C.1.b) Traffic Analysys 2: Scenario B: Zone-D-E ingress/download traffic, primary link ISP-D is cut.
1. Because ISP-D link is cut, ISP-A does not announce anymore the prefix-C 18.104.22.168/24 to ISP-D with the community value of 500:100.
Hence ISP-D will not broadcast the route: “22.214.171.124/24, LocalPref 100, as-path: 1000”.
2. Within ISP-B network, the ISP-B Ebgp speaker connected to ISP-A learns prefixes 126.96.36.199/24 with community value 300:80, and so tags it with LocalPref 80.
This route will be fixed as the best within ISP-B network. It will be sent by ISP-B to its peers which are ISP-C and ISP-D (see yellow arrow (a) in Figure J1).
It does not receive it anymore from ISP-D the older route “188.8.131.52/24, LocalPref 100, as-path: 1000”).
3. Within ISP-C network, the ISP-C Ebgp router connected to ISP-A learns also prefix-C 184.108.40.206/24 with community value 400:70.
It receives additionally two routes of this prefix, one from ISP-B and another one from ISP-D, which are marked with LocalPref 90 (prefixes received from peers).
The one read from ISP-D is coming from ISP-B too, so at same LocalPref value; the as-path is longer.
As a result, the best route will be the one received from ISP-B directly.
ISP-C will announce this route to its peers ISP-B and ISP-D (see grey arrow (b) in Figure J1)
4. ISP-D will also learn prefix 220.127.116.11/24 from ISP-B and will send this route again to its peers(ISP-C and ISP-B) (see blue arrow (c) in figure J1)
5. In summary
(a) within the bgp table of ISP-D Ebgp speaker, the route for prefix 18.104.22.168/24 is learned by 2 ways :
i. 22.214.171.124/24, localpref 85, as-path: 300 – 1000.
ii. 126.96.36.199/24, localpref 85, as-path: 400 – 300 – 1000.
(b) Within the bgp table of ISP-B Ebgp speaker, the prefix 188.8.131.52/24 is received by 3 ways :
i. 184.108.40.206/24, localpref 80, as-path: 1000.
ii. 220.127.116.11/24, localpref 86, as-path: 500 – 300- 1000.
iii. 18.104.22.168/24, localpref 86, as-path: 400 – 300 – 1000.
(c) Within the bgp table of ISP-C Ebgp speaker, the prefix is learned by 3 ways :
i. 22.214.171.124/24, localpref 70, as-path: 1000.
ii. 126.96.36.199/24, localpref 90, as-path: 300 – 1000.
best(higher localpref and shortest as-path)
iii. 188.8.131.52/24, localpref 90, as-path: 500 – 300 – 1000.
rejected(same localpref but longer as-path)
6. The configuration seems exact at this point; secondary backup link for Zone-D-E ingress traffic is ISP-B’s upstream/transit link. When ISP-D’s primary link for Zone-D-E is cut, the ingress traffic is coming back through ISP-B.
This also due to the fact that, when the ingress traffic destined to Zone-D-E is coming within ISP-D’s or ISP-C’s networks, it is uploaded to ISP-B (and then ISP-B sends it to ISP-A).
Below Figure C.2: Return Traffic of prefix-C 184.108.40.206/24 (Zone-D-E) when the primary link ISP-C, is cut.