Manual Summarization in EIGRP, BGP causes the creation of route to "null 0 interface".
The configuration of manual summarization in EIGRP and BGP causes the creation of route to "null 0 interface"
Enhanced Interior Gateway Routing Protocol (EIGRP) can summarize both internal routes and external routes. The type of route you choose determines which commands you issue. When a router running EIGRP is configured for route summarization using manual or automatic summarization, a summary route pointing to null 0 interface is created in the routing table of the router.
A similar behavior is observed with BGP too. When Border Gateway Protocol (BGP) is configured for manual summarization, a summary route pointing to the null 0 interface is created in the routing table of the router.
To configure a summary aggregate address for a specified interface, use the ip summary-address eigrp command in interface configuration mode. To disable a configuration, use the no form of this command.
ip summary-address eigrp as-number ip-address mask [admin-distance] [leak-map name]
no ip summary-address eigrp as-number ip-address mask
command under a specific interface adds a route for the summary being configured to the routing table of the router pointing to interface null 0 . This is also true when the router is configured for automatic summarization by issuing the command:auto-summary command
To restore the default behavior of automatic summarization of subnet routes into network-level routes, use the auto-summary command in router configuration mode. To disable this function and send subprefix routing information across classful network boundaries, use the no form of this command.
under the routing protocol configuration mode.
The route to null 0 interface is added to avoid routing loops or sending packets to a black hole when a less specific route, like a default route, is also available on the router performing the summarization. The default route is used for reaching networks that are not specifically available in the routing table.
For example, router R1 is connected to few other routers within a private network and connects to a public network, such as the Internet, through the border router R2. R1 is learning specific networks from the routers in the private network through EIGRP, but it advertises only a summary route to R2, since R1 is the only way for R2 to reach all those specific networks. This is achieved by configuring the EIGRP process on R1 for either manual or automatic summarization. R1 also relies on a default route pointing to R2 to reach the routes available in the public network.
If any of the specific networks stops learning by R1 through EIGRP, it is no longer available in its routing table. Packets that are arriving at R1 destined for this specific network will match the default route due to longest match lookup, and they will be forwarded to the wrong next-hop R2 instead of getting dropped. R2 still has the summary route in its routing table, since R1 continues advertising it through EIGRP as long as one of the specific networks is being learned. The packet reaching R2 now matches the summary route and is sent back to R1. This results in a routing loop.
Any packet forwarded to the null 0 interface on a router is dropped by the router. In the same scenario, when the summary route pointing to null 0 interface is installed in the routing table of R1, all packets destined for the specific network match the summary route on R1 instead of the default route. They use the longest match lookup and are dropped on R1 itself.
When the specific network is once again learned by R1 through EIGRP, packets destined for this network match this specific route instead of the summary route due to longest match lookup. They are forwarded to the router that advertised this network.
Similarly, when a router running BGP is configured for manual summarization using the aggregate-address command in the router configuration mode, a route pointing to interface null 0 for the summary being configured, is added to the routing table of the router. Here too, the addition of route pointing to null 0 is used to avoid routing loops and avoids forwarding packets to a black hole.
For more information on configuring automatic and manual summarization, refer to these documents: