Unit 1: Network Infrastructure
This will allow you to demonstrate your networking skills, knowledge, and abilities, with a focus on enterprise-level switching, routing, and multicast components that support cross-platform (inter)operability and integration with the most recent software-defined technologies.
One of the things BGP checks for path selection is whether we have an external BGP (eBGP) or an internal BGP (iBGP) path.
We’ll see in this topic that BGP prefers eBGP paths more than iBGP paths.
Configuration
We will be using the following topology:
Two ASes contain three routers:
• AS120 contains R1 and R2.
• AS300 contains R3.
The 3.3.3.3/32 prefix is advertised by R3 on a loopback interface in BGP. Through eBGP, R1 and R2 will learn about this prefix. Through iBGP, they will also learn about each other’s paths.
Verification
We will examine R1’s BGP table. Our first command will be “show ip bgp”:
R1#show ip bgp
BGP table version is 4, local router ID is 1.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
t secondary path,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
* i 3.3.3.3/32 192.168.230.3 0 100 0 300 i
*> 192.168.130.3 0 0 300 i
*> 192.168.130.0 0.0.0.0 0 32768 i
*>i 192.168.230.0 192.168.120.2 0 100 0 i
We can notice two pathways for 3.3.3.3/32 in the report above:
- Through R2 (192.168.230.3)
- Through R3 (192.168.130.3)
The symbol > indicates that R1 installed this path as the best path. The question is, why did it choose this path as the best path? To find an answer to that question, we will have to look over the list of attributes that are included in the optimal path selection method.
