Lab 3, Task 5.4 (How Do I Know?)

5.4. BGP Attribute Manipulation
• Advertise VLAN 29 into BGP on R2.
• R5 should see this prefix as follows:
Network Next Hop Metric LocPrf Weight Path
*> 136.X.29.0/24 136.X.245.2 0 100 300 i
• This configuration should not affect any other prefixes on R5.
2 Points

My notes:
**Weight is locally significant. Determine who R5 is learning about 136.x.29.0 from, and apply a route map for inbound traffic that sets the weights of the
136.x.29.0 prefix advertisements to be 300**

I understand the mechanics of task 5.4 (at least I think). However, my question is this. How do I know that R5 is not learning about the 136.x.29.0 prefix (VLAN29) from any other source
besides R2?

I understand that the prefix R5 sees comes from R2, but how do I know that one of R5's eBGP neighbors is not also advertising this prefix to R5?

Network Next Hop Metric LocPrf Weight Path
*> 136.X.29.0/24 136.X.245.2 0 100 300 i

Comments

  • You right, R5 will receive VLAN29 advertised thru R1, but that will have a longer ASPATH. 100 300, but from R2 to R5 it will be only 300, so that will be selected.

    Please refer to BGP Path selection.

    HTH
    =========
    Just to make it more clear, here's the output from R1

    Rack1R1#sh ip bgp neigh 136.1.15.5 advertised-routes
    BGP table version is 16, local router ID is 150.1.1.1
    Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
    r RIB-failure, S Stale
    Origin codes: i - IGP, e - EGP, ? - incomplete

    Network Next Hop Metric LocPrf Weight Path
    *>i136.1.29.0/24 136.1.23.2 0 100 0 300 i
    /*output omitted*/

    As mentioned earlier, R1 is in fact advertising VLAN29 to R5, but it won't be taken due to a longer ASPATH.
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