Load Balancing

Types of Load Balancing

Equal-Cost Load Balancing

By default, EIGRP performs equal-cost load balancing across up to 4 paths with identical metrics. This can be adjusted using the maximum-paths command.

router eigrp 100
 maximum-paths 6    ! Allow up to 6 equal-cost paths
 ! Default is 4, maximum is 16

Equal-Cost Example

    Router A
       /|\
      / | \
     /  |  \
Router B  C  D --- Destination 192.168.1.0/24
 (Cost 100) (Cost 100) (Cost 100)

Unequal-Cost Load Balancing

EIGRP's unique feature allows load balancing across paths with different metrics using the variance command. The variance multiplier determines which paths qualify for load balancing.

router eigrp 100
 variance 2         ! Allow paths up to 2x the best metric
 maximum-paths 6    ! Allow up to 6 total paths

Unequal-Cost Example

Router A to 192.168.1.0/24:
- Path 1 via Router B: Metric 100 (Best path)
- Path 2 via Router C: Metric 150 (1.5x best metric)
- Path 3 via Router D: Metric 250 (2.5x best metric)

With variance 2:
- Path 1: 100 ≤ (100 × 2) = 200 ✓ Qualifies
- Path 2: 150 ≤ (100 × 2) = 200 ✓ Qualifies  
- Path 3: 250 ≤ (100 × 2) = 200 ✗ Does not qualify

Traffic Distribution

EIGRP distributes traffic proportionally to the inverse of the path metrics. Lower metric paths carry more traffic.

Load Balancing Verification

# Check routing table
show ip route 192.168.1.0

# View topology table
show ip eigrp topology

# Check traffic share
show ip route eigrp

D    192.168.1.0/24 [90/100] 
       via 10.1.1.2, GigabitEthernet0/1
       [90/150] via 10.2.2.2, GigabitEthernet0/2
       
# Traffic share count shown

Load Balancing Best Practices

Common Issues

Route Summarization

Types of Summarization

Manual Route Summarization

Manual summarization is configured on interfaces and provides precise control over summary routes. It's the preferred method for modern networks.

interface GigabitEthernet0/0
 ip summary-address eigrp [as-number] [network] [mask] [admin-distance]
 
# Example:
interface GigabitEthernet0/0
 ip summary-address eigrp 100 192.168.0.0 255.255.240.0
 # Summarizes 192.168.0.0/20 (192.168.0.0 - 192.168.15.255)

Summarization Benefits

Summarization Example

Regional Office Networks:
192.168.0.0/24   - Branch 1
192.168.1.0/24   - Branch 2  
192.168.2.0/24   - Branch 3
192.168.3.0/24   - Branch 4

Summary Route: 192.168.0.0/22
Covers: 192.168.0.0 - 192.168.3.255

# Regional router advertising to core
interface Serial0/0
 description Link to Core Network
 ip summary-address eigrp 100 192.168.0.0 255.255.252.0
 # Advertises single /22 summary instead of four /24 routes

Null0 Route Creation

When a summary is configured, EIGRP automatically creates a Null0 route to prevent routing loops for destinations within the summary range that don't exist.

# Show routing table after summary configuration
show ip route

# Output shows:
D    192.168.0.0/22 is a summary, 00:05:23, Null0
D    192.168.0.0/24 [90/2172416] via 10.1.1.2, 00:05:20, Serial0/1
D    192.168.1.0/24 [90/2172416] via 10.1.2.2, 00:05:18, Serial0/2

Query Boundary Effect

Summary routes act as query boundaries, preventing DUAL queries from propagating beyond the summary point. This is crucial for network stability.

Hierarchical Summarization

Summarization Best Practices

Verification Commands

# Check summary routes
show ip eigrp topology summary

# View interface summaries
show ip eigrp interfaces detail

# Check Null0 routes
show ip route null0

# Debug summary creation
debug eigrp summary

# Monitor route advertisements
debug eigrp packets update

# Check query propagation
debug eigrp packets query

Common Summarization Issues

Stub Routing

What is EIGRP Stub?

EIGRP stub routing prevents a router from becoming a transit point for other routers. Stub routers receive routes but don't forward queries, improving network stability and reducing bandwidth usage.

Stub Router Behavior

Stub Configuration Options

router eigrp 100
 eigrp stub [connected] [summary] [static] [redistributed] [receive-only]
 
# Common configurations:
router eigrp 100
 eigrp stub connected summary     # Default behavior
 
router eigrp 100
 eigrp stub receive-only          # Receive routes only

Stub Options Explained

Typical Stub Deployment

         Core Network
              |
         [Hub Router]
         /     |     \
        /      |      \
   [Stub A] [Stub B] [Stub C]
      |        |        |
   Branch A  Branch B  Branch C

# Hub router - normal EIGRP configuration
router eigrp 100
 network 10.0.0.0 0.255.255.255
 # No stub configuration needed

# Branch router - stub configuration
router eigrp 100
 network 192.168.1.0 0.0.0.255
 network 10.1.1.0 0.0.0.3
 eigrp stub connected summary

Query Behavior with Stubs

Leak-Map Configuration

Leak-maps allow stub routers to advertise specific routes even when using receive-only mode.

# Create access list for leaked routes
access-list 10 permit 192.168.100.0 0.0.0.255
 
# Create route-map
route-map LEAK-ROUTES permit 10
 match ip address 10
 
# Apply leak-map to stub configuration
router eigrp 100
 eigrp stub receive-only
 eigrp stub leak-map LEAK-ROUTES

Stub Verification

# Check stub status
show ip eigrp neighbors detail

# View EIGRP configuration
show running-config | section eigrp

# Check advertised routes
show ip eigrp topology

Router# show ip eigrp neighbors detail
EIGRP neighbor is 10.1.1.1 on Serial0/0
 Version 12.4/1.2, Retrans: 0, Retries: 0
 Stub Peer Advertising (CONNECTED SUMMARY) Routes

Stub Benefits

Stub Limitations

Route Filtering

Types of Route Filtering

Distribute Lists

Distribute lists use access lists to filter routes in either inbound or outbound direction on specific interfaces or for all EIGRP neighbors.

# Create access list
access-list 10 deny 192.168.100.0 0.0.0.255
access-list 10 permit any

# Apply distribute list
router eigrp 100
 distribute-list 10 out GigabitEthernet0/0  # Outbound on interface
 distribute-list 10 in                      # Inbound from all neighbors

Distribute List Directions

Route Maps for Advanced Filtering

Route maps provide sophisticated filtering capabilities with multiple match criteria and the ability to modify route attributes.

# Create route map
route-map EIGRP-FILTER permit 10
 match ip address 20
 set metric 1000 100 255 1 1500

route-map EIGRP-FILTER deny 20
 match ip address 30

# Apply to EIGRP
router eigrp 100
 distribute-list route-map EIGRP-FILTER out

Route Map Match Criteria

Prefix Lists

Prefix lists provide more precise filtering than standard access lists by matching both the network address and subnet mask length.

# Create prefix list
ip prefix-list BRANCH-NETWORKS seq 5 permit 192.168.0.0/16 le 24
ip prefix-list BRANCH-NETWORKS seq 10 deny 0.0.0.0/0 le 32

# Apply to EIGRP
router eigrp 100
 distribute-list prefix BRANCH-NETWORKS out GigabitEthernet0/0

Prefix List Operators

Administrative Distance Filtering

Modify administrative distance for specific routes to control route preference and selection.

# Set administrative distance for specific routes
router eigrp 100
 distance 200 10.1.1.1 0.0.0.0 20  # AD=200 for routes from 10.1.1.1 matching ACL 20
 distance 150 0.0.0.0 255.255.255.255  # AD=150 for all other EIGRP routes

Filtering Examples

Example 1: Block Specific Networks

# Block guest network advertisements
access-list 25 deny 192.168.200.0 0.0.0.255
access-list 25 permit any

router eigrp 100
 distribute-list 25 out
 # Prevents advertising guest networks to any neighbor

Example 2: Allow Only Specific Subnets

# Allow only production networks
ip prefix-list PRODUCTION-ONLY seq 5 permit 10.100.0.0/16 le 24
ip prefix-list PRODUCTION-ONLY seq 10 permit 10.200.0.0/16 le 24
ip prefix-list PRODUCTION-ONLY seq 15 deny 0.0.0.0/0 le 32

router eigrp 100
 distribute-list prefix PRODUCTION-ONLY in

Example 3: Modify Route Metrics

# Make backup paths less preferred
access-list 30 permit 192.168.50.0 0.0.0.255

route-map BACKUP-PATHS permit 10
 match ip address 30
 set metric 10000 1000 255 1 1500

route-map BACKUP-PATHS permit 20

router eigrp 100
 distribute-list route-map BACKUP-PATHS in Serial0/1

Verification Commands

# Check applied distribute lists
show ip protocols

# View prefix lists
show ip prefix-list

# Check route maps
show route-map

# Check filtered routes
show ip eigrp topology

# Verify routing table
show ip route eigrp

# Debug filtering
debug ip routing

Filtering Best Practices

Common Filtering Mistakes

Authentication

Authentication Methods

MD5 Authentication Configuration

MD5 authentication uses a shared key to authenticate EIGRP packets. Each router must have matching key configurations.

# Configure key chain
key chain EIGRP-KEYS
 key 1
  key-string MySecretKey123
  accept-lifetime 00:00:00 Jan 1 2024 infinite
  send-lifetime 00:00:00 Jan 1 2024 infinite

# Apply to interface
interface GigabitEthernet0/0
 ip authentication mode eigrp 100 md5
 ip authentication key-chain eigrp 100 EIGRP-KEYS

Key Chain Components

SHA Authentication Configuration

SHA authentication provides stronger security than MD5 and is recommended for new deployments.

# Configure key chain with SHA
key chain EIGRP-SHA-KEYS
 key 1
  key-string SecurePassword456
  cryptographic-algorithm hmac-sha-256
  accept-lifetime 00:00:00 Jan 1 2024 infinite
  send-lifetime 00:00:00 Jan 1 2024 infinite

# Apply to interface
interface GigabitEthernet0/0
 ip authentication key-chain eigrp 100 EIGRP-SHA-KEYS

Key Rotation Strategy

Key rotation allows for secure key changes without disrupting EIGRP adjacencies by overlapping key lifetimes.

key chain EIGRP-ROTATION
 key 1
  key-string OldKey123
  send-lifetime 00:00:00 Jan 1 2024 23:59:59 Mar 31 2024
  accept-lifetime 00:00:00 Jan 1 2024 23:59:59 Apr 30 2024
 key 2
  key-string NewKey456
  send-lifetime 00:00:00 Mar 1 2024 infinite
  accept-lifetime 00:00:00 Mar 1 2024 infinite

Key Rotation Process

Authentication Verification

# Check neighbor authentication
show ip eigrp neighbors detail

# View interface authentication
show ip eigrp interfaces detail

# Check key chain configuration
show key chain

# Debug authentication issues
debug eigrp packets
debug ip eigrp neighbor

# Check authentication failures
show logging | include AUTH

Common Authentication Issues

Authentication Best Practices

Bandwidth Tuning

EIGRP Bandwidth Parameters

Interface Bandwidth Configuration

The bandwidth command sets the interface bandwidth value used by EIGRP for metric calculations. This should accurately reflect the actual link speed.

# Set interface bandwidth (in kilobits per second)
interface Serial0/0
 bandwidth 1544          # T1 link speed
 
interface Serial0/1
 bandwidth 256           # 256 Kbps link
 
interface GigabitEthernet0/0
 bandwidth 1000000       # Gigabit Ethernet

Common Bandwidth Values

EIGRP Bandwidth Percentage

Control how much interface bandwidth EIGRP can use for its own traffic (hellos, updates, queries, replies).

# Limit EIGRP to 25% of interface bandwidth
interface Serial0/0
 ip bandwidth-percent eigrp 100 25
 
# Allow EIGRP to use 75% of interface bandwidth
interface GigabitEthernet0/0
 ip bandwidth-percent eigrp 100 75

Bandwidth Percentage Guidelines

Hello Timer Tuning

Adjust hello intervals and hold times to optimize failure detection speed vs. bandwidth usage.

# Faster failure detection on critical links
interface GigabitEthernet0/0
 ip hello-interval eigrp 100 2      # Send hellos every 2 seconds
 ip hold-time eigrp 100 6           # Declare neighbor dead after 6 seconds
 
# Slower timers for bandwidth conservation
interface Serial0/0
 ip hello-interval eigrp 100 30     # Send hellos every 30 seconds
 ip hold-time eigrp 100 90          # Hold time 90 seconds

Timer Relationships

Delay Tuning for Path Selection

Use delay modification to influence EIGRP path selection without affecting bandwidth calculations.

# Increase delay to make path less preferred
interface Serial0/1
 delay 5000              # 50ms delay (value in tens of microseconds)
 
# Primary path with lower delay
interface Serial0/0
 delay 2000              # 20ms delay (more preferred)

Bandwidth Tuning Example

Router A --- T1 (Primary) --- Router B
    |                             |
     \-- 256K (Backup) ----------/
     
Goal: Optimize primary path, limit backup usage

# Primary T1 link optimized for performance
interface Serial0/0
 description Primary T1 to Router B
 bandwidth 1544
 ip bandwidth-percent eigrp 100 50
 ip hello-interval eigrp 100 5
 ip hold-time eigrp 100 15
 
# Backup link optimized for minimal overhead
interface Serial0/1
 description Backup 256K to Router B
 bandwidth 256
 ip bandwidth-percent eigrp 100 25
 ip hello-interval eigrp 100 30
 ip hold-time eigrp 100 90
 delay 5000                     # Make less preferred

Quality of Service Integration

Coordinate EIGRP bandwidth usage with QoS policies to ensure routing protocol doesn't interfere with critical applications.

# Reserve bandwidth for EIGRP in QoS policy
policy-map WAN-QOS
 class EIGRP-TRAFFIC
  bandwidth 64                  # Reserve 64K for EIGRP
 class VOICE
  priority 256                  # Voice gets priority
 class class-default
  fair-queue
  
# Limit EIGRP to reserved bandwidth
interface Serial0/0
 ip bandwidth-percent eigrp 100 25  # 25% of 256K = 64K

Verification and Monitoring

# Check interface bandwidth settings
show interface brief
show ip eigrp interfaces detail

# Monitor EIGRP traffic
show ip eigrp traffic

# Check hello timers
show ip eigrp neighbors detail

# Monitor interface utilization
show interface Serial0/0 | include load

# Check convergence times
show ip eigrp events

# Analyze route metrics
show ip eigrp topology all-links

Bandwidth Tuning Best Practices

Common Tuning Issues