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Traceroute and Not-so-Equal ECMP
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Présentation : I came across an odd little issue recently involving equal-cost multipath ECMP routing and traceroute. Traceroutes from within our network to destinations out on the Internet were following two different paths, with one path being one hop longer than the other. This resulted in mangled traceroute output, impeding our ability to troubleshoot. The relevant network topology comprises a mesh of two edge routers and two core switches. Each edge router has a number of transit circuits to different providers, and advertises a default route via OSPF to the two core switches below. The core switches each load-balance traffic across both default routes to either edge routers. topology.png Because each edge router has different providers, some destinations are routed out via edge1 and others via edge2, which means sometimes a packet will be routed to edge2 via edge1, or vice versa. two_paths.png Routers typically employ a hash function using layer three and four information from each packet to pseudo-randomly distribute traffic across equal links. Typically, all packets belonging to a flow e.g. all packets with the same source and destination IP and port numbers follow the same path. However, in this case traceroute packets were being split across two path of unequal length, which made traceroute output pretty much unreadable. We noticed that only UDP traceroutes were affected ICMP traceroutes reported one path as normal. Continue reading No comments
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