Computer NetworksRIP

RIP: The Routing Information Protocol

LevelIntermediate

Duration75 mins

TopicRIP

3 / 5

Hop Count Metric: The Simple Measure That Defined RIP

Counting Steps to the Destination

When you ask for directions to a coffee shop, the simplest response might be: "It's four blocks away." Not "0.8 miles through heavily trafficked streets" or "a 12-minute walk with a 3-minute wait at the traffic light"—just four blocks. This intuitive simplicity is exactly what RIP's hop count metric provides for routing.

In RIP, every path's cost is measured in hops—the number of routers a packet must traverse to reach its destination. A directly connected network is 1 hop away. A network two routers away is 2 hops. This couldn't be simpler.

But simplicity has trade-offs. A 10 Gbps fiber path and a 56 Kbps dialup connection both count as "1 hop" in RIP's eyes. The ancient modem and the cutting-edge backbone are considered equivalent. This radical simplification is simultaneously RIP's greatest strength and its most significant limitation.

What You Will Learn

By the end of this page, you will understand why RIP uses hop count as its metric, the mathematical properties that make 16 the "infinity" value, how the 15-hop limit constrains network design, and how hop count compares to more sophisticated metrics used by OSPF and EIGRP.

The Hop Count Design Decision

The choice of hop count as RIP's metric wasn't arbitrary—it emerged from the networking context of the 1980s and the specific design goals of the protocol.

Historical Context

When RIP was developed:

Network links were relatively homogeneous (mostly 56 Kbps to T1 speeds)
Link bandwidth wasn't exposed to routing protocols
CPU and memory were precious resources
Simple protocols were essential for limited hardware

Design Goals That Led to Hop Count

Hop Count Selection Rationale

•Simplicity — No bandwidth discovery needed; just count routers. Every router already knows its neighbor count.
•Universality — Works regardless of link technology. Ethernet, Token Ring, HDLC—all contribute one hop.
•Low Overhead — 4-byte integer for metric. No complex calculations, no floating point.
•Determinism — Metric is static unless topology changes. No fluctuation based on load or delay.
•Interoperability — All vendors can implement identical hop counting. No interpretation differences.

The Metric Formula

RIP's metric calculation is trivially simple:

Metric to destination = Metric from neighbor + 1

That's it. No bandwidth consideration, no delay measurement, no reliability assessment. Each router-to-router transition adds exactly one to the path cost.

Directly Connected Networks

For directly connected networks, RIP assigns metric 1 (not 0) because a packet must still enter the router to be processed before delivery:

Router R1 connected to Network 10.0.0.0/8:
  10.0.0.0/8 → Metric 1 (directly connected)

Router R2 receives update from R1:
  10.0.0.0/8 → Metric 1 + 1 = 2 (via R1)

Router R3 receives update from R2:
  10.0.0.0/8 → Metric 2 + 1 = 3 (via R2)

Metric 0 is Reserved

In RIP, metric 0 is never used for normal routes. A metric of 0 would indicate 'no cost to reach,' which only makes sense for the router itself. Some implementations use metric 0 in special contexts (like route request messages), but standard route advertisements always start at metric 1.

The Infinity Problem and Maximum 15

A fundamental challenge in distance vector routing is representing "unreachable" destinations. The Bellman-Ford algorithm needs an infinity value to handle missing paths, but computers can't truly represent infinity. RIP's solution: define 16 as infinity.

Why 16?

The choice of 16 balances several considerations:

Mathematical Properties:

Infinity Value Analysis
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# Why is 16 the optimal choice for RIP's infinity?
 
# Consideration 1: Packet size efficiency
# =========================================
# Metric field is 4 bytes (32 bits), but smaller values are more efficient
# 16 can be represented in 5 bits, leaving headroom for future use
# Values 0-15 = 4 bits (valid metrics), 16+ = infinity
 
# Consideration 2: Count-to-infinity bounds
# ==========================================
# Counting from 1 to infinity takes (infinity - starting_metric) iterations
# Each iteration = 30 seconds (update interval)
# 
# If infinity = 16 and starting metric = 1:
#   Worst case = 15 iterations × 30s = 450 seconds = 7.5 minutes
#
# If infinity = 128:
#   Worst case = 127 iterations × 30s = 3,810 seconds = 63.5 minutes!
 
def calculate_worst_case_convergence(infinity, update_interval=30):
    """Calculate worst-case counting-to-infinity time."""
    max_iterations = infinity - 1  # Starting from metric 1
    total_seconds = max_iterations * update_interval
    return total_seconds
 
print(f"Infinity=16: {calculate_worst_case_convergence(16)} seconds")
print(f"Infinity=32: {calculate_worst_case_convergence(32)} seconds")
print(f"Infinity=128: {calculate_worst_case_convergence(128)} seconds")
 
# Output:
# Infinity=16: 450 seconds (7.5 minutes) - Acceptable
# Infinity=32: 930 seconds (15.5 minutes) - Excessive
# Infinity=128: 3810 seconds (63.5 minutes) - Unacceptable
 
# Consideration 3: Network diameter trade-off
# ============================================
# Maximum valid metric = infinity - 1 = 15 hops
# This limits network diameter but bounds convergence time
 
# Consideration 4: Historical precedent
# =====================================
# ARPANET used similar values
# BSD routed implementation chose 16
# Became de facto standard before formal specification

The Trade-Off Matrix

Choosing an infinity value involves balancing competing concerns:

Infinity Value Trade-offs
If Infinity Too Low	If Infinity Too High
Network diameter is limited	Count-to-infinity takes forever
Valid paths marked unreachable	Routing loops persist longer
Network must be subdivided	Memory usage for tracking increases
Simple network design required	Convergence time unbounded

Metric 16: The Line Between Reachable and Unreachable

In RIP, metric 16 is special:

Metric 1-15: Valid, reachable destinations
Metric 16: Unreachable ("infinity")
Metric > 16: Treated as 16 (unreachable)

When a router processes an update:

if received_metric + 1 >= 16:
    metric = 16  # Mark as unreachable
    do not install in routing table for forwarding
else:
    metric = received_metric + 1
    install/update routing table

Advertising Unreachable Routes

When advertising routes, metric 16 explicitly means "I cannot reach this destination":

# Poison reverse example
Router R1 to R2: "Network 10.0.0.0/8, metric 16"

R2 interprets: "R1 says this is unreachable through it"
R2 action: Do not use R1 as next-hop for 10.0.0.0/8

The 15-Hop Limit is Fundamental

Because 16 is infinity and each hop adds 1, the maximum valid metric is 15. Any network where the longest path exceeds 15 hops simply cannot use RIP. This isn't a configuration option—it's a protocol limitation baked into every RIP implementation.

Network Diameter Constraints

The 15-hop limit has profound implications for network design. Understanding these constraints is essential for proper RIP deployment.

What is Network Diameter?

Network diameter is the longest shortest path between any two nodes in the network. It represents the maximum number of hops between the two most distant points.

Example: Linear topology
R1 -- R2 -- R3 -- R4 -- R5

Diameter = 4 hops (R1 to R5 or vice versa)

Network Size vs. Diameter Examples
Topology Type	Number of Routers	Typical Diameter	RIP Viable?
Star (hub and spoke)	Any	2	✓ Yes
Full mesh	Any	1	✓ Yes
Ring	10	5	✓ Yes
Ring	30	15	✓ Barely
Ring	32	16	✗ No
Linear chain	8	7	✓ Yes
Linear chain	16	15	✓ Barely
Linear chain	20	19	✗ No
Hierarchical tree	100	6	✓ Yes (if well-designed)
Arbitrary mesh	50	8-12	✓ Usually

Practical Impact on Network Design

1. Branch Office Connectivity

Consider a company with headquarters, regional offices, and branch offices:

HQ (core routers) → Regional (3 hops) → Branch (2 more hops)

Max path: Branch → Regional → HQ → Regional → Branch
Total: 2 + 3 + 3 + 2 = 10 hops (safe for RIP)

But if branches must communicate via dual-regional:
Branch → Regional1 → HQ → Regional2 → Branch

With more layers, 15 hops is quickly exceeded

2. Data Center Architectures

Modern data center designs (leaf-spine, fat-tree) typically have:

Spine layer: 2 hops to any leaf
Inter-DC traffic: 4-6 additional hops
Total: 6-8 hops (RIP viable)

But with east-west traffic patterns and multiple DC interconnects, hop counts can spiral.

3. Campus Networks

Large university or corporate campuses:

Building → Floor → Distribution → Core
Cross-campus traffic might traverse: Building1 → Floor → Dist → Core → Dist → Floor → Building2
Total: 6 hops minimum, often 8-10

Calculating Network Diameter
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# Tool to calculate network diameter and RIP viability
 
from collections import deque
 
def calculate_diameter(adjacency_list):
    """
    Calculate network diameter using BFS from each node.
    
    Args:
        adjacency_list: {node: [list of neighbors]}
    
    Returns:
        diameter: Maximum shortest path length
        endpoints: Tuple of nodes at maximum distance
    """
    max_distance = 0
    endpoints = None
    
    for start in adjacency_list:
        # BFS from each node
        distances = {start: 0}
        queue = deque([start])
        
        while queue:
            current = queue.popleft()
            for neighbor in adjacency_list[current]:
                if neighbor not in distances:
                    distances[neighbor] = distances[current] + 1
                    queue.append(neighbor)
                    
                    if distances[neighbor] > max_distance:
                        max_distance = distances[neighbor]
                        endpoints = (start, neighbor)
    
    return max_distance, endpoints
 
def check_rip_viability(diameter):
    """Determine if RIP can operate on this network."""
    if diameter <= 14:
        return "SAFE: Diameter well within RIP limits"
    elif diameter == 15:
        return "MARGINAL: Exactly at RIP limit, no room for growth"
    else:
        return f"INVALID: Diameter {diameter} exceeds RIP maximum of 15"
 
# Example: Ring network of 30 routers
ring_30 = {i: [(i-1) % 30, (i+1) % 30] for i in range(30)}
diameter, endpoints = calculate_diameter(ring_30)
print(f"30-router ring: Diameter = {diameter}")
print(f"  Endpoints: R{endpoints[0]} to R{endpoints[1]}")
print(f"  RIP Status: {check_rip_viability(diameter)}")
 
# Output:
# 30-router ring: Diameter = 15
# Endpoints: R0 to R15
# RIP Status: MARGINAL: Exactly at RIP limit, no room for growth

Design for Diameter Headroom

When designing RIP networks, aim for diameter ≤ 12, leaving room for backup paths that might add hops. If your network diameter approaches 14-15, consider OSPF or evaluate if network redesign can reduce diameter through additional links.

Hop Count Limitations

Beyond the 15-hop maximum, hop count as a metric has inherent limitations that affect routing quality. Understanding these limitations explains why more sophisticated protocols use composite metrics.

The Bandwidth Blindness Problem

Converting Mermaid diagram...

In the diagram above:

RIP's calculation:

Path A → B → E: 2 hops
Path A → C → E: 2 hops
Path A → D → E: 2 hops

RIP's decision: All three paths are equal! Traffic may be distributed across them.

Reality:

Path A → B → E: 10 Gbps capacity (10,000,000 Kbps)
Path A → C → E: 56 Kbps capacity (0.056 Mbps)
Path A → D → E: 56 Kbps capacity (0.056 Mbps)

The fiber path has 178,571 times more capacity than the dialup paths, yet RIP treats them identically.

What Hop Count Ignores

•Bandwidth — 1 Gbps fiber = 1 hop, 56 Kbps modem = 1 hop
•Delay — Satellite (500ms RTT) = 1 hop, fiber (1ms RTT) = 1 hop
•Reliability — 99.999% fiber = 1 hop, 90% wireless = 1 hop
•Load — Empty link = 1 hop, congested link = 1 hop
•Cost — Free peering = 1 hop, expensive transit = 1 hop

What Hop Count Provides

•Simplicity — No measurement or estimation required
•Stability — Metric only changes with topology
•Predictability — Path selection is deterministic
•Low overhead — No probes, no calculations
•Universality — Works across all link types

Problematic Scenarios

1. Asymmetric Network Design

Networks with high-speed cores and slow edge links:

Traffic takes slow edge path (fewer hops)
High-speed backbone is bypassed
Performance degrades unexpectedly

2. Geographic Distribution

Transcontinental links vs. local connections:

NYC → London (1 satellite hop, 500ms)
NYC → Boston → Philadelphia → DC → Atlanta (4 hops, 20ms)
RIP prefers the high-latency satellite link

3. Redundancy Architecture

Primary high-speed path plus backup slow path:

Primary: 10 Gbps, 3 hops
Backup: 1 Mbps, 2 hops
RIP uses backup even when primary is available!

When Hop Count Fails

The most dangerous scenario is when hop count leads RIP to select a clearly inferior path. Users experience poor performance, but the network appears "working" to administrators checking connectivity. The problem is invisible at layer 3 but obvious at layers above.

Comparison with Other Metrics

Modern routing protocols use more sophisticated metrics that address hop count's limitations. Understanding these alternatives provides context for RIP's design trade-offs.

Routing Protocol Metric Comparison
Protocol	Metric Type	Components	Formula/Calculation
RIP	Hop Count	Hops only	Metric = hops (max 15)
OSPF	Cost	Bandwidth	Cost = Reference_BW / Interface_BW
EIGRP	Composite	BW, Delay, (Load, Rel.)	Metric = f(BW, Delay, K-values)
IS-IS	Cost	Configurable	Sum of link costs (default 10)
BGP	Path attributes	AS Path, etc.	Complex policy-based selection

OSPF Cost Metric

OSPF calculates cost based on bandwidth:

Cost = Reference_Bandwidth / Interface_Bandwidth

Default Reference_Bandwidth = 100 Mbps (10^8 bps)

Examples:
  10 Gbps link: 100,000,000 / 10,000,000,000 = 0.01 → 1 (minimum)
  1 Gbps link:  100,000,000 / 1,000,000,000 = 0.1 → 1
  100 Mbps:     100,000,000 / 100,000,000 = 1
  10 Mbps:      100,000,000 / 10,000,000 = 10
  1 Mbps:       100,000,000 / 1,000,000 = 100
  56 Kbps:      100,000,000 / 56,000 = 1,785

With OSPF, our earlier example becomes:

Path A → B → E: Cost 1 + 1 = 2
Path A → C → E: Cost 1,785 + 1,785 = 3,570

OSPF correctly prefers the high-speed path.

EIGRP Composite Metric

EIGRP uses a complex formula:

Metric = 256 × [(K1 × BW) + (K2 × BW)/(256 - Load) + (K3 × Delay)] × [K5/(Reliability + K4)]

Default K-values: K1=1, K2=0, K3=1, K4=0, K5=0
Simplified: Metric = 256 × (BW + Delay)

Where:
  BW = 10^7 / minimum_bandwidth_in_kbps
  Delay = sum of delays / 10 (in microseconds)

EIGRP considers both bandwidth bottlenecks and cumulative delay.

Metric Calculation Examples
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# Compare path selection across protocols
 
def rip_metric(hops):
    """RIP: Simple hop count."""
    return min(hops, 16)
 
def ospf_cost(bandwidths_bps, reference_bw=100_000_000):
    """OSPF: Sum of interface costs based on bandwidth."""
    total_cost = 0
    for bw in bandwidths_bps:
        cost = max(1, int(reference_bw / bw))
        total_cost += cost
    return total_cost
 
def eigrp_metric_simplified(min_bandwidth_kbps, total_delay_us, k1=1, k3=1):
    """EIGRP: Composite metric (simplified, K2=K4=K5=0)."""
    bw_component = (10_000_000 / min_bandwidth_kbps) * 256
    delay_component = (total_delay_us / 10) * 256
    return int(k1 * bw_component + k3 * delay_component)
 
# Scenario: Two paths to destination
# Path 1: 10 Gbps fiber, 2 hops, 1ms delay each
# Path 2: 56 Kbps dialup, 1 hop, 50ms delay
 
print("Path 1 (10 Gbps, 2 hops, 2ms total):")
print(f"  RIP Metric:   {rip_metric(2)}")
print(f"  OSPF Cost:    {ospf_cost([10_000_000_000, 10_000_000_000])}")
print(f"  EIGRP Metric: {eigrp_metric_simplified(10_000_000, 2000)}")
 
print("
Path 2 (56 Kbps, 1 hop, 50ms):")
print(f"  RIP Metric:   {rip_metric(1)}")
print(f"  OSPF Cost:    {ospf_cost([56_000])}")
print(f"  EIGRP Metric: {eigrp_metric_simplified(56, 50000)}")
 
print("
Best Path Selection:")
print("  RIP chooses:   Path 2 (lower hop count)")
print("  OSPF chooses:  Path 1 (lower cost)")
print("  EIGRP chooses: Path 1 (better composite)")
 
# Output:
# Path 1 (10 Gbps, 2 hops, 2ms total):
#   RIP Metric:   2
#   OSPF Cost:    2
#   EIGRP Metric: 256256
#
# Path 2 (56 Kbps, 1 hop, 50ms):
#   RIP Metric:   1
#   OSPF Cost:    1785
#   EIGRP Metric: 46995970

No Metric is Perfect

Even sophisticated metrics have limitations. OSPF's cost is static (doesn't reflect current load). EIGRP can oscillate if load/reliability are included. The "best" metric depends on network requirements—RIP's simplicity remains valuable when its assumptions match reality.

Working Within Hop Count Constraints

Despite its limitations, hop count can work well when network design accommodates its characteristics. Here are strategies for effective RIP deployment.

Strategy 1: Homogeneous Link Speeds

If all inter-router links have similar bandwidth, hop count becomes a reasonable proxy for path quality:

All links: 1 Gbps
Path 1: A → B → C (2 hops, 2 Gbps potential)
Path 2: A → D → E → F → C (4 hops, 4 Gbps potential)

RIP selects Path 1: Fewer hops = less latency, acceptable

Strategy 2: Topology Design for RIP

RIP-Friendly Network Design Principles

•Flat hierarchies — Keep core-to-edge distance minimal. Three layers maximum: Core, Distribution, Access.
•Direct high-speed links — Connect major sites directly. Don't force traffic through slow intermediaries.
•Mesh where needed — Full mesh for critical sites reduces hop count. 5 sites fully meshed = 1 hop between any pair.
•Avoid linear chains — A chain of 10 routers has diameter 9. A ring of 10 has diameter 5.
•Eliminate slow links — If possible, upgrade or remove low-bandwidth links that RIP can't distinguish.

Strategy 3: Offset Lists (Where Supported)

Some RIP implementations allow offset lists—manually adding to the metric of specific routes:

# Cisco IOS example
# Add 5 to metric for routes learned from slower interface

router rip
  offset-list 10 in 5 Serial0/0

access-list 10 permit any

# Effect: Routes via Serial0/0 are 5 hops "worse"
# RIP prefers faster interfaces (shorter effective hop count)

This workaround lets administrators influence path selection when topology alone isn't sufficient.

Strategy 4: Passive Interfaces and Filtering

# Don't advertise routes on undesired interfaces
router rip
  passive-interface FastEthernet0/0  # No RIP on this interface
  
# Filter specific networks
router rip
  distribute-list 1 out Serial0/0
  
access-list 1 deny 10.0.0.0 0.255.255.255
access-list 1 permit any

# Routes to 10.0.0.0/8 not advertised via Serial0/0

RIP Deployment Scenarios and Strategies
Scenario	Challenge	Strategy
Mixed link speeds	RIP ignores bandwidth	Uniform core links, offset-lists for slow links
Deep hierarchy	15-hop limit approached	Flatten topology, add direct links
Remote branches	Many hops to central	Hub-and-spoke design, dedicated WAN links
Backup slow links	RIP uses backup inappropriately	Offset-list +8 on backup, force primary use
Multiple paths exist	RIP load-balances across unlike paths	Filter routes, manual redistribution

Know When to Switch Protocols

If you find yourself using many offset-lists and filters to make RIP behave correctly, consider migrating to OSPF or EIGRP. The complexity of workarounds can exceed the complexity of running a proper link-state protocol.

Metric in RIP Messages

Understanding how the metric field appears in actual RIP messages helps with troubleshooting and packet analysis.

Message Structure Review

Each route entry in a RIP message contains a 4-byte metric field (bytes 17-20 of the entry):

Route Entry (20 bytes):
Bytes 0-1:   Address Family (0x0002 for IP)
Bytes 2-3:   Route Tag (RIPv2) or Must Be Zero (RIPv1)
Bytes 4-7:   IP Address
Bytes 8-11:  Subnet Mask (RIPv2) or Must Be Zero (RIPv1)
Bytes 12-15: Next Hop (RIPv2) or Must Be Zero (RIPv1)
Bytes 16-19: Metric (1-16)

Packet Capture Analysis
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# Wireshark/tcpdump analysis of RIP metric field
 
# Sample RIP Response packet (hex dump)
# Command: 02 (Response)
# Version: 02 (RIPv2)
# 
# Route Entry 1:
#   00 02          Address Family: IP
#   00 00          Route Tag: 0
#   0a 00 01 00    IP Address: 10.0.1.0
#   ff ff ff 00    Subnet Mask: 255.255.255.0
#   00 00 00 00    Next Hop: 0.0.0.0 (use sender)
#   00 00 00 02    Metric: 2 <-- This network is 2 hops away
 
# Route Entry 2:
#   00 02          Address Family: IP
#   00 00          Route Tag: 0
#   c0 a8 00 00    IP Address: 192.168.0.0
#   ff ff 00 00    Subnet Mask: 255.255.0.0
#   00 00 00 00    Next Hop: 0.0.0.0
#   00 00 00 10    Metric: 16 <-- UNREACHABLE (0x10 = 16)
 
# Python: Parse RIP metric from packet bytes
def parse_rip_entry(entry_bytes):
    """Parse a single RIP route entry."""
    import struct
    
    af, tag = struct.unpack('!HH', entry_bytes[0:4])
    ip = '.'.join(str(b) for b in entry_bytes[4:8])
    mask = '.'.join(str(b) for b in entry_bytes[8:12])
    nexthop = '.'.join(str(b) for b in entry_bytes[12:16])
    metric = struct.unpack('!I', entry_bytes[16:20])[0]
    
    # Interpret metric
    if metric == 16:
        status = "UNREACHABLE"
    elif 1 <= metric <= 15:
        status = f"{metric} hops"
    else:
        status = f"INVALID ({metric})"
    
    return {
        'address_family': af,
        'ip': ip,
        'mask': mask,
        'next_hop': nexthop,
        'metric': metric,
        'status': status
    }
 
# Example usage
entry_hex = bytes.fromhex('0002 0000 0a000100 ffffff00 00000000 00000002'.replace(' ', ''))
result = parse_rip_entry(entry_hex)
print(f"Network: {result['ip']}/{result['mask']}")
print(f"Metric: {result['status']}")
 
# Output:
# Network: 10.0.1.0/255.255.255.0
# Metric: 2 hops

Common Metric Patterns in Packet Captures

Metric Values and Their Meaning
Metric Value (Hex)	Metric Value (Dec)	Interpretation
0x00000001	1	Directly connected network
0x00000002	2	One router away from sender
0x0000000F	15	Maximum valid metric (15 hops)
0x00000010	16	Unreachable (infinity)
0x00000010	16	Invalid (should be treated as 16)

Debugging with Metrics

When troubleshooting RIP, watching metric values in packet captures reveals the network's understanding of topology. If you see metric 16 for a network that should be reachable, track upstream to find where it was poisoned. If metrics seem wrong, trace the path to find where the calculation diverged.

Summary: The Hop Count Metric

We've thoroughly examined RIP's hop count metric—its design rationale, mathematical properties, and practical implications. Let's consolidate the key insights:

Key Takeaways

•Hop count was chosen for simplicity — No bandwidth discovery, no calculations, just counting routers. This was essential for 1980s hardware.
•16 is infinity in RIP — The maximum valid metric is 15. Metric 16 means unreachable. This bounds convergence time but limits network diameter.
•The 15-hop limit is fundamental — Networks with diameter > 15 cannot use RIP. This isn't configurable—it's baked into the protocol.
•Hop count ignores link quality — Bandwidth, delay, reliability, and load are invisible to RIP. A 10 Gbps fiber and 56 Kbps modem both count as 1 hop.
•Design must accommodate hop count — Flat hierarchies, homogeneous links, and hub-and-spoke topologies work better with RIP than deep hierarchies or mixed link speeds.
•Workarounds exist — Offset-lists can artificially increase metrics on slow paths. Filtering and passive interfaces can prevent poor path selection.
•Modern protocols use better metrics — OSPF (bandwidth-based cost), EIGRP (composite metric), and others address hop count's limitations.

What's Next

Now that we understand RIP's metric, we'll examine its timer mechanisms. RIP uses multiple timers to control update frequency, route expiration, and garbage collection. These timers define RIP's convergence behavior and are critical for network stability.

Page Complete

You now understand the hop count metric in depth—why it was chosen, how infinity is defined, what the 15-hop limit means for network design, and how it compares to more sophisticated metrics. You can design networks that work well with hop count and recognize when to consider alternative protocols.

3 / 5

Loading learning content...

Computer NetworksRIP

RIP: The Routing Information Protocol

LevelIntermediate

Duration75 mins

TopicRIP

3 / 5

Hop Count Metric: The Simple Measure That Defined RIP

Counting Steps to the Destination

What You Will Learn

The Hop Count Design Decision

The choice of hop count as RIP's metric wasn't arbitrary—it emerged from the networking context of the 1980s and the specific design goals of the protocol.

Historical Context

When RIP was developed:

Network links were relatively homogeneous (mostly 56 Kbps to T1 speeds)
Link bandwidth wasn't exposed to routing protocols
CPU and memory were precious resources
Simple protocols were essential for limited hardware

Design Goals That Led to Hop Count

Hop Count Selection Rationale

•Simplicity — No bandwidth discovery needed; just count routers. Every router already knows its neighbor count.
•Universality — Works regardless of link technology. Ethernet, Token Ring, HDLC—all contribute one hop.
•Low Overhead — 4-byte integer for metric. No complex calculations, no floating point.
•Determinism — Metric is static unless topology changes. No fluctuation based on load or delay.
•Interoperability — All vendors can implement identical hop counting. No interpretation differences.

The Metric Formula

RIP's metric calculation is trivially simple:

Metric to destination = Metric from neighbor + 1

That's it. No bandwidth consideration, no delay measurement, no reliability assessment. Each router-to-router transition adds exactly one to the path cost.

Directly Connected Networks

For directly connected networks, RIP assigns metric 1 (not 0) because a packet must still enter the router to be processed before delivery:

Router R1 connected to Network 10.0.0.0/8:
  10.0.0.0/8 → Metric 1 (directly connected)

Router R2 receives update from R1:
  10.0.0.0/8 → Metric 1 + 1 = 2 (via R1)

Router R3 receives update from R2:
  10.0.0.0/8 → Metric 2 + 1 = 3 (via R2)

Metric 0 is Reserved

The Infinity Problem and Maximum 15

Why 16?

The choice of 16 balances several considerations:

Mathematical Properties:

Infinity Value Analysis
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# Why is 16 the optimal choice for RIP's infinity?
 
# Consideration 1: Packet size efficiency
# =========================================
# Metric field is 4 bytes (32 bits), but smaller values are more efficient
# 16 can be represented in 5 bits, leaving headroom for future use
# Values 0-15 = 4 bits (valid metrics), 16+ = infinity
 
# Consideration 2: Count-to-infinity bounds
# ==========================================
# Counting from 1 to infinity takes (infinity - starting_metric) iterations
# Each iteration = 30 seconds (update interval)
# 
# If infinity = 16 and starting metric = 1:
#   Worst case = 15 iterations × 30s = 450 seconds = 7.5 minutes
#
# If infinity = 128:
#   Worst case = 127 iterations × 30s = 3,810 seconds = 63.5 minutes!
 
def calculate_worst_case_convergence(infinity, update_interval=30):
    """Calculate worst-case counting-to-infinity time."""
    max_iterations = infinity - 1  # Starting from metric 1
    total_seconds = max_iterations * update_interval
    return total_seconds
 
print(f"Infinity=16: {calculate_worst_case_convergence(16)} seconds")
print(f"Infinity=32: {calculate_worst_case_convergence(32)} seconds")
print(f"Infinity=128: {calculate_worst_case_convergence(128)} seconds")
 
# Output:
# Infinity=16: 450 seconds (7.5 minutes) - Acceptable
# Infinity=32: 930 seconds (15.5 minutes) - Excessive
# Infinity=128: 3810 seconds (63.5 minutes) - Unacceptable
 
# Consideration 3: Network diameter trade-off
# ============================================
# Maximum valid metric = infinity - 1 = 15 hops
# This limits network diameter but bounds convergence time
 
# Consideration 4: Historical precedent
# =====================================
# ARPANET used similar values
# BSD routed implementation chose 16
# Became de facto standard before formal specification

The Trade-Off Matrix

Choosing an infinity value involves balancing competing concerns:

Infinity Value Trade-offs
If Infinity Too Low	If Infinity Too High
Network diameter is limited	Count-to-infinity takes forever
Valid paths marked unreachable	Routing loops persist longer
Network must be subdivided	Memory usage for tracking increases
Simple network design required	Convergence time unbounded

Metric 16: The Line Between Reachable and Unreachable

In RIP, metric 16 is special:

Metric 1-15: Valid, reachable destinations
Metric 16: Unreachable ("infinity")
Metric > 16: Treated as 16 (unreachable)

When a router processes an update:

if received_metric + 1 >= 16:
    metric = 16  # Mark as unreachable
    do not install in routing table for forwarding
else:
    metric = received_metric + 1
    install/update routing table

Advertising Unreachable Routes

When advertising routes, metric 16 explicitly means "I cannot reach this destination":

# Poison reverse example
Router R1 to R2: "Network 10.0.0.0/8, metric 16"

R2 interprets: "R1 says this is unreachable through it"
R2 action: Do not use R1 as next-hop for 10.0.0.0/8

The 15-Hop Limit is Fundamental

Network Diameter Constraints

The 15-hop limit has profound implications for network design. Understanding these constraints is essential for proper RIP deployment.

What is Network Diameter?

Network diameter is the longest shortest path between any two nodes in the network. It represents the maximum number of hops between the two most distant points.

Example: Linear topology
R1 -- R2 -- R3 -- R4 -- R5

Diameter = 4 hops (R1 to R5 or vice versa)

Network Size vs. Diameter Examples
Topology Type	Number of Routers	Typical Diameter	RIP Viable?
Star (hub and spoke)	Any	2	✓ Yes
Full mesh	Any	1	✓ Yes
Ring	10	5	✓ Yes
Ring	30	15	✓ Barely
Ring	32	16	✗ No
Linear chain	8	7	✓ Yes
Linear chain	16	15	✓ Barely
Linear chain	20	19	✗ No
Hierarchical tree	100	6	✓ Yes (if well-designed)
Arbitrary mesh	50	8-12	✓ Usually

Practical Impact on Network Design

1. Branch Office Connectivity

Consider a company with headquarters, regional offices, and branch offices:

HQ (core routers) → Regional (3 hops) → Branch (2 more hops)

Max path: Branch → Regional → HQ → Regional → Branch
Total: 2 + 3 + 3 + 2 = 10 hops (safe for RIP)

But if branches must communicate via dual-regional:
Branch → Regional1 → HQ → Regional2 → Branch

With more layers, 15 hops is quickly exceeded

2. Data Center Architectures

Modern data center designs (leaf-spine, fat-tree) typically have:

Spine layer: 2 hops to any leaf
Inter-DC traffic: 4-6 additional hops
Total: 6-8 hops (RIP viable)

But with east-west traffic patterns and multiple DC interconnects, hop counts can spiral.

3. Campus Networks

Large university or corporate campuses:

Building → Floor → Distribution → Core
Cross-campus traffic might traverse: Building1 → Floor → Dist → Core → Dist → Floor → Building2
Total: 6 hops minimum, often 8-10

Calculating Network Diameter
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# Tool to calculate network diameter and RIP viability
 
from collections import deque
 
def calculate_diameter(adjacency_list):
    """
    Calculate network diameter using BFS from each node.
    
    Args:
        adjacency_list: {node: [list of neighbors]}
    
    Returns:
        diameter: Maximum shortest path length
        endpoints: Tuple of nodes at maximum distance
    """
    max_distance = 0
    endpoints = None
    
    for start in adjacency_list:
        # BFS from each node
        distances = {start: 0}
        queue = deque([start])
        
        while queue:
            current = queue.popleft()
            for neighbor in adjacency_list[current]:
                if neighbor not in distances:
                    distances[neighbor] = distances[current] + 1
                    queue.append(neighbor)
                    
                    if distances[neighbor] > max_distance:
                        max_distance = distances[neighbor]
                        endpoints = (start, neighbor)
    
    return max_distance, endpoints
 
def check_rip_viability(diameter):
    """Determine if RIP can operate on this network."""
    if diameter <= 14:
        return "SAFE: Diameter well within RIP limits"
    elif diameter == 15:
        return "MARGINAL: Exactly at RIP limit, no room for growth"
    else:
        return f"INVALID: Diameter {diameter} exceeds RIP maximum of 15"
 
# Example: Ring network of 30 routers
ring_30 = {i: [(i-1) % 30, (i+1) % 30] for i in range(30)}
diameter, endpoints = calculate_diameter(ring_30)
print(f"30-router ring: Diameter = {diameter}")
print(f"  Endpoints: R{endpoints[0]} to R{endpoints[1]}")
print(f"  RIP Status: {check_rip_viability(diameter)}")
 
# Output:
# 30-router ring: Diameter = 15
# Endpoints: R0 to R15
# RIP Status: MARGINAL: Exactly at RIP limit, no room for growth

Design for Diameter Headroom

Hop Count Limitations

Beyond the 15-hop maximum, hop count as a metric has inherent limitations that affect routing quality. Understanding these limitations explains why more sophisticated protocols use composite metrics.

The Bandwidth Blindness Problem

Converting Mermaid diagram...

In the diagram above:

RIP's calculation:

Path A → B → E: 2 hops
Path A → C → E: 2 hops
Path A → D → E: 2 hops

RIP's decision: All three paths are equal! Traffic may be distributed across them.

Reality:

Path A → B → E: 10 Gbps capacity (10,000,000 Kbps)
Path A → C → E: 56 Kbps capacity (0.056 Mbps)
Path A → D → E: 56 Kbps capacity (0.056 Mbps)

The fiber path has 178,571 times more capacity than the dialup paths, yet RIP treats them identically.

What Hop Count Ignores

•Bandwidth — 1 Gbps fiber = 1 hop, 56 Kbps modem = 1 hop
•Delay — Satellite (500ms RTT) = 1 hop, fiber (1ms RTT) = 1 hop
•Reliability — 99.999% fiber = 1 hop, 90% wireless = 1 hop
•Load — Empty link = 1 hop, congested link = 1 hop
•Cost — Free peering = 1 hop, expensive transit = 1 hop

What Hop Count Provides

•Simplicity — No measurement or estimation required
•Stability — Metric only changes with topology
•Predictability — Path selection is deterministic
•Low overhead — No probes, no calculations
•Universality — Works across all link types

Problematic Scenarios

1. Asymmetric Network Design

Networks with high-speed cores and slow edge links:

Traffic takes slow edge path (fewer hops)
High-speed backbone is bypassed
Performance degrades unexpectedly

2. Geographic Distribution

Transcontinental links vs. local connections:

NYC → London (1 satellite hop, 500ms)
NYC → Boston → Philadelphia → DC → Atlanta (4 hops, 20ms)
RIP prefers the high-latency satellite link

3. Redundancy Architecture

Primary high-speed path plus backup slow path:

Primary: 10 Gbps, 3 hops
Backup: 1 Mbps, 2 hops
RIP uses backup even when primary is available!

When Hop Count Fails

Comparison with Other Metrics

Modern routing protocols use more sophisticated metrics that address hop count's limitations. Understanding these alternatives provides context for RIP's design trade-offs.

Routing Protocol Metric Comparison
Protocol	Metric Type	Components	Formula/Calculation
RIP	Hop Count	Hops only	Metric = hops (max 15)
OSPF	Cost	Bandwidth	Cost = Reference_BW / Interface_BW
EIGRP	Composite	BW, Delay, (Load, Rel.)	Metric = f(BW, Delay, K-values)
IS-IS	Cost	Configurable	Sum of link costs (default 10)
BGP	Path attributes	AS Path, etc.	Complex policy-based selection

OSPF Cost Metric

OSPF calculates cost based on bandwidth:

Cost = Reference_Bandwidth / Interface_Bandwidth

Default Reference_Bandwidth = 100 Mbps (10^8 bps)

Examples:
  10 Gbps link: 100,000,000 / 10,000,000,000 = 0.01 → 1 (minimum)
  1 Gbps link:  100,000,000 / 1,000,000,000 = 0.1 → 1
  100 Mbps:     100,000,000 / 100,000,000 = 1
  10 Mbps:      100,000,000 / 10,000,000 = 10
  1 Mbps:       100,000,000 / 1,000,000 = 100
  56 Kbps:      100,000,000 / 56,000 = 1,785

With OSPF, our earlier example becomes:

Path A → B → E: Cost 1 + 1 = 2
Path A → C → E: Cost 1,785 + 1,785 = 3,570

OSPF correctly prefers the high-speed path.

EIGRP Composite Metric

EIGRP uses a complex formula:

Metric = 256 × [(K1 × BW) + (K2 × BW)/(256 - Load) + (K3 × Delay)] × [K5/(Reliability + K4)]

Default K-values: K1=1, K2=0, K3=1, K4=0, K5=0
Simplified: Metric = 256 × (BW + Delay)

Where:
  BW = 10^7 / minimum_bandwidth_in_kbps
  Delay = sum of delays / 10 (in microseconds)

EIGRP considers both bandwidth bottlenecks and cumulative delay.

Metric Calculation Examples
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# Compare path selection across protocols
 
def rip_metric(hops):
    """RIP: Simple hop count."""
    return min(hops, 16)
 
def ospf_cost(bandwidths_bps, reference_bw=100_000_000):
    """OSPF: Sum of interface costs based on bandwidth."""
    total_cost = 0
    for bw in bandwidths_bps:
        cost = max(1, int(reference_bw / bw))
        total_cost += cost
    return total_cost
 
def eigrp_metric_simplified(min_bandwidth_kbps, total_delay_us, k1=1, k3=1):
    """EIGRP: Composite metric (simplified, K2=K4=K5=0)."""
    bw_component = (10_000_000 / min_bandwidth_kbps) * 256
    delay_component = (total_delay_us / 10) * 256
    return int(k1 * bw_component + k3 * delay_component)
 
# Scenario: Two paths to destination
# Path 1: 10 Gbps fiber, 2 hops, 1ms delay each
# Path 2: 56 Kbps dialup, 1 hop, 50ms delay
 
print("Path 1 (10 Gbps, 2 hops, 2ms total):")
print(f"  RIP Metric:   {rip_metric(2)}")
print(f"  OSPF Cost:    {ospf_cost([10_000_000_000, 10_000_000_000])}")
print(f"  EIGRP Metric: {eigrp_metric_simplified(10_000_000, 2000)}")
 
print("
Path 2 (56 Kbps, 1 hop, 50ms):")
print(f"  RIP Metric:   {rip_metric(1)}")
print(f"  OSPF Cost:    {ospf_cost([56_000])}")
print(f"  EIGRP Metric: {eigrp_metric_simplified(56, 50000)}")
 
print("
Best Path Selection:")
print("  RIP chooses:   Path 2 (lower hop count)")
print("  OSPF chooses:  Path 1 (lower cost)")
print("  EIGRP chooses: Path 1 (better composite)")
 
# Output:
# Path 1 (10 Gbps, 2 hops, 2ms total):
#   RIP Metric:   2
#   OSPF Cost:    2
#   EIGRP Metric: 256256
#
# Path 2 (56 Kbps, 1 hop, 50ms):
#   RIP Metric:   1
#   OSPF Cost:    1785
#   EIGRP Metric: 46995970

No Metric is Perfect

Working Within Hop Count Constraints

Despite its limitations, hop count can work well when network design accommodates its characteristics. Here are strategies for effective RIP deployment.

Strategy 1: Homogeneous Link Speeds

If all inter-router links have similar bandwidth, hop count becomes a reasonable proxy for path quality:

All links: 1 Gbps
Path 1: A → B → C (2 hops, 2 Gbps potential)
Path 2: A → D → E → F → C (4 hops, 4 Gbps potential)

RIP selects Path 1: Fewer hops = less latency, acceptable

Strategy 2: Topology Design for RIP

RIP-Friendly Network Design Principles

•Flat hierarchies — Keep core-to-edge distance minimal. Three layers maximum: Core, Distribution, Access.
•Direct high-speed links — Connect major sites directly. Don't force traffic through slow intermediaries.
•Mesh where needed — Full mesh for critical sites reduces hop count. 5 sites fully meshed = 1 hop between any pair.
•Avoid linear chains — A chain of 10 routers has diameter 9. A ring of 10 has diameter 5.
•Eliminate slow links — If possible, upgrade or remove low-bandwidth links that RIP can't distinguish.

Strategy 3: Offset Lists (Where Supported)

Some RIP implementations allow offset lists—manually adding to the metric of specific routes:

# Cisco IOS example
# Add 5 to metric for routes learned from slower interface

router rip
  offset-list 10 in 5 Serial0/0

access-list 10 permit any

# Effect: Routes via Serial0/0 are 5 hops "worse"
# RIP prefers faster interfaces (shorter effective hop count)

This workaround lets administrators influence path selection when topology alone isn't sufficient.

Strategy 4: Passive Interfaces and Filtering

# Don't advertise routes on undesired interfaces
router rip
  passive-interface FastEthernet0/0  # No RIP on this interface
  
# Filter specific networks
router rip
  distribute-list 1 out Serial0/0
  
access-list 1 deny 10.0.0.0 0.255.255.255
access-list 1 permit any

# Routes to 10.0.0.0/8 not advertised via Serial0/0

RIP Deployment Scenarios and Strategies
Scenario	Challenge	Strategy
Mixed link speeds	RIP ignores bandwidth	Uniform core links, offset-lists for slow links
Deep hierarchy	15-hop limit approached	Flatten topology, add direct links
Remote branches	Many hops to central	Hub-and-spoke design, dedicated WAN links
Backup slow links	RIP uses backup inappropriately	Offset-list +8 on backup, force primary use
Multiple paths exist	RIP load-balances across unlike paths	Filter routes, manual redistribution

Know When to Switch Protocols

Metric in RIP Messages

Understanding how the metric field appears in actual RIP messages helps with troubleshooting and packet analysis.

Message Structure Review

Each route entry in a RIP message contains a 4-byte metric field (bytes 17-20 of the entry):

Route Entry (20 bytes):
Bytes 0-1:   Address Family (0x0002 for IP)
Bytes 2-3:   Route Tag (RIPv2) or Must Be Zero (RIPv1)
Bytes 4-7:   IP Address
Bytes 8-11:  Subnet Mask (RIPv2) or Must Be Zero (RIPv1)
Bytes 12-15: Next Hop (RIPv2) or Must Be Zero (RIPv1)
Bytes 16-19: Metric (1-16)

Packet Capture Analysis
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# Wireshark/tcpdump analysis of RIP metric field
 
# Sample RIP Response packet (hex dump)
# Command: 02 (Response)
# Version: 02 (RIPv2)
# 
# Route Entry 1:
#   00 02          Address Family: IP
#   00 00          Route Tag: 0
#   0a 00 01 00    IP Address: 10.0.1.0
#   ff ff ff 00    Subnet Mask: 255.255.255.0
#   00 00 00 00    Next Hop: 0.0.0.0 (use sender)
#   00 00 00 02    Metric: 2 <-- This network is 2 hops away
 
# Route Entry 2:
#   00 02          Address Family: IP
#   00 00          Route Tag: 0
#   c0 a8 00 00    IP Address: 192.168.0.0
#   ff ff 00 00    Subnet Mask: 255.255.0.0
#   00 00 00 00    Next Hop: 0.0.0.0
#   00 00 00 10    Metric: 16 <-- UNREACHABLE (0x10 = 16)
 
# Python: Parse RIP metric from packet bytes
def parse_rip_entry(entry_bytes):
    """Parse a single RIP route entry."""
    import struct
    
    af, tag = struct.unpack('!HH', entry_bytes[0:4])
    ip = '.'.join(str(b) for b in entry_bytes[4:8])
    mask = '.'.join(str(b) for b in entry_bytes[8:12])
    nexthop = '.'.join(str(b) for b in entry_bytes[12:16])
    metric = struct.unpack('!I', entry_bytes[16:20])[0]
    
    # Interpret metric
    if metric == 16:
        status = "UNREACHABLE"
    elif 1 <= metric <= 15:
        status = f"{metric} hops"
    else:
        status = f"INVALID ({metric})"
    
    return {
        'address_family': af,
        'ip': ip,
        'mask': mask,
        'next_hop': nexthop,
        'metric': metric,
        'status': status
    }
 
# Example usage
entry_hex = bytes.fromhex('0002 0000 0a000100 ffffff00 00000000 00000002'.replace(' ', ''))
result = parse_rip_entry(entry_hex)
print(f"Network: {result['ip']}/{result['mask']}")
print(f"Metric: {result['status']}")
 
# Output:
# Network: 10.0.1.0/255.255.255.0
# Metric: 2 hops

Common Metric Patterns in Packet Captures

Metric Values and Their Meaning
Metric Value (Hex)	Metric Value (Dec)	Interpretation
0x00000001	1	Directly connected network
0x00000002	2	One router away from sender
0x0000000F	15	Maximum valid metric (15 hops)
0x00000010	16	Unreachable (infinity)
0x00000010	16	Invalid (should be treated as 16)

Debugging with Metrics

Summary: The Hop Count Metric

We've thoroughly examined RIP's hop count metric—its design rationale, mathematical properties, and practical implications. Let's consolidate the key insights:

Key Takeaways

•Hop count was chosen for simplicity — No bandwidth discovery, no calculations, just counting routers. This was essential for 1980s hardware.
•16 is infinity in RIP — The maximum valid metric is 15. Metric 16 means unreachable. This bounds convergence time but limits network diameter.
•The 15-hop limit is fundamental — Networks with diameter > 15 cannot use RIP. This isn't configurable—it's baked into the protocol.
•Hop count ignores link quality — Bandwidth, delay, reliability, and load are invisible to RIP. A 10 Gbps fiber and 56 Kbps modem both count as 1 hop.
•Design must accommodate hop count — Flat hierarchies, homogeneous links, and hub-and-spoke topologies work better with RIP than deep hierarchies or mixed link speeds.
•Workarounds exist — Offset-lists can artificially increase metrics on slow paths. Filtering and passive interfaces can prevent poor path selection.
•Modern protocols use better metrics — OSPF (bandwidth-based cost), EIGRP (composite metric), and others address hop count's limitations.

What's Next

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