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D e f g link from a to e fails a adverses distance

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Unformatted text preview: s table C A 6 8 B’s table ID Cost A 4 D F D 12 Nextho p C 2 ID F E D 4 1 1 2 1 B F 8 Adap)ng to Failures G, 3, C B C G, ∞,- A 3,C 2, F D E F G, 2, D G, 1, G G, 4, A 3 G G, 4, A 1G G, ∞, - •  •  •  •  •  •  F- G fails F sets distance to G to infinity, propagates A sets distance to G to infinity A receives periodic update from C with 2- hop path to G A sets distance to G to 3 and propagates F sets distance to G to 4, through A Count- to- Infinity . . B C A . D E F •  •  •  •  •  •  •  G Link from A to E fails A adver)ses distance of infinity to E B and C adver)se a distance of 2 to E B decides it can reach E in 3 hops through C A decides it can reach E in 4 hops through B C decides it can reach E in 5 hops through A, … When does this stop? Good news travels fast 1 B 4 1 A C 10 •  A decrease in link cost has to be fresh informa)on •  Network converges at most in O(diameter) steps Bad news travels slowly 12 B 4 1 A C 10 •  An increase in cost may cause confusion with old informa)on •  May form loops How to avoid loops •  IP TTL field prevents a packet from living forever –  Does not repair a loop •  Simple approach: consider a small cost n (e.g., 16) to be infinity –  Ajer n rounds decide node is unavailable –  But rounds can be long, this takes )me Becer loop avoidance •  Split Horizon –  When sending updates to node A, don’t include routes you learned from A –  Prevents B and C from sending cost 2 to A •  Split Horizon with Poison Reverse –  Rather than not adver)sing routes learned from A, explicitly include cost of ∞. –  Faster to break out of...
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