BGP Multihoming and Multipath

Question 1

BGP Multihoming and Multipath

Answer 1

The simplest method of providing redundancy is to provide a second circuit.
Adding a second circuit and establishing a second BGP session across that peering link
is known as BGP multihoming because there are multiple sessions to learn
routes and establish connectivity.
BGP’s default behavior is to advertise only the best path to the routing information base
(RIB), which means that only one path for a network prefix is used when forwarding network
traffic to a destination.

BGP multipath allows for multiple paths to be installed into the RIB, so that both paths
can forward traffic to a network prefix at the same time. BGP multipath is an enhanced
form of BGP multihoming.

Question 2

BGP Multipath Types

Answer 2

There are three types of BGP multipath:

External BGP (EBGP)
Internal BGP (IBGP)
EIBGP multipath

EBGP multipath and IBGP multipath cannot be configured on the router at the same time.

Question 3

BGP Attributes that must match

Answer 3

In all three types of BGP multipath, the following BGP path attributes (PA) must match
for multipath to be eligible:

• Weight
• Local Preference
• AS-Path length and content
• Origin
• Multi-Exit Discriminator (MED)
• Advertisement method must match (iBGP or eBGP): If the prefix is learned via an iBGP advertisement, the Interior Gateway Protocol (IGP) routing protocol cost must match to be considered equal.

Question 4

Scenario 1

Answer 4

Scenario 1: R1 connects to two different Internet SPs with two different circuits. R1
cannot use eBGP multipath because the AS-Paths are different for any prefix on the
Internet. R1 is multihomed only.

Question 5

Scenario 2

Answer 5

Scenario 2: R1 connects to the same Internet SP with two different circuits. R1 can
use eBGP multipath for any Internet prefixes because the routes are learned via the
same service provider (regardless whether R1 connects to the same router or different
routers in service provider ABC). This assumes that all other BGP PAs are identical
between R1’s perspective.

Question 6

Scenario 3

Answer 6

Scenario 3: R3 connects via iBGP to R1 and R2. R1 connects to SP ABC, whereas R2
connects to SP XYZ. R3 cannot use iBGP multipath because the AS-Paths are differ-
ent for any Internet prefixes. R3 is multihomed only.

Question 7

Scenario 4

Answer 7

Scenario 4: R3 connects via iBGP to R1 and R2. R1 and R2 connect to the same
Internet SP. R3 uses iBGP multipath for any Internet prefixes because the AS-Path is
the same (along with the other required BGP PAs.)

Question 8

eBGP and iBGP Multipath Configuration

Answer 8

eBGP multipath is enabled on IOS and NX-OS devices 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.

The command maximum-paths ibgp number-paths sets the number of iBGP routes
to install in the RIB.

Question 9

Multipath Eligibility

Answer 9

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 10

AS-PATH Relax

Answer 10

BGP multipath works well if an organization uses the same service provider for Internet
connectivity. However, if an organization uses different service providers, the AS-Path is
not identical and does not meet the requirements for BGP multipath. The AS-Path relax
feature allows for BGP multipath to work when the AS-Path length is the same, but the
AS-Path content is different.

The AS-Path relax feature is configured with the BGP configuration command bgp
bestpath as-path multipath-relax on IOS, IOS XR, and NX-OS devices.