[j-nsp] Where and how to inject the default route in DFZ

Mark Smith markrefresh12 at gmail.com
Thu Nov 26 10:30:35 EST 2015

Hi list

This is best explained by an example.

Router R1 has a full bgp table (~550k prefixes). R1 needs to announce a
default route using OSPF and BGP. The worst issue is when R1 boots up.
Assume there is a static 0/0 route to discard. R1 brings up OSPF
adjacencies and starts announcing 0/0. Blackhole. Next R1 brings up BGP
adjacencies and starts announcing 0/0. Another blackhole. Only after R1 has
received the complete route table (and pushed all prefixes to FIB) will the
blackhole disappear.

What's the best solution to work around this? Where and how do you generate
the default?

With OSPF or ISIS one can use overload w/ timeout. It works.
What about BGP? out-delay? One can use "routing-options generate route ..."
for default. But what prefix to use as contributing route? One can announce
a dummy prefix from e.g. RR, but still this does not guarantee that R1 has
all necessary routes (in FIB) before announcing default. And this probably
leads to transient routing loop while R1 has default pointing to e.g.
loopback of RR, but routers in between see the OSPF default announced by R1.

Overload timeout + generate default?


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