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[Bug 377] eBGP multihop/nexthop problem

 

 

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bugzilla-daemon at allevil

Jun 10, 2008, 8:34 AM

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[Bug 377] eBGP multihop/nexthop problem

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http://bugzilla.quagga.net/show_bug.cgi?id=377


jrhett [at] netconsonance changed:

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CC| |jrhett [at] netconsonance




------- Additional Comments From paul [at] dishone 2008-06-10 16:34 -------
Jho,

Can you describe the topology of that setup in more detail? I.e. how are the
peers connected, and how are they connected to the nexthop? I don't quite
understand still.



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bugzilla-daemon at allevil

Jun 12, 2008, 1:23 PM

Post #2 of 6 (6766 views)
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[Bug 377] eBGP multihop/nexthop problem [In reply to]

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http://bugzilla.quagga.net/show_bug.cgi?id=377





------- Additional Comments From jrhett [at] netconsonance 2008-06-12 21:22 -------
Cogent standard installation is as follows:

/30 IP subnet for fiber transit
you announce all your prefixes to far side IP
you also announce a /32 they provide to you.
it announces to you only a single /32

You setup a second peering session, origin the /32 cogent assigned to you,
destination the /32 you receive on the BGP peering session. On this peering
session you announce nothing, but receive full tables.

This is how Cogent does it, and they won't make exceptions in this policy. The
gear you plug into can't handle a full table.




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bugzilla-daemon at allevil

Jun 13, 2008, 10:29 AM

Post #3 of 6 (6757 views)
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[Bug 377] eBGP multihop/nexthop problem [In reply to]

Please do not reply directly to this email. All additional
comments should be made in the comments box of this bug
report.

http://bugzilla.quagga.net/show_bug.cgi?id=377





------- Additional Comments From paul [at] dishone 2008-06-13 18:29 -------
Ok, I get it now. It's using BGP as a sort of 'AS-interface Gateway Protocol' to
distribute nexthop routing for multihop BGP (rather than statics, or some simple
IGP like RIP). Thanks Jo.

To address Eugen's comment, it isn't that the existing code is broken - its that
we don't support recursive lookups to this extent. Its not just a question of
code erroneously marking a nexthop as inactive, its that the code simply doesn't
know how to work out a route for 3.3.3.3:

# show ip route 0.0.0.0
Routing entry for 0.0.0.0/0
Known via "bgp", distance 20, metric 0
Last update 00:34:38 ago
3.3.3.3 inactive

E.g., if it were just a simple misapplication of an 'inactive', then the above
would have said '3.3.3.3 (recursive via 1.1.1.2) inactive' instead.

We have some support for recursive routes already. However, it relies on an
ordering of protocols to avoid cycles in the lookup (iBGP/multihop BGP ->
IGP/Static -> connected). To support BGP via BGP we'll need some other way to
detect cycles (e.g. floyd cycle finder, etc).

Another aspect to this bug is that the workaround is made harder because the BGP
route for the nexthop, though not considered for nexthop resolution, manages to
interfere withthe static - that's probably easier to fix.




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bugzilla-daemon at allevil

Jun 13, 2008, 1:38 PM

Post #4 of 6 (6757 views)
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[Bug 377] eBGP multihop/nexthop problem [In reply to]

Please do not reply directly to this email. All additional
comments should be made in the comments box of this bug
report.

http://bugzilla.quagga.net/show_bug.cgi?id=377


web [at] pilot changed:

What |Removed |Added
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CC| |web [at] pilot






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bugzilla-daemon at allevil

Jun 17, 2008, 6:27 AM

Post #5 of 6 (6769 views)
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[Bug 377] eBGP multihop/nexthop problem [In reply to]

Please do not reply directly to this email. All additional
comments should be made in the comments box of this bug
report.

http://bugzilla.quagga.net/show_bug.cgi?id=377





------- Additional Comments From web [at] pilot 2008-06-13 21:38 -------
I hope to produce some particular opinion on this matter after discussing the
setups above with my co-worker on Monday. Cheers!

------- Additional Comments From web [at] pilot 2008-06-17 14:27 -------
Well, if such configurations violate the RFC, we are against them.



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bugzilla-daemon at allevil

Jun 17, 2008, 9:20 AM

Post #6 of 6 (6754 views)
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[Bug 377] eBGP multihop/nexthop problem [In reply to]

Please do not reply directly to this email. All additional
comments should be made in the comments box of this bug
report.

http://bugzilla.quagga.net/show_bug.cgi?id=377





------- Additional Comments From jrhett [at] netconsonance 2008-06-17 17:20 -------
"Well, if such configurations violate the RFC, we are against them."

Who said that this violated an RFC?

Multi-hop BGP peering based on BGP routes violates no RFC.



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