NetworkTalk & BGP

C.1.a) Traffic Analysis 1: Scenario A: Zone-D_E ingress/download traffic, all Ebgp links are up.

C.1.a) Traffic Analysys 1:  Scenario A: Zone-D_E ingress/download traffic, all Ebgp links are up.

1. ISP-A advertises the prefix 11.11.229.0/24 to ISP-D with the community value of 500:100.
This route in ISP-D’s BGP table, is selected as the best.
Then, it is also announced to its Ebgp peers, in our case ISP-B and ISP-C (see blue arrow (a) in figureJ).

2. Within ISP-B network, the ISP-B’s Ebgp router connected to ISP-A learns the prefix 11.11.229.0/24 with community value 300:80, and so tags it with LocalPref 80.

It receives also a route of this prefix from ISP-D, since it is a route received from a peer, the LocalPref is set up to 86.
This route will be considered better to the previous one for the reason that the LocalPref value is higher (86>80), even if the as-path is longer (300-500-1000 than 300-1000).
Therefore, ISP-B will send this route to its peers too (ISP-C and ISP-D). (see yellow arrow (b) in Figure J).
This means traffic also that traffic destined to ISP-A, coming within ISP-B’s network, will be forwarded by ISP-B to ISP-D and not ISP-A.

3. Within ISP-C network, the ISP-C’s Ebgp router connected to ISP-A learns prefixes 11.11.229.0/24 with community value 400:70, and so tags it with LocalPref 70.

It receives also two routes of this prefix: one from ISP-D and another one ISP-B, and tags it both with LocalPref value 90.

The best route will be the one learned from ISP-D, since at same LocalPref number (90); the as-path is shorter (400-500-100 and 400-300-500-1000).
Thus ISP-C will announce it (learned from ISP-D) to its peers ISP-B and ISP-D (see grey arrow (c) in Figure J).

4. To summarize :

(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 (higher localpref)
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 prefix is learned by 3 ways :

i. 11.11.229.0/24, localpref 80, as-path: 1000.
rejected(lower localpref)
ii. 11.11.229.0/24, localpref 86, as-path: 500 – 1000.
best(higher localpref and shortest as-path)
iii. 11.11.229.0/24, localpref 86, as-path: 400 – 500 – 1000.
rejected(same localpref but longer as-path)

(c) Within the bgp table of ISP-C Ebgp speaker, the prefix is received by 3 ways :

i. 11.11.229.0/24, localpref 70, as-path: 1000.
rejected(lower localpref)
ii. 11.11.229.0/24, localpref 90, as-path: 500 – 1000.
best(higher localpref)
iii. 11.11.229.0/24, localpref 90, as-path: 300 – 500 – 1000.
rejected(same localpref but longer as-path)

5. Our configuration of Zone-D-E advertisements prefixes to our ISPs is right. Ingress traffic is coming back through ISP-D Ebgp link, since all traffic destined to Zone-D-E coming to ISP-B or ISP-C networks, is sent to ISP-D. Then ISP-D uploads these packets to ISP-A.

Below Figure C.1: Return Traffic of prefix-C 11.11.229.0/24 (Zone-D-E) when all ebgp speakers are up

top

Come back to Tutorial Index”