Black Hole Routing / Summary routes

[cgravell]

i understand why you might use black hole routing for privately addressed internets as a more efficient way of filtering inbound IP SA to guard against spoofing internal addressing, BUT, i don't understand why EIGRP installs them automatically and OSPF manually for summary routes on ABR's.

i don't see what the significance of them is, apart from not sending ICMP unreachables for broken routes...i'm missing something here because i just don't get it!

Please explain,

Chris

[mark-obrien]

Chris,

The idea behind having a route to the null interface (I think this is what you are referring to) is that an OSPF ABR or an EIGRP router performing summarization will not send packets destined for an unused address in the range via the default route. If it did, this would cause a routing loop, between the summarizing router and the router advertising a default.

Mark