The MPLS-OPS Archive

[Christopher Lewis <chrlewis@cisco.com>]

I fear there is still lack of clarity on this issue.

As stated, there is currently a 32 routing process limit in IOS.

As Geoff points out, these routing processes (called Process Descriptor 
Blocks or PDBs for short) are consumed every time you start a routing 
instance. In a typical MPLS VPN PE, it will look something like this.

Service Provider BGP = 1 PDB
Service Provider IGP = 1 PDB
Connected networks = 1PDB
Static networks = 1 PDB

So right off the bat, you are left with 28 routing processes for customer 
facing IGP instances used on the PE to CE links. Now as is the case with 
the large scale commercial deployments I am familiar with, the PE to CE 
routing is handled by eBGP, which supports address families, and therefore 
no matter how many customers there are on that PE, only one more PDB is 
consumed.

BGP is a better choice from the SP perspective than OSPF as it does not 
open the PE up to unnecessarily processing lots of LSAs from a misbehaving 
CE router. In these large scale deployments, the service has been deployed 
as a managed service, meaning the CE router is actually supplied and run by 
the service provider who is familiar with BGP (one hopes:-) and the 
enterprise does not have to be involved in running BGP to get the VPN service.

Chris

At 03:10 AM 7/6/2002, Geoff Zinderdine wrote:

>"The question of running lots of IGP instances comes in to play when you are
>looking at supporting lots of customers on a PE, then lots of customer
>instances of THEIR IGP may exist on the PE. Roger is correct in that there
>is currently a 32 routing process limit in IOS and if the PE to CE protocol
>is OSPF, that is the limit."
>
>Effectively, the limit is 30 processes as a process each is taken up for
>Static and Connected.
>
>Geoff Zinderdine
>
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