TRILL issue: Pseudonodes
Radia Perlman
Radia.Perlman@sun.com
December 2007
TRILL WG Vancouver
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What’s a pseudonode?
• It’s 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, that’s n2 links
December 2007
TRILL WG Vancouver
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TRILL WG Vancouver
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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 router’s “name” is system ID | “0”
• Pseudonode name is, typically, DR’s system
ID | number assigned by DR
December 2007
TRILL WG Vancouver
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With pseudonode
R1 names LAN “R1.25”
Without pseudonode
R6
R7
R1
R1
R5
R5
R2
R2
R4
R4
R3
R3
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
December 2007
R6
R7
R1.25: nbrs R1, R2, R3, R4, R5, R6, 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
TRILL WG Vancouver
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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
December 2007
TRILL WG Vancouver
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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: n+1 LSPs, 1 link reported in
n LSPs, n links reported in 1 LSP
– Without pseudonode: n LSPs, each with n-1
links reported
December 2007
TRILL WG Vancouver
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We’d like to do something sensible
• Don’t have pseudonodes if “very few”
RBridges on the link
• Do have pseudonodes if “a lot” of RBridges
on the link
• Don’t have configuration
• Don’t have disruption if RBridges go up
and down
December 2007
TRILL WG Vancouver
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Claim
• With only endnodes on a link, really don’t
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 it’s OK with or without pseudonode
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TRILL WG Vancouver
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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, it’s nice to have a recommendation
December 2007
TRILL WG Vancouver
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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
December 2007
TRILL WG Vancouver
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