NetworkTalk & BGP

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.

Figure A.1: PEERING AGREEMENT WITH LOCAL PREFERENCE

Figure A.1: PEERING AGREEMENT WITH LOCAL PREFERENCE

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:

Log C-2f-1

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.

Log C-2f-2

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.

 Log C-2f-3

Traceroute will provide vital information for the egress traffic of a customer network.

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