Pseudonode Reduction in IS-IS draft-isis-pseudo-perlman-roy-thomas-00.txt Matthew R Thomas [e-mail]

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Pseudonode Reduction in IS-IS draft-isis-pseudo-
perlman-roy-thomas-00.txtMatthew R
Thomasmrthom_at_essex.ac.uk
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Pseudonode overview

• Pseudonodes are useful with more than 4 nodes.
• With only 2 nodes, they are dramatically
  wasteful.
• 50 increase in nodes, more LSPs, costs to CPU,
  Bandwidth and memory

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TRILL Topologies and Pseudonodes

• Initially with RBridges mixed with lots of
  bridges, there are likely to be "links" with
  large numbers of RBridges -- clearly initially
  there is a need for pseudonodes in those cases.
• Once TRILL is well deployed, it is expected that
  almost all links will be pt-to-pt between
  RBridges

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TRILL deployment

• TRILL is aimed at deployment in locations that
  are comfortable with zero configuration. This is
  partly why RBridges are being used and not
  routers

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Setting reduction limits

• We are recommending using a pseudonode for any
  link with more than two nodes.
• Even though in theory it's a little wasteful to
  use a pseudonode with 3 nodes, on balance it
  seems reasonable to set the limit to three for
  use of pseudonode.

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Simple operation

• We are not talking about changing how flooding is
  done on the link-- just how the link is
  represented in the link state database
• DIS election, flooding, and the use of CSNPs and
  PSNPs remain unchanged on the link.

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Pseudonode operation

• A broadcast network with a pseudonode with two
  routers, R1 (DR) and R2   R1's LSP says "I'm
  attached to R1.25"   R2's LSP says "I'm attached
  to R1.25   R1.25's LSP says "Im attached to R1
  and R2"

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Pseudonode reduction mechanism

• Without the reduction mechanism, then in this
  case
• R1's LSP says "I'm attached to R2
• R2's LSP says "I'm attached to R1
• Local operation of the link remains the same

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NS and SS flags

• Two flags are used NS (pseudonode suppression
  capable) and SS (suppress pseudonode)
• The NS bit indicates the pseudonode suppression
  capability.
• All routers set this in their Hello for the link
  if they are pseudonode reduction capable
• The proposed TLV needs to be attached to the Hello

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NS and SS flags cont

• If ALL neighbors have the NS bit set in the Hello
  packet concerning the link then The DIS may
  decide to use pseudonode reduction.
• The DIS then sets the SS bit to signal to the
  routers to use pseudonode reduction
• We are proposing using Link Attribute sub-TLV 19
  (RFC 5029)
• This sub-TLV is part of TLV 22 (RFC 3784)

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Setting and Resetting the SS bit

• The link starts with no pseudonode and the DIS
  monitors the NS bit of the neighbors on the link.
• If there are lt2 ISIS nodes (ie one neighbor) on
  the link then the DIS may use pseudonode
  reduction.
• If a third node arrives then the DIS resets the
  SS bit and cancels the pseudonode reduction.

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Options with resetting the SS bit

• Option 1 Regardless of the loss of connectivity
  with one of the neighbors, the pseudonode
  reduction is not re-instated.
• Option 2 A variant is to have a long
  hysteresis, perhaps 2 hours, and if there remains
  no 3rd node then pseudonode reduction can be
  re-instated.
• Option 3 Have a simple upper and lower
  threshold, with stable state in-between. ie
  Reduction off if gt 3, and on if 2

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Advantages / Disadvantages

• Option 1
• Disadvantage Could end up with permanent
  pseudonode where it wasnt required
• Advantage Stability, and reduction of pseudonode
  in most cases.
• Option 2
• Disadvantage Possible pseudonode flapping
• Advantage Less LSAs if a router is removed
  permanently.
• Option 3
• Disadvantage variant on option 2
• Advantage variant on option 2

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Options considered for NS / SS location

• Option 1 Proposed solution sub-TLV
• The information is link specific.The sub-TLV can
  be used with TLV 22 and is standardised.
• Permission required to place this TLV in the ISIS
  Hello packets
• Option 2 Router Capability TLV 242 (RFC 4971)
• Using this TLV field was not possible
• The information is link specific. Although a
  router may have the same capability on all
  Ethernets the SS bit may be different on each
  link depending on the location of non-compatible
  nodes.
• Option 3 Use of specific bits in ISIS Hello
• Require changes to ISIS hello packet structure /
  new TLV in ISIS Hello packet.

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WG Issues