From: Robert Raszuk
Hi Jim,
The approach as I understand it is two deploy multiple channels to
disseminate routing state be it the 2,3,...nth path to some dest D..
Not necessarily at all. Multiple channels is just one of possible
deployment models.
Primary goal is to observe that while today having a pair of best path
RRs one could easily turn one of the reflector within such pair into a
backup RR, and without any need for any new IBGP session to be
provisioned or without any need to add new RRs disseminate backup path
to all clients.
This provides very easy to deploy mechanism where without any need for
PE upgrade you provide PEs additional paths for fast connectivity
restoration, PIC or load balancing needs.
Comments follow...
So do further replies ...
Jim Uttaro
Section 1.0
"The parallel route reflector planes solution brings very
significant
benefits at a negligible capex and opex deployment price as compared
to
the alternative techniques"
A number of points need to be clarified here. The first is the
SP/Operator needs to deploy n number of RR planes to disseminate N
paths. Assuming some form of redundancy we would have to of course
buy
the RRs or deploy some type of logical routers. How can this be
monetized?
Not required. You maintain connectivity redundancy even if turning
existing pair of RRs into primary and backup case. No new purchase
order
required nor need for any logical routers.
The draft describes RR planes to give the formalized description of
the
proposal, but I was hoping that it could be easily interpreted into
basic deployment styles.
Adding new set of RRs if someone needs to is also possible, but
completely not necessary for the diverse path deployment.
Does this approach assume that customers who want fast
restoration, load balancing, mitigation of oscillation would pay for
this. Or does the draft assume that the addl RRs are of such
negligible
capex cost that the operator would simply incur the cost.. This
model
does not usually sit well with the folks that write the checks.
Again .. this is not necessary at all. Contrary if you need to swap
your
all PEs into a new ones which support alternative ways to disseminate
more then one BGP path .. this is where the check would be rather
heavy :).
Comparing that with just code upgrade on the RRs seems clear where the
savings are.
From an
opex perspective we are putting in addl planes for each AS that is
under
the operators authority. So we not only need to pay for it we would
need
to establish coherent inter-AS strategies to manage, maintain these
addl
RRs.. Additionally the function of these devices is different than a
traditional RR which implies that OpS needs to be cognizant of the
difference and how they should be managed.. As described in the
draft
the 2,3..nth plane need not be as robust as it is not the primary
path..
This needs to be understood by OpS in terms of their response to
failure
or how to perform maintenance.. We are essentially introducing a new
device from these perspectives...
Again you are stuck with one particular deployment model. I take as a
recommendation to clarify this in the next version of the draft that
provider may simply without adding new RRs nor without touching PEs
turn
one of the existing RRs into a diverse-path RR and disseminate diverse
BGP paths to the clients.
All what is required on provider side is to upgrade RRs with new code
+
enable client sessions with new knob.
Section 2.1
"This new requirement has its own memory and processing cost.
Suffice
to say that by the middle of 2009 none of the commercial BGP
implementation can claim to support the new add-path behaviour in
production code, in part because of this resource overhead."
A bit confused by this statement.. My thoughts on this was add-paths
is
useful for a customer that is advertising multiple paths or at
peering
points. In both cases we would anticipate the use of routing policy
to
only select a subset of these routes. It is impractical to believe
that
we are going to duplicate the same state over and over again on each
plane.. This is not a function of the draft but how operators deploy
the
functionality..This functionality has been around a long time in
VPNV4
services and I believe it will eventually be used for IPV4
services..
At peering points I find it very common to set next hop self while
advertising towards IBGP peers so there is practically no reason to
advertise mutliple ebgp learned paths towards the core.
VPNv4 PEs require this day one .. many ISPs in their IPv4/IPv6 do it
by
default as well today.
One would need to also note that this quite a common practice to
provision external peerings on different ASBRs in order to avoid
single
ASBR going down a disconnect of number of external peering sessions.
" The add paths protocol extensions have to be implemented by all
the
routers within an AS in order for the system to work correctly."
Pls explain.. Why do you believe this? It is certainly not practical
and
I never envisioned a full upgrade across thousands of edges in
multiple
AS domains.. The approach we believe we could take is to deploy on a
subset of edges for some set of routes.
The question one needs to ask is what is overall goal ? As you have
observed the primary goal SPs are after is to provide fast
connectivity
restoration, load balancing or mitigate oscillations.
To accomplish this in MPLS networks or in IP encapsulation networks
you
need to push additional state to all edges/PEs otherwise you are
missing
the alternative paths where they need to be present.
Pushing more then best path with add-paths requires upgrade of PEs.
Distributing additional paths with diverse-path proposal does not
requires any touch to the PE.
So if you can clarify what is the point of using add-paths for "subset
of edges for some set of routes" ?
" It is intended as a way to buy more time allowing for a smoother
and
gradual migration where router upgrades will be required for perhaps
different reasons. It will also allow the time required where
standard
RP/RE memory size can easily accommodate the associated overhead
with
other techniques without any compromises.'
His statement seems to conflict with the one above.. Above you state
that it is needed everywhere to work correctly here the statement is
we
can buy time to gradually migrate.. Why don't we just gradually
migrate
and eliminate this middle step??
The gradual migration may take 10 years. Do you want to offer inferior
services as compared with other SPs over the next 10 years ?
In my true opinion both add-paths and diverse-path are complimentary
to
each other. Yes they both enable operator to distribute more then
overall best path, but they do it differently.
I imagine that RRs could be capable of supporting peers which are
upgraded with the add-paths code as well as those clients which are
not
yet upgraded, but still would benefit with receiving more then best
path
only.
In fact I think that most of the current applications can be easily
satisfied with just 2nd best path which dissemination diverse path
proposal addresses.
Section 4
" The proposed solution is based on the use of additional route
reflectors or new functionality enabled on the existing route
reflectors
that instead of distributing the best path for each route will
distribute an alternative path other then best. "
Would like to drill down on this a bit..In the first case where addl
deployment of RRs are done I am assuming that these RRs would
somehow
prefer the second best path of the first.. How would this be done
customers use many different mechanisms to identify primary,
secondary,
etc... AS-PATH prepend, Local Pref, IGP cost etc... are all
used..How is
this done on the secondary plane? Regardless of either of these
approaches changes to the BGP implementation to select a different
POI
is needed. But where how do you know how a customer is identifying?
Pls
expand on this. It would seem that although the protocol definition
does
not change the operator needs to ensure that this functionality is
constructed the same way across all the vendors.. Will this require
another draft?
BGP best path algorithm is quite consistent today across vendors. That
means that calculating best path is consistent. That also means that
calculating 2nd best path would also be consistent.
So when this could break ?
It could break in step 9 of best path on non co-located RRs where we
consider IGP metric to a BGP next hop.
In order to address this point there are number of options:
* Make sure from IGP point of view primary RR and backup RR are on the
same point in the network - nothing additional is needed - that is
also
very often the case in control plane RRs in tunneled networks
* Disable IGP metric check step on RRs - as a matter of fact RRs
making
decision on best path from their point of view makes only sense when
RRs
are in the data plane on the POP to core boundaries. In all other RR
placements somewhere in the core it is really not necessary.
* No need to worry about any IGP metric step but allow backup RR to
learn primary RR's best path and accommodate this knowledge when
advertising diverse path towards clients. Again no need to add any new
RR is needed nor modify even a single line of configuration of the
clients.
Any other BGP mechanism like AS-PATH prepend, Local Pref etc ... would
be treated identically on both primary and backup RR so no issue.
" The best path (main) reflector plane distributes the best path for
each route as it does today. The second plane distributes the
second
best path for each route and so on. Distribution of N paths for
each
route can be
achieved by using N reflector planes."
How is this done when it is the IGP cost that is the deciding
factor..
Will we have to correctly place the Nth plane corresponding to IGP
correctly in the IGP??
See above.
" It is easy to observe that the installation of one or more
additional
route reflector control planes is much cheaper and an easier than
the
need of upgrading 100s of routers in the entire network to support
different protocol encoding."
See Above I do not believe it is all or nothing..
Also see above :) And by installation please do not think of physical
RR
installation. Under this I meant to indicate turning existing set of
RR
into a backup RR plane as well.
" Diverse path route reflectors need the new ability to calculate
and
propagate the Nth best path instead of the overall best path. An
implementation is encouraged to enable this new functionality on a
per
neighbor basis."
Encouraged? I think it would be required..
I agree it is preferred and I am supporting that.
But one could observe that especially in topologies where you have
very
good POP symmetry towards pairs of RRs or when you would prefer to add
RR as backup that you may want to turn diverse-path functionality on
such backup RR on a per SAFI basis.
Section 4.1. Co-located best and backup path RRs
"To simplify the description let's assume that we only use two route
reflector planes (N=2). When co-located the additional 2nd best
path
reflectors are connected to the network at the same points from the
perspective of the IGP as the existing best path RRs.'
Based upon implementation this may require ports on existing core
router
to terminate and a costing paradigm that duplicates the original the
latter may be simple the former would require that there is
availability
at these locations.. Doesn't this also imply full symmetry? We could
not
deploy a subset for the nth plane and mimic the IGP decision making
of
the first?? The draft states that full symmetry is not needed.. Pls
Clarify..
As indicated above full symmetry only applies when you want to make
sure
that IGP point of RRs is the same on primary and backup RR. As
described
earlier this is just one of 3 ways to make sure backup RR calculates
correct backup paths towards it's clients.
And also as described above in this example addition of second plane
may
be as simple as upgrading one of your existing RRs and enabling it to
distribute diverse path towards the clients.
Initially to one or few on a per session basis .. while to other
clients
still sending duplicate of best path like today - later with more
experience gained to more and more clients being served by this
cluster.
" One of the deployment model of this scenario can be achieved by
simple
upgrade of the existing route reflectors without the need to deploy
any
new logical or physical platforms. Such upgrade would allow route
reflectors to service both upgraded to add-paths peers as well as
those
peers which can not be immediately upgraded while in
the same time allowing to distribute more than single best path."
The implication here is that the same primary RR would have to
"hold"
and disseminate multiple paths to D.. Would this create a
scalability
problem on this RR as it would have to hold these addl routes. Even
though the number of BGP routes for the internet is small in
comparison
to VPNV4 this should be accounted for when RR platforms are
selected.
I think for RRs platforms scalability concerns for number of routes
and
number of sessions are no longer the issue. Talk to your favorite
vendor
for up to date RR's scalability numbers :) But you are very correct.
Those need to be considered when RR platforms are selected.
Section 4.2. Randomly located best and backup path RRs
" The basic premise of this mode of deployment assumes that all
reflector planes have the same information to choose from which
includes
the same set of BGP paths. It also requires the ability to skip the
comparison of the IGP metric to reach the bgp next hop during
best-path
calculation."
Scalability concerns.We would be putting our main primary RRs at
risk.
Not sure what risk you are referring to. As indicated earlier for
control plane RRs it is really not necessary step in the best path
since
day one.
Again I am confused about the IGP metric.. If the paths are equal up
to
the IGP metric how do decide which is primary/secondary.. The
secondary
RR needs to select one of the paths how does it do that??Is it
router-id
or something of that nature..
See above.
"4. Fully meshing newly added RRs' with the all other reflectors in
both planes. That condition does not apply if the newly added
RR'(s)
already have peering to all ASBRs/PEs."
I cannot see creating BGP sessions to all ASBR/PEs. There are BGP
session limits that also must be accounted for so I do not see that
as a
viable alternative in a large network.. So I guess we would have to
fully mesh to all RRs. This is similar to a full mesh of PEs in
terms of
getting all the routes on the secondary to make a decision.
This is normal introduction process of new RR into the network. But as
said already few times this is optional.
" Any of the existing routers that are not already members of the
best
path route reflector plane can be easily configured to serve the 2nd
plane either via using a logical / virtual router partition or by
local
implementation hooks."
The term "Easily" is used to liberally. Getting complex
functionality
configured on our most important parts of the network is never easy.
It
requires a lot of test certification and coordination between OpS,
maintenance, etc... to get deployed
One needs to pick the right set of tools which he can accomplish the
task with in the most easy way. My goal is to deliver various
deployment
options and assist in selection of the best set of tools to complete
the
job.
That's why I am not saying to do this one way .. Depending on network
size, scale, complexity one may find adding a new RR as trivial
exercise, on the other hand someone else may think of existing RR
upgrade at the next upgrade window as pretty much free operation which
needs to be performed anyway. Then enabling diverse path to some
clients
and seeing how it works seems like a very smooth and gradual
deployment
- much easier then any RR based alternatives I can think of today.
" The additional planes of route reflectors do not need to be fully
redundant as the primary one does. If we are preparing for a single
network failure event, a failure of a non backed up N-th best-path
route
reflector would not result in an connectivity outage of the actual
data
plane. The reason is that this would at most affect the presence of
a
backup path (not an active one) on same parts of the network. If
the
operator chooses to build the N-th best path plane redundantly by
installing not one, but two or more route reflectors serving each
additional plane the additional robustness will be achieved."
Yes that may be true but we envision add-paths as being
functionality
that not only enables fast restoration but the ability to provide
customer with load balancing. Probably good to be specific about the
goals of the draft in the intro/abstract.
So do I. Diverse path accommodates both goals just fine.
> Probably good to be specific about the
> goals of the draft in the intro/abstract.
Ack.
Section 4.3. Multi plane route servers for Internet Exchanges
" In such cases 100s of ISPs are interconnected on a common LAN.
Instead
of having 100s of direct EBGP sessions on each exchange client, a
single
peering is created to the transparent route server. The route server
can
only propagate a single best path. Mandating the upgrade for 100s
of
different service providers in order to implement add-path may be
much
more difficult as compared to asking them for provisioning one new
EBGP
session to an Nth best-path route server plane."
I do not understand. Are you saying that each eBGP session is nailed
up
to each plane. Are you implying that we deploy 100 planes? If not
how do
we know which one of the 100 ISPs should get the benefit of having
routes source by them propagated through the network??
No :) I am saying that if you have 100s of IX of clients by default
route server would send only one overall best. So if you have two RS
(and this is common for redundancy) one may send overall best and the
other one diverse path to the IX customers. 3rd best path would be
also
easy to achieve.
I will clarify this section.
Jim - Many thx for your excellent comments and review,
R.
_______________________________________________
GROW mailing list
[email protected]
https://www.ietf.org/mailman/listinfo/grow
