C.2.f) “Traceroute and looking glass information”
To make the explanation easy, the paragraphs below will focus on network 10.10.201.0/24.
1. All the debug and monitoring commands described above were not able to give us clearly, which communities values were applied to each network. This information is necessary, when you design a traffic engineering case with redundancy option. In the case of a network, we would like to know which LocalPref value has the ISP transit/upstream apply. For that purpose, we can use the looking glass provided by the ISP. These bgp looking glasses will provide vital information for the inbound traffic on a network advertised to an ISP.
a. Let’s take the example of network 10.10.201.0/24. Its default ingress traffic should be ISP-B Ebgp speaker. As we can see below, since the community value applied to this network is 300:100, when we advertised it to ISP-B, within ISP-B’s network, the LocalPref will be fixed at 100.
b. On the ISP-B’s looking-glass website we have the following outputs:
c. In the case of ISP-C, the community value applied to this network is 400:70, so we can deduct that the LocalPref will be fixed at 70. However the following input of ISP-C’s looking glass will show that ingress path to reach 10.10.200.0/24 is going to ISP-B (check as-path 400 500 1000) when all links are up. So Localpref will be 90.
2. Now we would like to get information, for the outbound traffic of 10.10.200.0/24. The most reliable way is to make a trace-route from an IP which belongs to 10.10.200.0/24.
The trace-route will give the outbound path, but also specify if there is any additional setup on the next-hop. The following output show that next-hop is untouched, and that best ingress path for 10.10.200.0/24 is ISP-B Ebgp speaker.
Traceroute will provide vital information for the egress traffic of a customer network.