Hi everyone
R1 received 1.1.1.0/24 as an ebgp update with next hop .2 .3 .4 as expected but why that`s not happened with 2.2.2.0/24 and 3.3.3.0/24 and 4.4.4.0/24
why i see 3.3.3.3 3 times ? 4.4.4.4 3 time ?same next hop
R3 is going to advertise 3.3.3.0/24 to R1 with next hop 10.1.12.3 only one time why R3 advertise it 3 times to R1 ?because i see the next hop 10.1.12.3 3 times
That looks rather buggy to me. On R1, when you issue the command, “show ip bgp 2.2.2.0/24” do you actually see three entries? Is there anything that makes each one different from the others?
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sir,do you think it`s a bug ?
Okay…here’s what’s going on (I’ll just explain it for the 2.2.2.0/24 prefix but the same principle applies for 3.3.3.0/24 and 4.4.4.0/24)
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You have a BGP “network 2.2.2.0 mask 255.255.255.0” statement in R2. So this means that R2 will advertise this prefix:
—a. via eBGP directly to R1
—b. via iBGP directly to R3
—c. via iBGP directly to R4
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When R2 sends this prefix via eBGP to R1 it sets the next-hop attribute to itself (10.1.12.2)
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When R3 receives the iBGP update from R2, R3 then transmits this as an eBGP update to R1. Because of the “third party next-hop” rule R3 does NOT change the next-hop to itself but instead leaves the next-hop as 10.1.12.2. This is because R3 knows that R1 and R2 are in the same subnet.
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The same thing I described above happens on R4. It receives the route via R2 as iBGP and re-transmits it to R1 as eBGP. It does not change the next-hop because of the “Third party next-hop” rule.
For more info on that rule, please see this INE Blog posting:
https://ine.com/blog/2010-09-02-understanding-third-party-next-hop
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