[cisco-bba] Multiple LNSs: favourite IGP or iBGP?

Matthew Melbourne matt at melbourne.org.uk
Wed Jun 8 15:34:10 EDT 2011

How do providers typically implement routing for multiple LNSs (terminating
ADSL user sessions)? Given that a set of LNSes could be carrying many
(thousands?) of host routes (or small subnets), is it preferable to
integrate them directly into iBGP for scalability, or isolate them and run
an IGP and perform some redistribution at (say) a redundant pair of LNS
aggregation routers? 


I can see good scalability arguments for iBGP (and it is said to be good
practice to keep customer prefixes in iBGP), but would be concerned over the
BGP churn and hence convergence for large numbers of injected and withdrawn
routes (e.g. user sessions reconnecting and/or moving between LNSs).


An IGP (e.g. OSPF NSSA) could potentially converge much quicker in a similar
scenario but perhaps doesn't scale as well as iBGP. The problem is less of a
concern if each LNS only has its own pool of address from which dynamic
users are allocated addresses (as the summarisation placement is more
obvious), but it's not so clear when there are many users with static
addresses or require more that a single static address.







Matthew Melbourne


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