Now that specific Telia router is having a BGP session with Charter router (since Charter is their downstream customer network) and that Telia router has multiple broadcast domains. We get ICMP replies because we just followed chain, and in chain last router before Charter was Telia which is announcing its address space normally and we are able to reach it. Then how we are getting ICMP replies during initial trace to destination IP? Basically many providers like Charter (and many ISPs) do not announce address space allocated to their backbone routers which are middle in chain to avoid possibility of packet flooding and possibly some other attacks. Route-server> sh bgp ipv4 unicast 96.34.0.98Д®verything is just fine in such setup. HereвЂ™s another major backbone router in US: So technically no one is announcing any subnet in global IPv4 table which covers address space forВ 96.34.0.98. These prefixes are not announced in global routing table via BGP. WhatвЂ™s bit confusing here is the fact that we are able to reach a destination IP sayВ 71.89.140.11 as taken in this example and middle routers just seem normal but if we try to explicitly reach these middle routers then we donвЂ™t see a route.Д«ecause thereвЂ™s just no route. This situation is the one this blog post is about! рџ™‚ Checking route for IPВ 96.34.0.98 in Tata AS6453 routing table: Why? LetвЂ™s ask itвЂ™s neighbor upstream router Tata AS6453. (Surprising?) Well as we see вЂ“ we canвЂ™t go beyond Tata-VSNL AS4755 border router in Mumbai. LetвЂ™s try to do a trace to this IP to see where exactly is connection breaking.ДЄnurag:~ anurag$ traceroute -a 96.34.0.98 This confuses lot of people as we canвЂ™t have a firewall blocking ICMP packets here since we did had ICMP based traceroute with ICMP replies from 15th hop in last trace. Next, letвЂ™s try pinging hop 15th IP which is with a major cable company Charter operating in US East вЂ“В 96.34.0.98 LetвЂ™s try pinging IP on 14th hop (which is with a major backbone Telia) вЂ“В 213.248.89.46 I have seen this topic in discussion twice on NANOG and once on Linux Delhi user group. Sometimes we see interesting IPвЂ™s in traceroute & they confuse lot of people.

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