Sim 07: BGP Path Selection Policy: Local Preference, AS-Path Prepend and Transit Filtering¶
Blueprint 1.2, 1.11.c, 1.11.e
| Difficulty | Time target | Devices | Faults |
|---|---|---|---|
| Hard | 30 minutes | 5 x IOSv | 3 |
Scenario¶
AS 65010 is dual-homed. R1 connects to ISP-A (R3, AS 65001) and R2 connects to ISP-B (R4, AS 65002). Both ISPs connect to R5 (AS 65100), which stands in for the rest of the Internet and advertises 203.0.113.0/24. ISP-A also originates its own customer network 198.51.100.0/24. R1 and R2 run OSPF between them for internal reachability and an iBGP session between loopbacks.
The routing policy is:
| Policy | Mechanism | Where |
|---|---|---|
| Outbound traffic: prefer ISP-A for every destination | Local preference 200 on routes received from ISP-A | R1, inbound from R3 |
| Inbound traffic to 192.0.2.0/24: prefer ISP-A | Prepend AS 65010 twice on the prefix sent to ISP-B | R2, outbound to R4 |
| AS 65010 must never be a transit AS | Only advertise locally originated routes (AS path ^$) |
R1 and R2, outbound to both ISPs |
Download the lab
CML lab file · EVE-NG lab file
5 nodes. The lab boots already broken (the faults are in the start-up configs, so skip Break it). CML: Import a lab, then start it. EVE-NG: import the .unl from the lab list; if your image folder names differ, edit each node's image.
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