1. BGP Multipath in Inter-AS Option-B draft-mohanty-bess-mutipath-interas-00
Satya Mohanty, Arjun Sreekantiah, Dhananjaya Rao,
Cisco Systems
Keyur Patel, Arrcus
IETF 99, Prague, Czech Republic

2. Introduction
BGP Multipath: More than one path to the destination in the forwarding
Standard Concept, All vendors support it. But no standardization has been done.
Implementations take into account the uniqueness of the next-hop.
No label taken into consideration

3. Route-reflectors do not set the next-hop to themselves
/32 advertised from R1
Route-reflectors do not set the next-hop to themselves R2
RR2
RR1 R1
/32
At R2, path received from RR2 is discarded as a multi-path, as path from RR1 has the same next-hop R2
RR2
RR1 R1
/32
/32 advertised from R1
Route-reflectors set the next-hop to themselves
At R2, path received from RR2 is accepted as a multi-path, as path from RR1 has a different next-hop

4. Inter-as Option B Defined in RFC 2547
Control and Forwarding over the Autonomous System Boundary Router (ASBR)
Next-hop Reset at the advertising ASBR

5. Option B Inter-AS Topology
/32
RD1: /32,
NH = PE1, L1
RD1: /32,
NH = ASBR1, L2
RD1: /32,
NH = ASBR1, L2
/32
PE2
CE2
CE1
PE1
ASBR1
ASBR2
AS 100
AS 200

6. Option B Inter-AS Topology
RD1: /32,
NH = PE1, L1
PE1
/32
RD1: /32,
NH = ASBR1, L3
RD1: /32,
NH = ASBR1, L3
/32
PE3
CE2
CE1
ASBR1
ASBR2
RD2: /32,
NH = PE2, L2
RD2: /32,
NH = ASBR1, L4
RD2: /32,
NH = ASBR1, L4
/32
PE2
AS 100
AS 200

7. Proposed Solutions
Use a different next-hop via policy at the ASBR. But this next-hop has to be injected into the receiving AS. Also Configuration overhead
Use the tuple (next-hop, label) when determining the multi-path at the receiving PE

8. Option B Inter-AS Topology
RD1: /32,
NH = PE1, L1
PE1
/32
RD1: /32,
NH = ASBR1, L3
RD1: /32,
NH = ASBR, L3
/32
PE3
CE2
CE1
ASBR1
ASBR2
RD2: /32,
NH = PE2, L2
RD2: /32,
NH = ASBR1, L4
RD2: /32,
NH = ASBR1, L4
/32
PE2
AS 100
AS 200