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.

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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) run an RIP with OSPF 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.