2009 May 03 - Sun
Routing Within An ISP
Many ISP's I've seen have had two routing protocols implemented: BGP to talk to the
'internet' with the external /24 and shorter prefixes, and an internal routing protocol such
as EIGRP or OSPF to handle the internal /24 and longer prefixes. The internal protocol
would be running on all ISP devices and would handle all infrastructure devices and customer
links. For a multi-homed ISP, BGP would need to be running on all internal devices that
form internal paths from one external link to another. This provides an ability to choose
an appropriate exit point for any traffic generated from within an ISP destined for the
external network. Some ISP's 'cheat' by generating default routes to the nearest
exit and having BGP reside only on edge devices. Some optimum paths will be missed using
this simplified arrangement, particularily if an ISP is connected to non-transit neighbors.
Current best practices make expanded use of BGP. BGP, known as IBGP, is used
extensively within the ISP to carry customer prefixes. The internal routing protocol such
as OSPF or EIGRP is used simply for carrying infrastructure routes such as loopback
addresses and link addresses.
With this arrangement, it is then easy to make use of MP-BGP (Multi-Protocol BGP) to
handle the various requirements for carrying MPLS links.
One presentation at RIPE shows some basics of
BGP Best Practices.
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