From: Peter Van Oene (pvo@xxxxxxxxxxxx)
Date: Thu Apr 05 2001 - 13:26:56 GMT-3
that should say "rather unrelated to EGP/IGP synch" I really need to read my o
wn msgs before posting!
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On 4/5/2001 at 11:34 AM Peter Van Oene wrote:
>I'm not sure about this though I don't work that much with Cisco routers.
> I would suggest that ensuring reachability to EBGP Next_Hop addresses is
>pretty much fundamental to designing BGP networks and pretty rather to
>EGP/IGP synchronization.
>
>As far as how routes get into the BGP table, I'm not sure I agree with
>you. It is my understanding that all BGP paths received from a neighbor
>will populate the BGP RIB-IN table irrespective of whether they meet the
>criteria for re advertisement or use. With synch enabled, the NLRI
>information (prefixes) must match IGP provided routes in the routing table
>for those prefixes to be posted in the routing table. Reachability to the
>Next_Hop is also a criteria at this point. Whether synch is enabled or
>disabled however, that path information will certainly populate the
>RIB-IN.
>
>As far as synch being beautiful, I am somewhat confused. I assume you
>mean that because you have IGP reachability for a prefix, if that prefix
>happens to not be posted via BGP due to Next_Hop reachability not being
>met, that the router will still forward packets based on the IGP? I would
>suggest that this is a corner case benefit that does more harm than good.
>Its like saying a default route will enable packets to flow even if you
>mess up your routing configuration. In this case, I would think it far
>better for things not to work so that the designer could immediately
>notice and resolve the configuration issue. Cludged networks that work
>cause more grief than ones that don't as the need to troubleshoot is much
>more obvious :)
>
>Keep in mind that the networks that sync was designed to support no longer
>exist. Those networks provided transit services to an internet with so
>few prefixes that the IGP could handle them. I just wanted to reiterate
>the sync is obsolete and that one shouldn't waste ones time trying to
>figure out how to make it work :)
>
>Pete
>
>
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>
>On 4/5/2001 at 9:51 AM Mark Salmon wrote:
>
>>I have a caveat to the sync question. I would make sure that ALL IP nets
>>(include the WAN/LAN EBGP) links are reachable by all BGP routers in your
>>AS. The beauty about sync is no IP subnets/CIDR nets will appear in the
>>BGP table unless they appear first in the IGP table. With Sync off (no
>>sync) that will not happen. If your design is not done properly (ie all
>>IP nets in the BGP table include next hop) is not reachable, then packets
>>will be dropped.
>>
>>Peter Van Oene wrote:
>>
>>> Inline comments
>>>
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>>>
>>> On 4/5/2001 at 10:49 AM Oscar Diaz Poveda wrote:
>>>
>>> >Dear all,
>>> >
>>> >Inside an AS where there are routers that are not running IBPG:
>>>
>>> Obviously a stub AS as opposed to a transit? If transit, you should run
>>IBGP. If full routes internet transit there is no other option here
>>(assuming scalability tools like rr's/confeds etc used as well)
>>>
>>> >
>>> >When you should redistribute BGP into the IGP and when you should turn
>>off
>>> >synchronization???
>>>
>>> Synch is only relevant when you run a transit AS and don't' fully mesh
>>with IBGP. However, you should NOT do this. This is bad. Very bad in
>>fact. If you are running a transit service, use IBGP and disable synch.
>>Synch is an obsolete feature that should never be turned on. In fact, I
>>highly doubt that any transit AS in the world has synch enabled. Further,
>>I suggest that since it has received little to no programming attention
>>(educated guess) I expect it doesn't even work flawlessly. If a proctor
>>asks you to enable synch, I'd call an exorcist immediately.
>>>
>>> For what its worth, Juniper (who make routers ostensibly for transit
>>as's) do not even have a knob to enable synch.
>>>
>>> Pete
>>>
>>> >Thank you for your advice in advance.
>>> >
>>> > Best regards,
>>> >
>>> >Oscar.
>>> >
>>> >
>>>
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