>Hi Folks,
>Can anyone ( specially the likes of Berkowitzs, Forsyths, Oppenheimers,
>etc ) comment on the advantages of having multiple "IP routing tables" in a
>router such as featured by Juniper in its M-Series machines. Would it not
>consume comparatively more hardware resources on a router in terms of RAM,
>CPU cycles, etc ? Thanks in advance.
>
>Aziz S. Islam
I'm not an expert on Juniper's implementation, but I am going to
assume, by routing table, you refer to multiple routing information
bases (RIB), not multiple forwarding information bases (FIB).
Multiple FIBs are necessary in any high-performance implementation
that uses distributed forwarding, such as dCEF.
Frankly, there's a religious war among protocol architects about
whether to have one main routing table and process into which you
load more and more state, or multiple routing tables/instances for
different communities of interest. These issues are especially heated
when dealing with VPNs. If you look through IETF drafts, you will
find lots of different opinions.
RFC2547 style VPNs, for example, load lots and lots of VPN state into
a provider's BGP. Admittedly, not all this state information is
propagated to other providers. Even if you are doing a multiprovider
VPN, only subsets will be advertised outside the provider. But there
are certainly concerns about the amount of state and complexity this
adds to BGP.
There are some proposals to do a 2547-variant using IGPs to
disseminate information within providers. I like this somewhat
better, although I haven't done detailed analysis.
A comment from the audience at the last NANOG, with respect to 2547:
"if this is the answer, it must have been a pretty stupid question."
The multiple virtual router approach reduces the amount of state that
any single process has to maintain, but may have operational
limitations (or operational benefits, depending on your point of
view) in that it doesn't show a consistent, provider-wide view of
routing.
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