Refer to the exhibit. An engineer must design a WAN solution so that ISP-1 is always preferred over ISP-2. The path via ISP-2 is considered as a backup and must be used only when the path to ISP-1 is down. Which solution must the engineer choose?
A.
R1: - Routes advertised to ISP-1: 0x AS-path prepend - Routes received from ISP-1: HIGH local-preference - Routes advertised to R2: no action - Routes received from R2: community NO-EXPORT R2: - Routes advertised to ISP-2:5x AS-path prepend - Routes received from ISP-2: LOW local-preference - Routes advertised to R1: community NO-ADVERTISE - Routes received from R1: no action
B.
R1: - Routes advertised to ISP-1: 0x AS-path prepend - Routes received from ISP-1: HIGH local-preference - Routes advertised to R2: community NO-EXPORT - Routes received from R2: no action R2: - Routes advertised to ISP-2: 5x AS-path prepend - Routes received from ISP-2: LOW local-preference - Routes advertised to R1: no action - Routes received from R1: no action
C.
R1: - Routes advertised to ISP-1: 0x AS-path prepend - Routes received from ISP-1: LOW local-preference - Routes advertised to R2: community NO-ADVERTISE - Routes received from R2: no action R2: - Routes advertised to ISP-2: 5x AS-path prepend - Routes received from ISP-2: HIGH local-preference - Routes advertised to R1: no action - Routes received from R1: community NO-ADVERTISE
D.
R1: - Routes advertised to ISP-1: 5x AS-path prepend - Routes received from ISP-1: LOW local-preference - Routes advertised to R2: community NO-ADVERTISE - Routes received from R2: no action R2: - Routes advertised to ISP-2: 0x AS-path prepend - Routes received from ISP-2: HIGH local-preference - Routes advertised to R1: community NO-EXPORT - Routes received from R1: no action
B is correct.
- Routes advertised to ISP-1 do NOT need to have AS-path prepend
- Routes advertised to ISP-2 need to have AS-path prepend
- Routes received from ISP-1 need to have HIGH local-preference
- Routes received from ISP-2 need to have LOW local-preference
---> This already rules out C and D
- NO-ADVERTISE to internal routers is wrong (R1 and R2 should each know both paths)
--> This rules out A.
I am not sure, about your last sentence. Its not that R2 with NO-ADVERTISE will not send its network to R1, it will but R1 will not redistribute them anywhere else (e/iBGP), as we sends this community from R2. Based on the drawing, and having limited konwledge of rest of the config, it can rule out that R1 is still able to reach Internet via R2, and inform about this R3 which will preffer it as its eBGP and still act as gateway. I will vote for A.
Ok and now I just found that A/B differ also in direction in quesiton...
A: R2 out>R1 (mark as NO-ADV), while R2> in R1 (mark as NO-EX)
B: R1 our>R2 (mark as NO-EX)
I do not need to address C and D, because it is easy to see ISP1 needs no AS-path prepend and higher local preference and ISP2 needs the opposite to favor ISP1.
The confusion is with A and B, which differ for the path from/to AS-5000. So for A, it is telling R1 not to advertise anything from R2 and R2 is telling R1 the same. The diagram is showing that the primary path should go through R3 to R1, but R2 still can advertise R1's ISP to R4 and then R4 can go through R2-R1-ISP1.
With answer B, R1 is telling R2 not to pass on any advertisements it gives it to anyone outside of the AS. So, now R3 and R4 will only know the path to ISP1 through R1 and will also still get the ISP2 path through R2.
from Enterprise LAN point, if I advertised to R2 prefixes with community no export and R1 is down, what happens is that, INTERNET doesn't known my LAN so there is no conectivity, instead If advertised to R1 as no-advertise community doesn't happen anything in AS 65444, the election will be in INTERNET, and here will select the short AS-PATH atribute.
R1:
- Routes advertised to ISP-1: 0x AS-path prepend
- Routes received from ISP-1: HIGH local-preference
- Routes advertised to R2: community NO-EXPORT
- Routes received from R2: no action
R2:
- Routes advertised to ISP-2: 5x AS-path prepend
- Routes received from ISP-2: LOW local-preference
- Routes advertised to R1: no action
- Routes received from R1: no action
Answers should look like this. R2 will still receive the LAN routes from R4.
i dont know.......B is correct on all except the NO-EXPORT to R2. In case of ISP1 failure, how will internal routes be advertised to ISP2 if routes advertised to R2 have NO-EXPORT ?? internet will not be able to have customer routes. Am I missing something ?
In answer B:
"Routes advertised to R2: community NO-EXPORT"
This seems very weird, because if we give out this community that would mean that Route 2 wouldn't advertise the routes towards ISP2 (AS-6000) even if the route towards R1 and ISP1 failed...
So ALL the answers seem to be incorrect (but I guess answer B makes the most sense)
Please correct me if I'm wrong tho
Doesn't advertise learned routes to ebgp, but can stilff form ebgp peers with another AS. I guess we use the no-export to avoid R2 becoming a transit AS
it also uses the no-export to advertise to ISP's ibgp
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