CCNA Part 2

What is a Floating Static Route and what is its function?

• By default, a static route has an AD of 1.

• A Floating Static Route is configured with an administrative distance higher than the running dynamic routing protocol (e.g., setting AD to 120 when OSPF is running with AD 110).

• Function: The static route remains inactive (not installed in the routing table) as long as the dynamic route is active. If the dynamic route fails, the floating static route immediately enters the routing table as a backup

How does a Cisco router decide which route to use? There are 4 routing decisions it goes through.

1. Longest Prefix Match (Most Specific Route): Always wins first when forwarding traffic for a specific target IP address, regardless of AD or metric.

2. Administrative Distance (AD): Used when deciding which route source to install into the routing table when learned from different protocols for the exact same destination subnet.

3. Metric: Used when deciding between paths learned from the same protocol for the exact same destination subnet.

4. ECMP: Applied when multiple paths from the same protocol have the exact same metric.

Router A receives a packet destined for 10.20.0.14. Its routing table contains the following entries:

• S 10.20.0.0/22 [AD: 1, Metric: 0]

• R 10.20.0.0/24 [AD: 120, Metric: 2]

• D 10.20.0.0/26 [AD: 90, Metric: 2560]

• O 10.20.0.0/28 [AD: 110, Metric: 10]

Which route will Router A use to forward the packet?

• Answer: The OSPF route (10.20.0.0/28).

• Explanation: Although the static route has the lowest AD and EIGRP has a lower AD than OSPF, these entries are for different subnet mask lengths (prefix lengths). The Longest Prefix Match rule takes precedence above AD and Metric. The destination IP 10.20.0.14 matches /28 (range 10.20.0.0 - 10.20.0.15), which is the most specific mask.

Note: RIP (Routing Information Protocol)

• Type: Distance Vector Interior Gateway Protocol (IGP).

• Standard: Industry standard (non-proprietary).

• Metric: Hop Count (Maximum = 15 hops; 16 hops is considered unreachable) [00:02:43]. Link bandwidth is completely ignored (e.g., a 10 Gbps link and a 10 Mbps link both equal 1 hop).

What is the update interval of RIP?

Update Interval: Sends routing table updates every 30 seconds

What is the AD of RIP?

120

What is the difference between RIPv1 and RIPv2 in terms of addressing support, subnet mask info, and messaging type?

Feature	RIPv1	RIPv2
Addressing Support	Classful only (Class A, B, C)	Classless (Supports VLSM & CIDR)
Subnet Mask Info	Not sent in advertisements	Included in advertisements
Messaging Type	Broadcast (255.255.255.255)	Multicast (224.0.0.9)

What are the commands to configure RIPv2 on a router?

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)# passive-interface g2/0

Router(config-router)# default-information originate

• no auto-summary: Disables automatic classful network summarization.

• The network command behavior:

  • Commands are classful (e.g., entering 10.12.0.0 automatically truncates to 10.0.0.0).

  • Function: Identifies local interfaces whose IP addresses fall within the specified network range. It activates RIP on those interfaces to form adjacencies and advertises the interface's exact prefix length (not the classful network).

• passive-interface <interface>: Prevents RIP update messages from being sent out of an interface while still advertising the interface’s subnet to neighbors.

• default-information originate: Advertises a local default route (0.0.0.0/0) to downstream RIP neighbors.

Note: EIGRP (Enhanced Interior Gateway Routing Protocol)

• Type: Advanced/Hybrid Distance Vector IGP.

• Vendor Support: Originally Cisco proprietary; now an open standard, though predominantly deployed on Cisco equipment

What is the multicast address for EIGRP?

Multicast Address: 224.0.0.10

What metric calculation does EIGRP use?

Metric Calculation: Uses Bandwidth (slowest link in the path) and Delay (sum of delays) by default (K1=1, K3=1)

What is the AD for EIGRP internal routes? What about for external routes?

• Internal routes: 90

• External routes: 170

What does EIGRP support in addition to Equal-Cost Multi-Path (ECMP)?

Unique Feature: Supports Unequal Cost Load Balancing

EIGRP and OSPF use wildcard masks (inverted subnet masks) in network statements. How do you calculate the wildcard mask?

Just subtract each octet of the normal subnet mask from 255

• /24 (255.255.255.0) —> 0.0.0.255

• /28 (255.255.255.240) —> 0.0.0.15

• /32 (255.255.255.255) —> 0.0.0.0

What is the selection order for how EIGRP chooses what the router ID will be?

1. Manual Configuration: eigrp router-id <id> command.

2. Highest Loopback IP: Highest IP address among configured virtual loopback interfaces.

3. Highest Physical IP: Highest IP address among active physical interfaces

What are the commands to configure EIGRP?

Router(config)# router eigrp 1

Router(config-router)# no auto-summary

Router(config-router)# passive-interface GigabitEthernet2/0

Router(config-router)# network 10.0.0.0

Router(config-router)# network 172.16.1.0 0.0.0.15

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

• Note: The Autonomous System (AS) number (e.g., 1) must match between neighboring routers to form adjacencies

What does the network command do?

A common point of confusion is thinking the network statement tells EIGRP "go talk to a router at this remote network."

Instead, the network command acts as an internal filter for the local router:

1. Scans local interfaces: The router checks its own interfaces to see if any local IP address matches the network statement.

2. Enables EIGRP on matching interfaces: If an interface matches, EIGRP turns on for that specific port.

3. Starts sending Hellos: The router begins sending EIGRP Hello packets out that interface to 224.0.0.10 to discover neighbors.

4. Advertises the attached subnet: The router includes that interface's subnet in its EIGRP routing updates to its neighbors.

For two routers to form an EIGRP neighbor relationship and exchange routes, their configurations must agree on 4 parameters. What are they?

Parameter	Constraint	What Happens If It Mismatches?
Autonomous System (AS) Number	Must match exactly (e.g., both use router eigrp 1).	Routers ignore each other's Hellos entirely.
Primary IP Subnet	Neighboring interfaces must be on the same IP subnet.	Routers report a "neighbor not on same subnet" error.
K-Values (Metrics)	Metric weight constants must match (default: K1​=1, K3​=1, others 0).	Neighbors will continuously form and immediately drop.
Authentication	If configured, keys/passwords must match.	Hellos are rejected; neighbors won't form.

Summary: The network engineer configures the AS Number and the local network statements to enable EIGRP on specific physical/virtual interfaces. Once enabled, the router automatically uses multicast (224.0.0.10) to find any neighbor on the same link that shares that AS Number!

note

What commands lets you verify RIP, EIGRP, and OSPF and see the configuration details?

‘show ip protocols’

What command lets you view the routes that formed from RIP and EIGRP?

show ip route

• R: Code for RIP routes.

• D: Code for EIGRP routes

In what scenario does a router use Administrative Distance (AD) to make a route selection decision?

• A) When receiving multiple routes to different destinations from different protocols.

• B) When receiving multiple routes to the same destination from the same protocol.

• C) When receiving multiple routes to the same destination from different protocols.

• D) When load balancing across equal-cost links.

• C) When receiving multiple routes to the same destination from different protocols.

Simple EIGRP config example

Here are the exact network commands for R4 based on the topology and instructions in the image.

First, identify all interfaces on R4 that need to participate in EIGRP:

1. Loopback 0 Interface (Instruction #2): 4.4.4.4/32

2. FastEthernet 1/0 Interface (connected to R2): 10.0.24.0/30

3. FastEthernet 2/0 Interface (connected to R3): 10.0.34.0/30

4. GigabitEthernet 0/0 Interface (connected to SW1/PC1): 192.168.4.0/24

Option 1: Standard Method (Matching Subnet & Wildcard Mask)

Under router eigrp 100:

Plaintext

R4(config)# router eigrp 100

R4(config-router)# no auto-summary

R4(config-router)# network 4.4.4.4 0.0.0.0

R4(config-router)# network 10.0.24.0 0.0.0.3

R4(config-router)# network 10.0.34.0 0.0.0.3

R4(config-router)# network 192.168.4.0 0.0.0.255

Option 2: The Exact IP Method (0.0.0.0 Wildcard)

If you prefer using exact interface IP (interface on R4) addresses to avoid wildcard calculations:

Plaintext

R4(config)# router eigrp 100

R4(config-router)# no auto-summary

R4(config-router)# network 4.4.4.4 0.0.0.0

R4(config-router)# network 10.0.24.2 0.0.0.0

R4(config-router)# network 10.0.34.2 0.0.0.0

R4(config-router)# network 192.168.4.254 0.0.0.0

Note: In EIGRP, R1 knows that every destination network exists and how far away it is (the metric), but it has no idea how the routers behind its neighbors are physically wired together. It relies entirely on the word of its neighbors.

• EIGRP is like road signs (Distance-Vector): R1 pulls up to an intersection and sees two signs:

  • Left (via R2): "Destin, FL — 500 miles"

  • Right (via R3): "Destin, FL — 550 miles"

R1 doesn't know if the road to the left is a straight highway or a winding backroad, nor does it know how many towns lie between R2 and Destin. It simply trusts R2's sign post, adds the distance to get to R2, and picks the shorter route.

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Define Link-State routing (OSPF, IS-IS) and the pros and cons of it.

• Link-State (e.g., OSPF, IS-IS): Every router builds and maintains a full connectivity map (topology) of the network. Each router independently executes the Shortest Path First (SPF) algorithm to compute the best path to every destination.

  • Pros: Faster convergence, highly accurate routing.

  • Cons: Higher RAM and CPU consumption due to maintaining complete network topology data.

What is the shortest path first algorithm that link state routers use to compute the best path to every destination?

Algorithm: OSPF uses Dijkstra’s Shortest Path First (SPF) Algorithm

What are Link State Advertisements (LSAs)?

Data packets sent by routers containing information about local links, connected networks, and link metrics

What is the Link State Database (LSDB)?

A repository storing all received LSAs. All routers within the same OSPF area share an identical LSDB

Define Flooding

Process of sending LSAs to all OSPF neighbors so every router in an area converges on the exact same topology map

Define LSA Refresh Rate

LSAs have a default aging timer of 30 minutes, after which they are re-flooded

Summarize the OSPF operation in 3 steps

1. Become Neighbors: Form adjacencies with neighboring routers on shared link segments.

2. Exchange LSAs: Flood LSAs so all area routers form matching LSDBs.

3. Calculate Best Paths: Run Dijkstra’s SPF algorithm locally to insert optimal routes into the routing table

Why use areas? What is the problem with running a single OSPF area in large networks (500+ routers)?

In large networks (e.g., 500+ routers), running a single OSPF area causes:

• Excessive CPU load from frequent SPF re-calculations.

• High memory usage for huge LSDBs.

• Network-wide LSA flooding when any single link fluctuates.

Dividing the network into smaller areas confines SPF calculations and LSA flooding within each boundary

Define Backbone Area (Area 0)

The central area to which all other OSPF areas must connect

Define Internal Router

A router with all interfaces in a single area

Define Area Border Router (ABR)

A router with interfaces in multiple areas (e.g., Area 0 and Area 1). It maintains a separate LSDB for each area

Define Autonomous System Boundary Router (ASBR)

An OSPF router connected to an external network (such as an ISP or another routing protocol domain)

Define Backbone Router

Any router with at least one interface in Area 0 (includes ABRs connected to Area 0)

Define Intra-Area Route

A route to a destination within the same OSPF area

Define Inter-Area Route

A route to a destination in a different OSPF area

Rules for OSPF area: Define rule Contiguous Area

An area must be physically connected together, not split into separated parts

Rules for OSPF area: Define rule Backbone Connection

All non-backbone areas must directly attach to Area 0 via an ABR

Rules for OSPF area: Define rule Subnet Matching Area

Interfaces on the same IP subnet must belong to the same OSPF area to become neighbors

What are the commands to configure OSPF?

Entering OSPF configuration mode requires a Process ID (locally significant and does not need to match between neighbors):

Router(config)# router ospf 1

Router(config-router)# network 10.0.12.0 0.0.0.3 area 0

Router(config-router)# network 10.0.13.0 0.0.0.3 area 0

Router(config-router)# network 172.16.1.0 0.0.0.15 area 0

Note: OSPF uses wildcard masks (inverse subnet masks) and requires specifying the area parameter in the network statement

What does the network command tell OSPF to do?

1. Look for any interfaces with an IP address contained in the range specified in the network command

2. Activate OSPF on the interface in the specified area

3. The router will then try to become OSPF neighbors with other OSPF activated neighbor routers

What is an OSPF passive interface and how is it configured?

Prevents OSPF Hello messages from being sent out host-facing interfaces while still advertising that subnet to neighbors:

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

What is the command/commands to propagate a default static route to all downstream OSPF routers?

To propagate a default static route to all downstream OSPF routers:

Router(config)# ip route 0.0.0.0 0.0.0.0 203.0.113.2

Router(config)# router ospf 1

Router(config-router)# default-information-originate

OSPF identifies each router using a 32-bit IPv4-formatted Router ID. In what order does OSPF go about selecting the Router ID for each router?

1. Manual Configuration: Set via router-id <x.x.x.x> 

2. Highest Active Loopback IP: Highest IP address among all loopback interfaces.

3. Highest Active Physical IP: Highest IP address among active physical interfaces

What is the command to manually set the router-id?

router-id 1.1.1.1

What verification command displays information such as process and router id, AD, ASBR status, and ECMP?

‘show ip protocols’

• Process ID & Router ID

• Administrative Distance (AD): 110 for OSPF

• ASBR Status: Displays if the router is redistributing external routes or default routes

• Equal-Cost Multi-Path (ECMP): Defaults to 4 paths (max paths adjustable up to 16/32 depending on IOS)

If you run the ‘default-information-originate’ command on a router, what does that router transform into?

Executing default-information-originate transforms the router into an ASBR by importing an external default route into the OSPF domain.

What does OSPF use as its routing metric?

OSPF uses Cost as its routing metric. Cost is inversely proportional to bandwidth—faster links have lower costs.

What is the OSPF Cost Calculation Formula?

Cost = Reference Bandwidth / Interface Bandwidth

Interface Type	Default Speed	Formula	Calculated Cost	Actual OSPF Cost
Ethernet	10 Mbps	100 / 10	10	10
Fast Ethernet	100 Mbps	100 / 100	1	1
Gigabit Ethernet	1000 Mbps	100 / 1000	0.1	1
10G Ethernet	10,000 Mbps	100 / 10000	0.01	1

What is the problem with the OSPF reference bandwidth defaulting to 100?

Fast Ethernet, Gigabit, and 10G Ethernet all have an equal cost of 1 by default, preventing OSPF from choosing optimal high-speed paths

What is the command to modify OSPF cost using auto-cost reference bandwidth (recommended for global scaling)?

• Mode: Router OSPF configuration mode (config-router#).

• Command: auto-cost reference-bandwidth <bandwidth-in-Mbps>

• Example: Setting reference bandwidth to 100,000 Mbps (100 Gbps) makes Gigabit Ethernet cost = 100 and Fast Ethernet cost = 1000.

• Critical Rule: Must be configured identically on all routers in the OSPF domain

What is the command to manually modify the OSPF cost on an individual interface?

• Mode: Interface configuration mode (config-if#).

• Command: ip ospf cost <cost-value>

• Overrides automatically calculated costs.

• Recommended for individual link tuning

How is the total OSPF cost to a destination calculated?

Total OSPF cost to a destination is the cumulative sum of all outgoing (exit) interface costs along the path

What is the default OSPF cost of loopback interfaces?

1

Routers exchange OSPF ____ ____ to discover neighbors and negotiate adjacencies

Hello messages

What is the Hello multicast IP?

224.0.0.5 (All OSPF Routers)

What is the Hello IP Protocol Number?

89

What are the default Hello timers for ethernet (Hello, Dead)?

Hello = 10 sec, Dead = 40 sec (4x Hello timer)

The 7 OSPF Neighbor States:

What is the first OSPF neighbor state and what occurs in this state?

1. Down State

2. It is the initial state. No Hello packets have been received from the neighbor.

The 7 OSPF Neighbor States:

What is the second OSPF neighbor state and what occurs in this state?

1. Init State

2. A Hello is received. A Hello packet is received from a potential neighbor, but the receiving router's own Router ID (RID) is not listed in the Hello message.

The 7 OSPF Neighbor States:

What is the third OSPF neighbor state and what occurs in this state?

1. 2-Way State

2. Bidirectional communication is established. Both routers receive a Hello packet with its own RID in it. Both routers are now official neighbors. Designated Router (DR) and Backup Designated Router (BDR) elections occur at this stage on multi-access networks.

The 7 OSPF Neighbor States:

What is the fourth OSPF neighbor state and what occurs in this state?

1. ExStart State

2. Master/Slave negotiation. The two routers prepare to exchange information about their LSDB. Before that, they have to choose which one will start the exchange. They do this in the Exstart state. The router with the higher RID will become the Master and initiate the exchange. The router with the lower RID will become the slave.

The 7 OSPF Neighbor States:

What is the fifth OSPF neighbor state and what occurs in this state?

1. Exchange State

2. Database description exchange. Routers exchange DBD packets containing headers/summaries of their LSAs. Routers compare received summaries with their own LSDB to identify missing LSAs.

The 7 OSPF Neighbor States:

What is the sixth OSPF neighbor state and what occurs in this state?

1. Loading State

2. Database synchronization. The routers send Link State Request (LSR) messages to request that their neighbors send them any LSAs they don’t have. LSAs are sent in Link State Update (LSU) messages. The routers send LSAck messages to acknowledge that they received the LSAs.

The 7 OSPF Neighbor States:

What is the seventh OSPF neighbor state and what occurs in this state?

1. Full State

2. Full adjacency achieved. The routers have a full OSPF adjacency and identical LSDBs. They continue to send and listen for Hello packets (every 10 seconds by default) to maintain the neighbor adjacency. Every time a Hello packet is received, the ‘Dead’ timer (40 seconds by default) is reset. If the Dead timer counts down to 0 and no Hello message is received, the neighbor is removed. The routers will continue to share LSAs as the network changes to make sure each router has a complete and accurate map of the network (LSDB).

Define the Hello message type

Discovers and maintains neighbor relationships

Define the DBD (Database Description) message type

Summary list of router's LSDB used during Exchange state

Define the LSR (Link-State Request) message type

Requests specific full LSAs from a neighbor

Define the LSU (Link-State Update) message type

Carries requested LSAs to sync databases

Define the LSAck (Link-State Acknowledgment) message type

Explicitly acknowledges receipt of LSUs

Instead of using network commands under router ospf <process-id>, OSPF can be enabled directly on individual interfaces. What are the commands that let you do this?

Router(config)# interface gigabitethernet 0/0

Router(config-if)# ip ospf 1 area 0

What are the commands that let you secure all user-facing interfaces without manually disabling OSPF on each?

Router(config)# router ospf 1

Router(config-router)# passive-interface default

Router(config-router)# no passive-interface gigabitethernet 0/0

What verification command displays neighbor Router IDs, state (e.g., FULL/DR), dead timer countdown, and interface?

show ip ospf neighbor

What verification command provides a quick concise summary of OSPF-enabled interfaces, process ID, area, cost, and neighbor count?

show ip ospf interface brief

What verification command displays detailed interface parameters including Hello/Dead timers, cost, and DR/BDR roles?

show ip ospf interface <interface-id>

Note: Think of the DR like the teacher in a classroom, the BDR like the substitute teacher, and the DROthers like the students

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Define DROthers (Students)

Students (DROthers) only talk directly to the teacher and substitute. They don't exchange full OSPF databases with each other. Their relationship with other students stops at the 2-WAY state.

Define DR (Teacher) and BDR (Substitute)

The Teacher (DR) and Substitute (BDR) must talk to every single student to gather homework and pass out assignments. Therefore, the teacher is FULL with every student.

In OSPF, what is the difference between a Neighbor and an Adjacency (FULL)?

• Neighbor: Any router on the same segment that you have discovered via Hello packets and established a two-way communication with (the 2-WAY state).

• Adjacency (FULL): A neighbor with whom you have fully synchronized Link-State Databases (LSDBs) (the FULL state).

Define loopback interfaces. What are their purpose? How do they correlate with OSPF?

• Definition: A virtual interface inside the router that stays in an up/up state unless manually shut down.

• Purpose: Provides a stable, permanent IP address that does not depend on hardware or physical links.

• OSPF Integration: Automatically chosen as the OSPF Router ID (RID) if no explicit ID is configured, ensuring a stable identity across the network.

OSPF behaves differently depending on the underlying ___ ___ ___ type

OSPF behaves differently depending on the underlying Layer 2 media type

What is the first OSPF network type and what is its default media type?

1. Broadcast Network Type

2. Default Media: Ethernet and FDDI interfaces

Broadcast Network Type: Neighbor Discovery

Neighbor Discovery: Dynamic via multicast address 224.0.0.5

Broadcast Network Type: DR/BDR Election

• Designated Router (DR) and Backup Designated Router (BDR) are elected per subnet to reduce LSA flooding.

• Non-DR/BDR routers become DROther

Broadcast Network Type: Adjacency Behavior

• DROthers form FULL adjacencies only with the DR and BDR.

• DROthers remain in the 2-WAY state with each other

Broadcast Network Type: Multicast Communication

• All OSPF Routers: 224.0.0.5

• DR and BDR only: 224.0.0.6

Broadcast Network Type: How does the DR/BDR election work?

1. Highest OSPF Interface Priority wins (Default = 1, Range = 0–255). Priority 0 prevents a router from becoming DR/BDR.

2. Highest Router ID (RID) serves as the tiebreaker.

3. Non-Preemptive: Once elected, the DR/BDR roles persist until the OSPF process resets or the interface fails.

4. Step-Up Rule: If the current DR fails, the BDR immediately becomes the new DR, and a new BDR election is held

What is the second OSPF network type and what is its default media?

1. Point-to-Point Network Type

2. Default Media: Serial links using PPP or HDLC encapsulation

Point-to-Point Network Type: Neighbor Discovery

Neighbor Discovery: Dynamic via 224.0.0.5

Point-to-Point Network Type: DR/BDR Election

DR/BDR Election: None. Directly connected routers establish a direct FULL adjacency

Point-to-Point Network Type: Timers

Timers: Default Hello = 10s, Dead = 40s (same as Broadcast)

What is the third OSPF network type and what is its default media type?

1. Non-Broadcast Network Type

2. Default Media: Frame Relay and X.25

Non-Broadcast Network Type: Neighbor Discovery

Neighbor Discovery: Manual configuration required (neighbor command)

Non-Broadcast Network Type: Timers

Timers: Default Hello = 30s, Dead = 120s

To successfully establish neighbor relationships and exchange routing updates, routers must satisfy 8 matching parameters. What are these parameters?

1. Area Number	Must match on interconnecting interfaces.
2. IP Subnet	Interfaces must reside on the same network subnet/mask.
3. Process Status	OSPF process must not be administratively shut down.
4. Unique Router IDs	Duplicate RIDs prevent adjacency formation.
5. Timers Match	Hello and Dead intervals must match on both ends.
6. Authentication	Passwords and authentication methods must match.
7. MTU Match	Mismatched MTU causes neighbor state to get stuck in EXSTART/EXCHANGE.
8. Network Type	Mismatched types can show FULL status but fail to populate routes in the routing table.

What command lets you fix duplicate router ID?

no router-id

What command lets you fix Hello or Dead Interval Mismatch?

no ip ospf hello-interval

no ip ospf dead-interval