Day 25 RIP and EIGRP

RIP (Routing Information Protocol) is an industry-standard distance vector interior gateway protocol that uses hop count as its metric to determine the best path to a destination network. RIP operates on the principle of "routing-by-rumor," where routers share their complete routing tables with directly connected neighbors, and those neighbors then share what they learned with their neighbors.

EIGRP (Enhanced Interior Gateway Routing Protocol) is an advanced or hybrid distance vector routing protocol originally developed by Cisco that uses a composite metric based on bandwidth and delay to calculate the best path. EIGRP is considered superior to RIP because it converges faster, supports larger networks, and can perform unequal-cost load balancing.

The NETWORK command in RIP, EIGRP, and OSPF does not directly specify which networks to advertise; instead, it tells the router which interfaces to activate the routing protocol on based on IP address matching, and the router then advertises the actual network prefixes configured on those interfaces.

Wildcard masks are inverted subnet masks used in EIGRP and OSPF network commands where binary 0s indicate bits that must match and binary 1s indicate bits that don't need to match between the interface IP address and the network command.

Passive interfaces are interfaces configured to stop sending routing protocol advertisements while still allowing the network prefix of that interface to be advertised to neighbors through other interfaces.

RIP Key Points

• Maximum hop count is 15 (16 = unreachable)

• Updates sent every 30 seconds by default

• Administrative Distance: 120

• ECMP load-balancing over 4 paths by default

• Uses Request and Response messages

EIGRP Key Points

• Multicast address: 224.0.0.10

• Administrative Distance: 90 (internal) / 170 (external)

• Only IGP that supports unequal-cost load balancing

• No hop count limit (supports large networks)

• Faster convergence than RIP

• Uses bandwidth (slowest link) and delay (sum of all links) for metric by default

• AS numbers must match for adjacency formation

• ECMP load-balancing over 4 paths by default

1. NETWORK command misconception: The NETWORK command does NOT specify what networks to advertise—it specifies which interfaces to activate the protocol on. The router advertises the actual prefix on those interfaces.

2. Classful NETWORK command: Even with RIPv2 or EIGRP, the NETWORK command itself is classful unless you specify a wildcard mask (EIGRP only).

3. Auto-summary trap: If auto-summary is enabled, routes are advertised as classful networks regardless of actual configuration. Always use no auto-summary.

4. EIGRP route symbol: EIGRP routes show as "D" in the routing table, NOT "E" (E is for EGP).

5. Wildcard mask confusion: Remember wildcard masks are INVERTED—0 means "must match," opposite of ACLs logic for some students.

6. Router ID selection: Manual configuration > Loopback IP > Physical Interface IP (highest wins in each category).

7. AS number mismatch: EIGRP routers won't form adjacency if AS numbers differ.

8. Passive interface behavior: Stops SENDING advertisements but the network is still ADVERTISED to other neighbors.

Wildcard Mask Shortcut

Subtract each octet from 255:

• /24 (255.255.255.0) → 0.0.0.255

• /28 (255.255.255.240) → 0.0.0.15

• /21 (255.255.248.0) → 0.0.7.255

Multicast Address Memory Trick

• RIPv2 = 224.0.0.9 (2 + 7 = 9, think "RIP 2 heaven at cloud 9")

• EIGRP = 224.0.0.10 (one more than RIP, EIGRP is "one better")

AD Values Memory

• RIP = 120 ("RIP is old and slow, high AD like 120 years old")

• EIGRP = 90 ("EIGRP is Enhanced, lower/better AD of 90")

• EIGRP External = 170 ("External routes less trusted")

Router ID Priority

"Manual Loops over Physical"

1. Manual configuration

2. Loopback interface (highest IP)

3. Physical interface (highest IP)

EIGRP Metric Components (Default)

"Bandwidth and Delay" - K1 and K3 = 1

• Bandwidth: Slowest link in path

• Delay: Sum of all links

Example 1: RIP Basic Configuration

Router(config)# router rip

Router(config-router)# version 2

Router(config-router)# no auto-summary

Router(config-router)# network 10.0.0.0

Router(config-router)# network 172.16.0.0

Router(config-router)# passive-interface g2/0

Router(config-router)# default-information originate

Example 2: EIGRP Configuration with Wildcard Mask

Router(config)# router eigrp 1

Router(config-router)# no auto-summary

Router(config-router)# network 10.0.0.0

Router(config-router)# network 172.16.1.0 0.0.0.15

Router(config-router)# passive-interface g2/0

Router(config-router)# eigrp router-id 1.1.1.1

Example 3: Wildcard Mask Calculation

Convert /28 subnet mask to wildcard:

• Subnet mask: 255.255.255.240

• Calculation: 255-255=0, 255-255=0, 255-255=0, 255-240=15

• Wildcard mask: 0.0.0.15

Example 4: Network Command Matching

Interface IP: 172.16.1.14/28 Command: network 172.16.1.0 0.0.0.15

• First 28 bits must match (wildcard 0s)

• 172.16.1.14 and 172.16.1.0 share first 28 bits ✓

• EIGRP activates on interface

A network administrator configures RIP on a router with the command network 192.168.50.0. The router has the following interfaces:

• G0/0: 192.168.50.1/26

• G0/1: 192.168.50.65/26

• G0/2: 10.0.0.1/24

On which interface(s) will RIP be activated?

A. G0/0 only

B. G0/0 and G0/1

C. G0/0, G0/1, and G0/2

D. None of the interfaces

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Answer: B

Explanation: The NETWORK command is classful. 192.168.50.0 is in the Class C range, so it's interpreted as 192.168.50.0/24. Both G0/0 (192.168.50.1) and G0/1 (192.168.50.65) fall within this range. G0/2 (10.0.0.1) does not match and won't have RIP activated.

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Which of the following statements about EIGRP is TRUE?

A. EIGRP uses hop count as its primary metric

B. EIGRP routes appear with the letter "E" in the routing table

C. EIGRP is the only IGP that supports unequal-cost load balancing

D. EIGRP has a maximum hop count of 15

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Answer: C

Explanation: EIGRP is unique among IGPs in supporting unequal-cost load balancing. Option A is incorrect (EIGRP uses bandwidth and delay). Option B is incorrect (EIGRP uses "D" for routes). Option D is incorrect (that's RIP's limitation; EIGRP has no practical hop limit).

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What is the wildcard mask equivalent of the subnet mask 255.255.252.0?

A. 0.0.3.255

B. 0.0.4.255

C. 0.0.252.255

D. 0.0.0.255

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Answer: A

Explanation: Subtract each octet from 255:

• 255 - 255 = 0

• 255 - 255 = 0

• 255 - 252 = 3

• 255 - 0 = 255

Result: 0.0.3.255 (This represents a /22 prefix)

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Router R1 has the following configuration:

router eigrp 100

network 10.0.0.0

Router R2 has the following configuration:

router eigrp 200

network 10.0.0.0

Both routers are connected via their interfaces in the 10.0.0.0/24 network. What will happen?

A. The routers will form an EIGRP adjacency and share routes

B. The routers will not form an adjacency because the AS numbers don't match

C. The routers will form an adjacency but won't share routes

D. R1 will become the designated router

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Answer: B

Explanation: EIGRP routers must have matching AS (Autonomous System) numbers to form an adjacency. R1 uses AS 100 while R2 uses AS 200, so they will not become neighbors or exchange routing information.

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A router has the following interfaces configured:

• Loopback0: 10.10.10.10

• G0/0: 192.168.1.1

• G0/1: 172.16.1.1

If EIGRP is enabled without manual router-ID configuration, what will be the EIGRP router ID?

A. 10.10.10.10

B. 192.168.1.1

C. 172.16.1.1

D. 0.0.0.0

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Answer: A

Explanation: Router ID selection priority: 1) Manual configuration, 2) Highest loopback IP, 3) Highest physical interface IP. Since there's a loopback interface (10.10.10.10), it takes precedence over physical interfaces regardless of which IP is numerically higher.

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Which command would you use to advertise a default route to RIP neighbors?

A. ip route 0.0.0.0 0.0.0.0 <next-hop>

B. network 0.0.0.0

C. default-information originate

D. redistribute static

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Answer: C

Explanation: The default-information originate command (used in RIP configuration mode) advertises an existing default route to RIP neighbors. Option A creates a default route but doesn't advertise it. Option B would not work as intended. Option D is for route redistribution, a more advanced topic.

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Refer to the following partial output of show ip protocols:

Routing Protocol is "eigrp 1"

Metric weight K1=1, K2=0, K3=1, K4=0, K5=0

Which metrics are used to calculate EIGRP routes by default?

A. Bandwidth and reliability

B. Bandwidth and delay

C. Delay and load

D. Hop count and bandwidth

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Answer: B

Explanation: K1 represents bandwidth and K3 represents delay. When set to 1, these values are used in metric calculation. K2 (load), K4 (reliability), and K5 (MTU) are set to 0 and not used by default.

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What is the administrative distance of an external EIGRP route?

A. 90

B. 110

C. 120

D. 170

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Answer: D

Explanation: EIGRP has two AD values: 90 for internal routes (learned within EIGRP) and 170 for external routes (redistributed into EIGRP from another source). This makes external routes less preferred than internal routes.

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A network administrator issues the command network 172.16.1.8 0.0.0.7 under EIGRP configuration. Which interface IP addresses would match this command? (Choose two)

A. 172.16.1.1

B. 172.16.1.9

C. 172.16.1.14

D. 172.16.1.16

E. 172.16.1.8

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Answer: B, C, E

Explanation: Wildcard mask 0.0.0.7 means the first 29 bits must match. The range 172.16.1.8 with /29 covers 172.16.1.8 through 172.16.1.15. Options B (172.16.1.9), C (172.16.1.14), and E (172.16.1.8) fall within this range. Option A (172.16.1.1) and D (172.16.1.16) do not.

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