Sachin,
You have asked a very important question as it gets to
the
heart of the scalability of a PE deployment. In
a traditional router
architecture (e.g. Cisco,
Juniper, Unisphere), although FIBs are
distributed
out to individual line cards, the RIB is kept on a central
module. There is only one RIB and therefore all updates whether for
the
Local RIB, or for each VRF must go through this
RIB.
Obviously the RIB is divided into pieces so as
to keep the
VRFs separate, but the whole thing is
bounded by the
amount of memory and CPU cycles
available to the routing plane.
Prior to BGP/MPLS
VPNs, update performance of the RIB was
not a large
concern, so the fact that it was still centralized did not matter.
However BGP/MPLS VPNs place a new burden on traditional
routers.
VRFs are by definition specific to a
particular subscriber network and
must only contain
routes specific to that network; traditional routers
partition their RIBs and FIBs in order to maintain this separation.
This
can become a problem for traditional routers
since the RIB is maintained
in a central module and
serviced by a single processor. These centralized
resources (memory and CPU) become a bottleneck as the number
of subscribers increases, especially when the PE-CE
connections use chatty
routing protocols such as RIP
or computationally intensive routing protocols
such
as OSPF. For example, when using OSPF as the
CE-PE
routing protocol, a Cisco router can only sustain approx 20 to 30
instances per device. With a distributed architecture, like
the
CoSine IPSX platform, you can have more than
1500 such instances
per blade.
In a truly distributed device architecture, the RIBs are
distributed
with the VRFs, and new resources (CPUs
and Memory) can be
added as needed. Thus you
avoid the limitations imposed by
traditional routing
architectures.
The base architecture of a PE device is absolutely critical
in
determining how many BGP/MPLS VPNs the device can
handle.
Trying to retrofit an existing architecture
for a new network design
can lead to limitations
that purposely designed hardware doesn't suffer from.
--------
At 12:49 PM 4/23/2002 -0400, Liu, Chia J (Charlie), ALCNS
wrote:
>Sachin,
>
>There are two aspects:
>* Memory Usage in
GRP/RSP: In the lab, we saw data of ~935 bytes per VPN prefix in
VRF, compared to 500-600 bytes per internet prefix in global routing
table. I heard there is additional 60KB-70KB overhead per
VRF.
>* Memory Usage in
Line Card: We are particularly concerned about the VRF memory
overhead in PSA/TLU of E2 16xOC-3 in GSR. It looks like
the number is different in different IOS releases. I am
interested in knowing if anyone has number on this.
Thanx.
>
>C.J. (Charlie) Liu
>
>
>> -----Original Message-----
>>
From: Sachin Kalra [SMTP:skalra@opnet.com]
>>
Sent: Tuesday, April 23, 2002 11:51 AM
>>
To: mpls@UU.NET
>>
Subject: Memory at PE
>>
>> Dear Group:
>>
>> I would
appreciate if I can get answer to the following question regarding
>> BGP/MPLS VPNs (RFC2547bis)
>>
>> I understand that PE router
maintains a separate VRF table for each VPN
>>
site connected to it. I wanted to know if a PE also maintain separate RIBs
>> for each VPN site, apart from its main
Local RIB? Or, does it maintain only
>> one
single RIB?
>>
>> Actually, I was looking from the perspective of amount of
memory required
>> at PE, if it has to
maintain many VRFs and many RIBs.
>>
>> Thanks for your response.
>> Sachin Kalra
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