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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