NetworkTalk & BGP

C.1.d) Traffic Analysis 4: Scenario D: Zone-D-E ingress/download traffic, primary link ISP-D is restored however secondary backup link ISP-B is still down.

C.1.d) Traffic Analysys 4: Scenario D: Zone-D-E ingress/download traffic, primary link ISP-D is restored however secondary backup link ISP-B is still down.

1. This scenario described the following events through this order:

(a) ISP-D is cut. Ingress traffic at destination of ZONE-D-E customers is streaming from ISP-B to ISP-A.
 
(b) ISP-D is cut, and then ISP-B is also cut. Ingress traffic for ZONE-D-E is coming back through ISP-C to ISP-A.
 
(c) ISP-D is now restored but ISP-B link is still down. Ingress traffic for ZONE-D-E is coming back again from ISP-D to ISP-A.

This is because the prefix-C 11.11.229.0/24 is advertised again by ISP-A to ISP-D with the community value 500:100.
So the best route is once more the following one: “11.11.229.0/24, localpref 100, as-path: 1000”.

We come back then to same situation of paragraph C.1.a)

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 3 ways :

i. 11.11.229.0/24, localpref 100, as-path: 1000.
best
ii. 11.11.229.0/24, localpref 85, as-path: 300 – 500 – 1000.
rejected(as-loop issue)
iii. 11.11.229.0/24, localpref 85, as-path: 400 – 500 – 1000.
rejected(as-loop issue)

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

i. 11.11.229.0/24, localpref 86, as-path: 500 – 1000.
best (shortest as-path)
ii. 11.11.229.0/24, localpref 86, as-path: 400 – 500 – 1000. rejected(longer 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.
rejected (lower locapref)
ii. 11.11.229.0/24, localpref 90, as-path: 300 – 500 – 1000.
rejected(same locapref but longer as-path)
iii. 11.11.229.0/24, localpref 90, as-path: 500 – 1000.
best(same localpref but shortest as-path)

See Below Figure C.4 : Return Traffic of prefix-C 11.11.229.0/24 (Zone-D-E) when its primary link ISP-D is restored but secondary backup link from ISP-B is still cut.

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