[f-nsp] Multipath iBGP question

[David Freedman]

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Hello,


I know that ECMP (Equal cost multipath) for iBGP is a bad idea,
so this is only a "lab" question (i.e this is not in production!)



here is the question:

(apologies if this ASCII art is not formatted correctly)


[Router A]------------------[Router B]
~   |                         |
~   |                         |
[Access Router A]      [Access Router B]
~   |                         |
~   |                         |
[Customer Prefix]     [Customer Prefix]
~ 10.0.0.1/32            10.0.0.1/32


Both Router A and B are BI4K MGR4 Ironcore devices
with the following configured:


!
ip load-sharing 4
!
router bgp
maximum paths 4
!

Access Router A and B connected to them
are belonging to $other_vendor,
redistributing their statics for the customer prefix (10.0.0.1/32)
into iBGP.


How do the Bigiron RouterA/B know what the correct next-hop
is for the customer prefix using multipath?

In the lab,  traffic arriving on Router A destined for the customer
prefix (10.0.0.1) is being sent to Router B
Upon arriving at Router B its then being sent back to Router A
sometimes!!!

Both devices are running 07.06.01B and have also been running 07.06.05

I would have assumed that the process would have instructed the CAMs
to have been built in such a way as specifically NOT to send traffic
back in the direction from which it came.

Is this not the case?


The implementation of multipath seems to work well on the $other_vendor
kit....


Thanks in advance.


- --
David Freedman
Network Engineering Department
Claranet UK Limited
http://www.clara.net
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