Software and hardware annotations 2008 October
This document contains only my personal opinions and calls of
judgement, and where any comment is made as to the quality of
anybody's work, the comment is an opinion, in my judgement.
[file this blog page at:
- 081014 Tue
Routing with OSPF and ECMP
- Recently I have been quite happy to reconfigure a set of
switches to become routers, for added resiliency and load
sharing: the idea being to mimic the the resilience of the
Internet, creating a number of small networks dual with uplinks
to two independent backbones, just like a number of independent
networks may be uplinks to two independent ISPs.
Ensuring that the ISPs be truly independent is not an easy
task, but in my case the two backbones have been designed to be
independent to a substantial degree: there are independent cable
runs mostly following different paths to data rooms located in
different parts of campurs, with mostly independent cooling and
on different power substations.
Complementing this is a logical architecture in which only
the backbone routers have explicit routes to the rest of the
world (for now, then to be replaced by BGP provided routes);
the backbone routers, the local routers, and even several
servers (critical one attached directly to both backbone router)
enabled. This means that on each network (both backbones and
each local network) each router broadcasts the routes it is
aware of, and eventually all routes propagate, and traffic gets
split across balanced links. There is similar functionality
for exterior routing with BGP and ECMP.
Designs based on OSPF and ECMP for interior routing are
fairly common for large internetworks with dozens of routers,
simply because other alternatives are impractical; but it is
perhaps less well understood that it is fairly easy to setup and
convenient even for small internetworks, because the
configuration is rather simple.
In a single network single router situation it is not
difficult to distribute the address of the single router
dynamically via DHCP; in a single network, multiple router one
it is conceivably possible to use link local router discovery
(which is somewhat rarely supported for IPv4 and native in
IPv6). But for any internetwork, and even for a single network
OSPF is simple enough, even if its flexibility is not needed.