BGP Equal-Cost Multipath

Question 1

BGP Equal-Cost Multipath

Answer 1

All the IGP routing protocols support equal-cost multipath (ECMP).

ECMP provides load balancing by installing multiple paths into the RIB for that protocol.

BGP selects only one best path, but it allows for the installation of multiple routes into the RIB.

Enabling BGP multipathing does not alter the best-path algorithm or change the behavior of paths advertisement to other BGP peers. Only the BGP best path is advertised to peers.

Question 2

What need to match the additional paths?

Answer 2

When you configure BGP multipathing, the additional paths need to match the following best-path BGP path attributes:

• Weight
• Local Preference
• AS_Path Length
• Origin
• MED
• Advertisement method (iBGP or eBGP)

Multipath eligibility begins only after the BGP best path algorithm executes. Alternate paths must match the exact BGP best path attributes listed earlier for BGP multipath consideration.

Question 3

eBGP Multipath

Answer 3

eBGP multipath is enabled on IOS routers with the BGP configuration command maximum-paths number-paths.

The number of paths indicates the allowed number of eBGP paths to install in the RIB.

IOS XR routers set the number of paths for installation into the RIB with the BGP configuration command maximum-paths {ebgp | ibgp} number-paths [unequal-cost].

The unequal-cost keyword allows for iBGP paths to install if the metrics to the next hop do not match.

Question 4

eBGP Multipath Example

Answer 4

R1 advertises the 172.16.0.0/24 network, and transits AS200 to reach AS400.

XR4 and R5 enable BGP multipath for two eBGP paths to install into the RIB.

RP/0/0/CPU0:XR4#show route bgp
B 172.16.0.0/24 [20/0] via 10.34.1.3, 00:01:57
[20/0] via 10.24.1.2, 00:01:57

R5#show ip route bgp
B 172.16.0.0 [20/0] via 10.35.1.3, 00:01:38
[20/0] via 10.25.1.2, 00:01:38