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This document provides an overview of pseudonodes in TRILL, discussing their significance in optimizing routing efficiency. Originally stemming from IS-IS concepts, pseudonodes help manage routing overhead when Ethernet LANs are introduced, particularly with multiple routers. The paper explains how a designated router creates a pseudonode name, simplifying neighbor reporting among routers. It outlines when to implement pseudonodes based on the number of RBridges present, aiming for sensible configurations to minimize overhead while accommodating varying network scales.
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TRILL issue: Pseudonodes Radia Perlman Radia.Perlman@sun.com TRILL WG Vancouver
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 TRILL WG Vancouver
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 TRILL WG Vancouver
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 TRILL WG Vancouver
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 TRILL WG Vancouver
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 TRILL WG Vancouver
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 TRILL WG Vancouver
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 TRILL WG Vancouver
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 TRILL WG Vancouver
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 TRILL WG Vancouver
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 TRILL WG Vancouver
