"Holey" routes with TRRP


A "holey" route is a shorter prefix announced by one AS from which a longer prefix announced by a second AS has punched a hole. For example, if AS1 announces and AS2 announces then is a holey route because AS2 has overridden (punched a hole in) part of the prefix.

A route is considered traffic engineering if the longer override prefix is from the same AS as the shorter prefix. Traffic engineering routes are unholey as they will still reach the correct AS even if the longer prefix is filtered.

Handling "Holey" routes in TRRP

An end-of-the-line ITR only catches packets destined for addresses which are not advertised via BGP at all. If the longer override prefix in a holey route is filtered but the holey route is not, packets destined for the override prefix will not reach either the ITR or the intended destination.

A site implementing TRRP must not accept BGP announcements for holey routes in which any of the hole networks have implemented TRRP. This means that holey routes may not be designated "globally routable" for the purpose of assigning addresses to ETRs and DNS Route Servers.

A route negation list is necessary to allow ASes which implement TRRP to remove holey routes from their tables so that the packets correctly reach an ITR for retransmission to their destinations. There are at least a couple ways to implement a route negation list.

eBGP with the "Holey Route Authority"

A central route authority keeps track of all holey routes and peers them with all ASes which implement an ITR. Each such AS implements a route-map to force those prefixes to head the same direction as the default route.

iBGP from the ITR

The end-of-the-line ITR downloads the route negation list from the Holey Route Authority once per day. It advertises these routes within the AS via iBGP. The iBGP routes override the equivalent eBGP routes so that the traffic for them comes back to the ITR.


Any entity which receives a CIDR address block directly from a regional registry has the authority to designate as "holey" the prefix for its block, any longer prefix within it and any shorter prefix which overlaps it.

An entity which receives a CIDR address block from a local registry (aka ISP) has the authority to designate as "holey" the prefix for its block and any longer prefix within it. Shorter overlapping prefixes may only be designated holey by the ISP.

"Holey" designations will not be accepted or maintained for prefixes which do not otherwise appear within the DFZ routing table.