Hello,
I'd like to ask a couple of
questions regarding a scalability of LDP,
especially when it is deployed
in an ATM-LSR, adopting Downstream on
Demand distribution with conservative
label retension and ordered control,
where labels (VPI/VCI) are a
relatively scarce resource that must be conserved
in ATM switch. The
VPI/VCI is a particularly valuable and limited resource that
should be
saved when an ATM LSR is not able to support VC-merging
capability.
Unfortunately the system I'm managing has no VC-merging
capability.
As defined in the standard, LDP attempts to setup or release a
hop-by-hop LSP
whenever a new route is added or deleted in an ATM LER
through routing protocols,
mainly IGP such as OSPF, ISIS or RIP.
Let me
assume a sample ATM-MPLS network as follows.
Network1 - R1 <->
LER1 <-> LSR1 <-> LSR2 <-> LER2 <-> R2 -
Network2
<---IP Network---><------------MPLS
Network---------><---IP Network--->
R1, R2: Router operating
IGP (OSPF or RIP)
LER1, LER2: Edge ATM LSR operating LDP and IGP (iBGP if
needed)
LSR1, LSR2: ATM LSR operating LDP and IGP (OSPF or
IS-IS)
OSPF or RIP is used in the IP Network and OSPF or IS-IS is
deployed
in the MPLS Network. An i-BGP session could be setup between LER1
and LER2
if needed, but not necessarily since LER1 and LER2 are not
supposed to provide
BGP/MPLS functionality, in other words, they don't
work as a PE(Provider Edge) router.
LER1, LER2, LSR1 and LSR2 support only
LDP as an MPLS signaling protocol.
The problem is that the number of
LSP's from LER1 to LER2 increases
as the number of networks increases in
the Network2, meaning that the number of
routing entries in LER1 increases
accordingly, wherease the number of LSPs supported
by LER1 is a lot smaller
than the number of its route entries.
Eventually, as the number of LSPs
setup exceeds the maximum number of LSP's,
we'll not be able to setup an
LSP any more even though new routes are added.
As I mentioned above, LSR1
and LSR2 don't support VC-merging capability,
thus the scalability becomes
quite servere.
Therefore w're trying to figure out how to support a lot of
routing entries using
a limited number of LSP's. We are trying to come up
with a mechanism that setups
just one LSP between LER1 and LER2, which
could be shared between all best-effort
traffic between Network1 and
Networks2.
The problem we faced is how to setup only 1 LSPs between
LERs automatically,
and then how to map a lot of route entries to the
shared LSP.
Although two LER's are shown in the example, the number of
LSP's between LER's
can be quite large as the number of LER's
increases.
We're considering to setup a full-mesh iBGP sessions between all
LER's, and
to use BGP next hop gateway in order to get an FEC-to-NHFLE
mapping.
In other words, routes from Network2 will be redistributed to LER1
via iBGP session
between LER1 and LER2, and then LER1 will setup just one
LSP for the BGP next hop
address and map all routes from Network1 to the
LSP.
However it enforces us to setup iBGP sessions between all of LER's
even though
we don't need them unless we deploy MPLS. We're also wondering
whether
we'll be able to solve the problem using any IGP capability, like
tag field of AS-external
LSA in OSPF.
Please enlighten us on this
issue.
Any experience or idea will be greatly appreciated.
Thanks in
advance.
Regards,
Jung.