NetworkTalk & BGP

C.1.c) Traffic Analysis 3: Scenario C: Zone-D-E ingress/download traffic: primary link ISP-D is cut, secondary backup link ISP-B is cut too.

C.1.c) Traffic Analysys 3: Scenario C: Zone-D-E ingress/download traffic: primary link ISP-D is cut, secondary backup link ISP-B is cut too.

1. ISP-D and ISP-B Ebgp speakers are down, meaning that only ISP-C receives the prefix-C 11.11.229.0/24 and tags it with LocalPref 70. This route will be set as the best and announced to its peers too.

2. In summary :

(a) within the BGP table of ISP-D Ebgp speaker, the route for prefix 11.11.229.0/24 is learned by two ways:

i. 11.11.229.0/24, localpref 85, as-path: 400 – 1000.
best
ii. 11.11.229.0/24, localpref 85, as-path: 300 – 400 – 1000.
rejected(as-path longer)

(b) Within the bgp table of ISP-B Ebgp speaker, the prefix 11.11.229.0/24 is received by 2 ways :

i. 11.11.229.0/24, localpref 86, as-path: 500 – 400- 1000.
rejected(as-path longer)
ii. 11.11.229.0/24, localpref 86, as-path: 400 – 1000.
best(shortest as-path)

(c) Within the bgp table of ISP-C Ebgp speaker, the route for prefix 11.11.229.0/24 is learned by 3 ways :

i. 11.11.229.0/24, localpref 70, as-path: 1000.
best
ii. 11.11.229.0/24, localpref 90, as-path: 300 – 400 – 1000.
rejected(as-loop issue)
iii. 11.11.229.0/24, localpref 90, as-path: 500 – 400 – 1000.
rejected(as-loop issue)

Consequently all ingress traffic will come back from ISP-C Ebgp speaker.

Below Figure C.3: Return Traffic of prefix-C 11.11.229.0/24 (Zone-D-E) when its primary and secondary links are cut.

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