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Hakujou
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EBGP routes not advertised to IBGP peer

Mon Aug 24, 2020 5:45 pm

Hello,

I have a very simple routing setup like this:
R1 <-- IBGP (GRE) --> R2 <-- EBGP (multihop over physical link) --> TRANSIT PROVIDER
R1 and R2 are mikrotik routers with rOS 6.47.2 (latest stable) installed.

R1 has an IPv6 prefix advertised to R2 with my ASN. R2 then advertises it to the transit provider, still with my ASN.
This works correctly.

The transit providers send my full view IPv6 routes over BGP (he's using a private ASN), which are inserted in R2 route table and are preferred routes (there's no other ones, in fact), however, he does not advertises them to R1.
There's no filter involved and everything runs into a single BGP instance everywhere.

I cannot figure why this doesn't work.

I saw this post: viewtopic.php?t=82738 but it looks like it's the opposite issue (IBGP routes not sent to EBGP) and on an older version of RouterOS than mine.

Any idea of why this would happen ?
Looks like a bug to me.

Thanks !
 
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IPAsupport
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Re: EBGP routes not advertised to IBGP peer

Mon Aug 24, 2020 5:57 pm

1) The very first thing to check when BGP routes are not propagating especially when moving from iBGP to eBGP is to see if the next hop is reachable.

The border router in the iBGP ASN needs to either set next hop self for the peering towards other iBGP routers or a Route Reflector.....or the subnet used for eBGP peering needs to be advertised in the IGP used for iBGP

2) If different instances are used for iBGP and eBGP, they won't pass routes between them without redistribution.
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Hakujou
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Re: EBGP routes not advertised to IBGP peer

Mon Aug 24, 2020 6:31 pm

1) Indeed it looks like Transit gateway is marked unreachable in R2 route table. I assumed it was working since it was inserted in Mikrotik route table, but looks like it inserts it no matter if it's reachable or not.
Now I don't really understand why it is marked as unreachable, since I can ping it and reach IPv6 networks using those routes.

There's one thing, however, that I noticed, it's that the IPv6 default route does not appear in route table (I get IPv6 using RA from my provider, I have no choice). There is a default route, I can ping IPv6 websites without issues (with or without BGP learned routes), it just doesn't appear, but I can't get the default gateway since it does not appear on rOS routing table (I heard this was a thing with rOS and IPv6 RA), so maybe the issue is related to that ?

Is there a way I can propagate those routes no matter what? I tried to set nexthop-choice force-self on R2 GRE link to R1, it did not change anything.

2) All BGP peers are on the same instance (default).

Thanks for the help, I really appreciate it.

EDIT: I got my default gateway using traceroute and added a static route to the IP of the BGP peer in R2, which marked all the routes as reachable, but it did not help, R1 is still not getting routes from R2.
Not to mention I'm really not sure this is a good idea since the provider might change its gateway without notice.
 
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Re: EBGP routes not advertised to IBGP peer  [SOLVED]

Mon Aug 24, 2020 7:00 pm

I just noticed you're talking about IPv6. In IPv4, this works

However woth Ipv6, recursive routing does not work in RouterOS 6.x so iBGP does not work...the routes will advertise but are not reachable. You'll have to use OSPFv3 past the BGP Border router to advertise routing reachability. This has been a known limitation for a number of years due to the older Linux kernel 6.x uses

viewtopic.php?p=688104

v7 beta solves this problem but is not considered ready for production. Use it at your own risk.
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Hakujou
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Re: EBGP routes not advertised to IBGP peer

Mon Aug 24, 2020 9:11 pm

Thanks for your answer. I guess I have no other choice than try v7 then.

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