BGP%20VPLS%20Multi-homing

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BGP VPLS Multi-homing

• Kireeti Kompella
• Bhupesh Kothari
• Thomas Spencer

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Problem Statement
CE2
BGP VPLS
PE3
PE1
CE1
PE2
PE4
CE3
CE1 wants resilient connectivity as a VPLS CE, so
it is dual-homed to PE1 and PE2
Simple dual connectivity leads to loops and
duplicate packets. One answer is to run STP on
CE1
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Solution

• Let PE1 and PE2 know that they are connected to
  the same site
• Configure the VPLS VE ID to be the same
• Use BGP path selection to arbitrate among PE1 and
  PE2 as to which should be used to reach CE1
• And which forwards packets from CE1

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Multi-homing in 4364 VPNs
CE2
BGP VPN
PE3
LP200
PE1
10.1/16
CE1
PE2
10.1/16
PE4
CE3
LP100
CE1 wants resilient connectivity as a VPN CE, so
it is dual-homed to PE1 and PE2
Choices pick one of PE1 or PE2 to service CE1
or use both (ECMP)
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Differences in VPLS

• Cannot do ECMP must pick one of PE1 or PE2 (say
  PE1), and must do so consistently from all
  other PEs
• Both PE3 and PE4 must both choose PE1
• In particular, the losing PE (here, PE2) must
  know that it lost
• The winning PE (here, PE1) is called the
  designated PE

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Changes to BGP Path Selection

• When are prefixes comparable?
• What part of the NLRI is to be compared?
• Who wins, and how can we ensure that everyone
  picks consistently?
• Simple answer dont use IGP metric in the
  tie-breaking rules
• Control? (via Local Preference)
• Details in draft

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Result

• Designated PE forwards packets from and to CE
• Non-designated PEs (losing PEs) drop packets from
  CE as well as from other PEs
• The effect is as if CE was single-homed to just
  the designated PE

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Re-homing

• If the designated PE fails, or if the CE-PE link
  breaks, the designated PE withdraws its route
• All other PEs (including non-designated PEs) redo
  path selection and pick a new designated PE
• Traffic is re-established via the new PE

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Effect of Route Reflection
CE2
PE3
PE1
RR
CE1
PE2
PE4
CE3
If there is an RR in the path, the RR also does
path selection this can affect convergence. To
get around this, use different RDs on each PE (as
in 4364 VPNs)
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Status

• RFC 4761, section 3.5 talks about multi-homing
  and how to accomplish it
• However, there arent enough details on how
  prefixes are to be compared, and how BGP path
  selection should be done
• This draft attempts to rectify this

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Questions?