BGP Multipath & ECMP Load Balancing Explained

The Single Best Path Problem

By default, BGP selects only one best path per prefix and installs it in the routing table. Even if you have four transit links, all traffic for a given destination uses just one of them.

Default BGP behavior:
  Path 1 via Provider A: SELECTED (best path)
  Path 2 via Provider B: backup only
  Path 3 via IXP peer:   backup only

Result: Provider A carries all traffic. Others sit idle.

BGP Multipath: Using Multiple Paths

BGP multipath allows the router to install multiple equal-cost BGP paths into the forwarding table, distributing traffic across them using ECMP (Equal-Cost Multi-Path) hashing.

With multipath enabled:
  Path 1 via Provider A: INSTALLED (best path)
  Path 2 via Provider B: INSTALLED (multipath)

Result: Traffic is load-balanced 50/50 across both providers.

Configuring BGP Multipath

# Cisco IOS: eBGP multipath (up to 4 paths)
router bgp 64512
  address-family ipv4 unicast
    maximum-paths 4

# Cisco IOS: iBGP multipath
router bgp 64512
  address-family ipv4 unicast
    maximum-paths ibgp 4

# Junos: Enable multipath
routing-options {
    autonomous-system 64512;
}
protocols bgp {
    group TRANSIT {
        multipath;
    }
}

# BIRD 2
protocol bgp {
    merge paths on;
}

Requirements for Multipath

For two BGP paths to be considered "equal" for multipath, they must match on several attributes. The specific requirements depend on the implementation:

Attribute	Must Match?	Notes
Weight	Yes	Cisco-specific, must be equal
Local Preference	Yes	Must be identical
AS Path Length	Yes	Must be same length
Origin	Yes	Must be same (IGP/EGP/Incomplete)
MED	Yes (usually)	Unless `bgp bestpath med missing-as-worst`
Next-hop	Must differ	Each path needs a different next-hop
IGP metric to next-hop	Depends	Some implementations require equal IGP cost

ECMP Hashing

Once multiple paths are installed, the router uses a hash function to distribute flows across them. The hash typically uses:

Source IP address
Destination IP address
IP protocol number
Source port
Destination port

This ensures all packets for a single TCP flow take the same path (preventing out-of-order delivery), while different flows are distributed across paths.

Flow A (src: 10.0.0.1 dst: 8.8.8.8) -> Hash -> Path 1
Flow B (src: 10.0.0.2 dst: 8.8.8.8) -> Hash -> Path 2
Flow C (src: 10.0.0.1 dst: 1.1.1.1) -> Hash -> Path 1
Flow D (src: 10.0.0.3 dst: 9.9.9.9) -> Hash -> Path 2

Unequal-Cost Multipath

Some routers support unequal-cost multipath, distributing traffic proportionally based on link bandwidth or configured weights:

# Cisco: DMZ link bandwidth
neighbor 10.0.0.1 dmzlink-bw
neighbor 10.0.0.2 dmzlink-bw

# Traffic distributed proportionally:
# 10 Gbps link -> 10/(10+1) = 91% of traffic
# 1 Gbps link  -> 1/(10+1) = 9% of traffic

Multipath Best Practices

Start with 2 paths and increase gradually while monitoring.
Use BGP add-path (RFC 7911) to advertise multiple paths to iBGP peers instead of just the best.
Monitor per-path utilization to verify balanced distribution.
Test failover -- disable one path and verify traffic shifts cleanly.

BGP Multipath and ECMP Load Balancing