Advertising ECMP Routes in BGP

1
Advertising ECMP Routes in BGP

• Joel Halpern
• Manav Bhatia
• Paul Jakma

65th IETF Dallas, TX
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Requirements for an ECMP algorithm

• MUST be able to advertise individual BGP paths to
  other multipath capable peers.
• MUST preserve maximum path information and should
  be regular expression friendly
• Path length MUST be retained when advertising
  routes to non multipath capable peers

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ECMP Capability

• Uses MULTIPLE_HOP Capability
• Defines a new bit in the bitmask of flags
• ----------------------------
  -------
• Bit 7 6 5 4 3 2
  1 0
• ----------------------------
  -------
• flag R R R R R R
  ECMP AE
• ----------------------------
  -------

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How does this work within an AS?
R1
10.0.0.0/8 AS_PATH A B C NEXT_HOP N2
AS 65530
10.0.0.0/8 AS_PATH A B C MULTIPLE_HOP N2
R3
RR
10.0.0.0/8 AS_PATH X Y Z NEXT_HOP N1
10.0.0.0/8 AS_PATH X Y Z NEXT_HOP N1
(BGP RIB) gti 10.0.0.0/8 N2 A B C ? gti
10.0.0.0/8 N1 X Y Z ?
sh ip route B 10.0.0.0 170/0 via nexthop to
the RR
Assuming that R3 can reach N1 and N2
independently and not via the RR
R2
sh ip route B 10.0.0.0 170/0 via N1 B
10.0.0.0 170/0 via N1
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Advertising to an EBGP/non Multipath capable IBGP
peer
AS 61000
AS 65530
10.0.0.0/8 ORIGIN INCOMPLETE AS_PATH 65530 A
X B Y C Z
R3
R4
gti 10.0.0.0/8 N2 A B C ? gti 10.0.0.0/8
N1 X Y Z ?