TRILL issue: Pseudonodes

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TRILL issue Pseudonodes

• Radia Perlman
• Radia.Perlman_at_sun.com

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Whats a pseudonode?

• Its an IS-IS concept for efficiency
• Originally there were just pt-to-pt links
• Along came Ethernet
• Overhead of routing algorithm proportional to
  of links
• If there were a fully connected LAN with n
  routers, thats n2 links

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(No Transcript)
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How a pseudonode works

• One router is appointed Designated Router
• That router gives a 7-byte name to the LAN
• Each router has a 6-byte system ID
• A routers name is system ID 0
• Pseudonode name is, typically, DRs system ID
  number assigned by DR

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With pseudonode R1 names LAN R1.25
Without pseudonode
R6
R6
R7
R7
R1
R1
R5
R5
R2
R4
R2
R4
R3
R3
R1.25 nbrs R1, R2, R3, R4, R5, R6, R7
R1 nbrs R2, R3, R4, R5, R6, R7 R2 nbrs R1, R3,
R4, R5, R6, R7 R3 nbrs R1, R2, R4, R5, R6,
R7 R4 nbrs R1, R2, R3, R5, R6, R7 R5 nbrs R1,
R2, R3, R4, R6, R7 R6 nbrs R1, R2, R3, R4, R5, R7
R1 nbr R1.25 R2 nbr R1.25 R3 nbr R1.25 R4 nbr
R1.25 R5 nbr R1.25 R6 nbr R1.25 R7 nbr R1.25
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LSPs

• Without pseudonode, each router reports each
  other router on the link as a neighbor
• With pseudonode, each router on the link reports
  one neighbor (the pseudonode), and the Designated
  Router additionally issues and LSP claiming to be
  the pseudonode, listing the n routers on the link

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But not all Ethernets are huge LANs

• With RBridges, most ports will be pt-to-pt
• If every port were a pseudonode, even those to
  endnodes, that would be ridiculous
• 2 RBridges on a link
• With pseudonode, 3 LSPs
• Without, 2 LSPs

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Overhead with n RBridges

• N RBridges
• With pseudonode n1 LSPs, 1 link reported in n
  LSPs, n links reported in 1 LSP
• Without pseudonode n LSPs, each with n-1 links
  reported

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Wed like to do something sensible

• Dont have pseudonodes if very few RBridges on
  the link
• Do have pseudonodes if a lot of RBridges on the
  link
• Dont have configuration
• Dont have disruption if RBridges go up and down

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Claim

• With only endnodes on a link, really dont want
  pseudonode
• With few RBridges, preferable to not have
  pseudonode
• With a lot of RBridges, much better to have
  pseudonode
• Large grey area between few and a lot where
  its OK with or without pseudonode

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Observation

• It would be bad if different RBs on the link
  simultaneously made different decisions re
  pseudonode or not
• But we have one RB, the DRB, that can dictate to
  the rest
• There does not have to be a single agreed-upon
  algorithm for how the DRB decides
• However, its nice to have a recommendation

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Recommendation in the spec

• If DRB has one or zero RBridge neighbors, no
  pseudonode
• If DRB has 4 or more RBridge neigbors, use
  pseudonode
• If DRB has 2 or 3 RBridge neighbors, keep state
  the same