Routing comment

[Morgan Dollard <]

Hello, 
In reading the IOS Essentials for ISP's PDF, ver 2.84
 http://www.cisco.com/public/cons/isp/documents/IOSEssentialsPDF.zip

 i came across something I believe might need a little bit of clarification.

I dont know if many ISP's use this method or not, but i find it particularly
usefull. 


On page 29, under the Caveats section, specifically the routing protocols
part, it is stated that :
". Routing Protocols. If a routing protocol needs to be run over this link,
it is operationally much easier to use IP
addresses. Don't use "ip unnumbered" if the customer is peering with you
using BGP across the link, or if the link is
an internal backbone link. Simply use a network with a /30 address mask.
(Routing will work over unnumbered links
but the extra management and operational complexity probably outweighs the
small address space advantage gained.) "


Using Ip unnumbered and BGP is something that works extremely well, in the
following case. 
 The wan interface is unnumbered to the ethernet interface. The ethernet
interface is using a primary IP that is the one that should be configured on
the WAN interface, and a secondary IP that is the LAN IP 

In most cases, the Provider will want to peer its BGP session to the IP it
assigned the customer. This setup allows the BGP session to be peered to the
Ethernet interface, which in turns leeds to an end to end point of failure,
from the Ethernet Interface, to the Wan Interface. This succesfully allows
the ISP to monitor the customer's network availability without the use of
piningin methods, and relying solely upon the well being of the BGP session.


If this message is off list topic, i apologize for the inconvenience. 

Morgan Dollard






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