Re: [Idr] draft-uttaro-idr-bgp-persistence-00

[<[e-mail]>]

Robert,

>Jim,
>
>When one during design phase of a routing protocol or routing protocol
>extension or modification to it already realizes that enabling such
>feature may cause real network issue if not done carefully - that
should
>trigger the alarm to rethink the solution and explore alternative
>approaches to the problem space.
>
>We as operators have already hard time to relate enabling a feature
>within our intradomain boundaries to make sure such rollout is network
>wide. Here you are asking for the same level of awareness across ebgp
>boundaries. This is practically unrealistic IMHO.
>
>Back to the proposal ... I think that if anything needs to be done is
to
>employ per prefix GR with longer and locally configurable timer. That
>would address information persistence across direct IBGP sessions.
>
>On the RRs use case of this draft we may perhaps agree to disagree, but
>I do not see large enough probability of correctly engineered RR plane
>to experience simultaneous multiple ibgp session drops. If that happens
>the RR placement, platforms or deployment model should be
re-engineered.

The low chance / probability of the failure of both RR planes is indeed
a good point. I can also agree that this is expected to be low. I
disagree that it never happens. E.g. with current BGP error handling,
the same BGP attribute in "error" would cause both iBGP plane to go
down. Hopefully work is being done to improve this. But I would not bet
that this will cover all future cases. In such catastrophic cases, some
routing is better than no routing.
On a side note, considering that the RR and hence their failure are
independent and therefore will quite never fail at the same time, is
questionable. To start with, they process they mostly process the same
input (BGP updates) and are part of the same network. 

>Summary .. I do not think that IDR WG should adopt this document. Just
>adding a warning to the deployment section is not sufficient.
>
>Best regards,
>R.
>
>
>> Robert,
>>
>> The introduction of this technology needs to be carefully evaluated
>> when being deployed into the network. Your example clearly calls out
>> how a series of independent design can culminate in incorrect
>> behavior. Certainly the deployment of persistence on a router that
>> has interaction with a router that does not needs to be clearly
>> understood by the network designer. The goal of this draft is to
>> provide a fairly sophisticated tool that will protect the majority of
>> customers in the event of a catastrophic failure.. The premise being
>> the perfect is not the enemy of the good.. I will add text in the
>> deployment considerations section to better articulate that..
>>
>> Thanks, Jim Uttaro
>>
>> -----Original Message----- From: [e-mail]
>> [mailto:[e-mail]] On Behalf Of Robert Raszuk Sent:
>> Sunday, October 23, 2011 5:32 PM To: [e-mail] List Subject: [Idr]
>> draft-uttaro-idr-bgp-persistence-00
>>
>> Authors,
>>
>> Actually when discussing this draft a new concern surfaced which I
>> would like to get your answer on.
>>
>> The draft in section 4.2 says as one of the forwarding rules:
>>
>> o  Forwarding to a "stale" route is only used if there are no other
>> paths available to that route.  In other words an active path always
>> wins regardless of path selection.  "Stale" state is always
>> considered to be less preferred when compared with an active path.
>>
>> In the light of the above rule let's consider a very simple case of
>> dual PE attached site of L3VPN service. Two CEs would inject into
>> their IBGP mesh routes to the remote destination: one marked as STALE
>> and  one not marked at all. (Each CE is connected to different PE and
>> each PE RT imports only a single route to a remote hub headquarter to
>> support geographic load balancing).
>>
>> Let me illustrate:
>>
>> VPN Customer HUB
>>
>> PE3      PE4 SP PE1      PE2 |        | |        | CE1      CE2 |
>> | 1|        |10 |        | R1 ------ R2 1
>>
>> CE1,CE2,R1,R2 are in IBGP mesh. IGP metric of CE1-R1 and R1-R2 are 1
>> and R2-CE2 is 10.
>>
>> Prefix X is advertised by remote hub in the given VPN such that PE1
>> vrf towards CE1 only has X via PE3 and PE2's vrf towards CE2 only has
>> X via PE4.
>>
>> Let's assume EBGP sessions PE3 to HUB went down, but ethernet link
>> is up, next hop is in the RIB while data plane is gone. Assume no
>> data plane real validation too. /* That is why in my former message
>> I suggested that data plane validation would be necessary */.
>>
>> R1 has X via PE1/S (stale) and X via PE2/A (active) - it understands
>> STALE so selects in his forwarding table path via CE2.
>>
>> R2 has X via PE1/S (stale) and X via PE2/A (active) - it does not
>> understand STALE, never was upgraded to support the forwarding rule
>> stated above in the draft and chooses X via CE1 (NH metric 2 vs 10).
>>
>> R1--R2 produce data plane loop as long as STALE paths are present in
>> the system. Quite fun to troubleshoot too as the issue of PE3
>> injecting such STALE paths may be on the opposite site of the world.
>>
>> The issue occurs when some routers within the customer site will be
>> able to recognize STALE transitive community and prefer non stale
>> paths in their forwarding planes (or BGP planes for that matter)
>> while others will not as well as when both stale and non stale paths
>> will be present.
>>
>> Question 1: How do you prevent forwarding loop in such case ?
>>
>> Question 2: How do you prevent forwarding loop in the case when
>> customer would have backup connectivity to his sites or connectivity
>> via different VPN provider yet routers in his site only partially
>> understand the STALE community and only partially follow the
>> forwarding rules ?
>>
>> In general as the rule is about mandating some particular order of
>> path forwarding selection what is the mechanism in distributed
>> systems like today's routing to be able to achieve any assurance that
>> such rule is active and enforced across _all_ routers behind EBGP
>> PE-CE L3VPN boundaries in customer sites ?
>>
>> Best regards, R.
>>
>>
>> -------- Original Message -------- Subject: [Idr]
>> draft-uttaro-idr-bgp-persistence-00 Date: Sat, 22 Oct 2011 00:23:55
>> +0200 From: Robert Raszuk<[e-mail]> Reply-To:
>> [e-mail] To: [e-mail] List<[e-mail]>
>>
>> Hi,
>>
>> I have read the draft and have one question and one observation.
>>
>> Question:
>>
>> What is the point of defining DO_NOT_PERSIST community ? In other
>> words why not having PERSIST community set would not mean the same as
>> having path marked with DO_NOT_PERSIST.
>>
>> Observation:
>>
>> I found the below statement in section 4.2:
>>
>> o  Forwarding must ensure that the Next Hop to a "stale" route is
>> viable.
>>
>> Of course I agree. But since we stating obvious in the forwarding
>> section I think it would be good to explicitly also state this in
>> the best path selection that next hop to STALE best path must be
>> valid.
>>
>> However sessions especially those between loopbacks do not go down
>> for no reason. Most likely there is network problem which may have
>> caused those sessions to go down. It is therefor likely that LDP
>> session went also down between any of the LSRs in the data path and
>> that in spite of having the paths in BGP and next hops in IGP the LSP
>> required for both quoted L2/L3VPN applications is broken. That may
>> particularly happen when network chooses to use independent control
>> mode for label allocation.
>>
>> I would suggest to at least add the recommendation statement to the
>> document that during best path selection especially for stale paths
>> a validity of required forwarding paradigm to next hop of stale
>> paths should be verified.
>>
>> For example using techniques as described in:
>> draft-ietf-idr-bgp-bestpath-selection-criteria
>>
>> Best regards, R.
>>
>>
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