

show ip route 192.168.1.0
! Expect: D 192.168.1.0/24 via 10.10.10.2 ...

Explanation: The static route had lower AD and thus overrode EIGRP; removing it returns control to dynamic routing.

Warning: In production, accidentally configured static routes can cause traffic blackholing if the next hop is unreachable. Always verify static next-hops are reachable or use tracking mechanisms.

Verification Checklist

• EIGRP 111 is enabled and neighbors are up (check show ip eigrp neighbors).
• Loopbacks for 192.168.1/2/3 are present and advertised.
• R2 advertises 192.168.0.0/22 summary toward R1 (show ip route on R1 shows the summarized prefix).
• Static route on R1 (if configured) overrides EIGRP learned prefix; removing it restores EIGRP path.

Common Mistakes

Symptom	Cause	Fix
No EIGRP routes appear on R1	Forgot network statements or no auto-summary mismatch	Add correct network under router eigrp 111 and verify interfaces are in those nets
R1 only sees summary and cannot reach specific hosts	Summary applied in the direction of R1 suppressed specifics	Remove or adjust ip summary-address or place summary on a different interface
Static route unintentionally overrides dynamic route	Static has lower AD (1) than EIGRP (90)	Remove static or change to a floating static with higher AD

Challenge Task

Configure R2 to summarize 192.168.1.0/24 and 192.168.2.0/24 into 192.168.0.0/23 (255.255.254.0) instead of /22, and observe which /24 addresses fall inside the new summary and which do not. Explain which destinations use the summary vs specific routes and why.

Important real-world note: In data centers and large WANs, AD and longest-prefix match determine where traffic goes; engineers use summarization to reduce route churn, but must carefully design summaries to avoid hiding important specifics. Think of AD as “trust score” (lower is more trusted) and longest-prefix as “the most precise address you have.”