What is Discard Route or NULL0 Route?

What is Discard Routes and how does it help to stop layer 3 routing loops? As we know the fact that loops are not only originates in layer2 network but could also originate in layer 3 networks. We have various mechanisms to prevent layer 3 routing loops with spit horizon and route poisoning.

Would like to understand; IS there any kind of other scenario where in routing loop could exist? See the below infographic

As depicted above, routing loop could occur whenever a summary route is announced towards the peer. This loop could be stopped by creating a manual discard route pointing towards null interface.

IP Route 10.2.0.0 255.255.0.0 null0

OSPF, IS-IS and EIGRP automatically creates discard route whenever summary is performed on the routers.


Below are the configurations for disabling discard routes:-
OSPF
router ospf 1
no discard route

EIGRP
interface serial0 ip summary-address eigrp 1 10.2.0.0 255.255.0.0 255


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Solution Of Multihomed Redundancy

Most of the answers received for quiz is that configure the secondary route on PE2 with higher metric for CE1 lan in case of static routing or configure dynamic routing. But I am more concerned towards the static routing solution. If static routes are configured on secondary PE2 with higher metric will not come into play because the local originated weight of route is 32768 but the route which is received by MP-iBGP is having weight of 0, so the metric will not play and by default traffic forwarding for CE1 lan will happen from PE2 back door link configured for CE1. The higher metric route will be selected during that time when it is being received by multiple sources. To overcome this problem I have tested the solution by configuring the route map with weight 0 for CE1 lan. This route map is called under address family of vpn with redistribute static. Once this will be done one can see the weight of local originated route will become 0. There after configure the static route for CE1 lan with metric more than 200. Once this will be done PE2 will receive the routes from RR and local originated route and compare both the routes having same weight and then check for the lowest AD. The lowest AD configured route is receiving by RR, os it will be preferred and traffic will start flowing by PE1 instead of PE2. Once the PE1 primary link fails, that floating static route will add in the table.
For dynamic back door links refer to the previous post of BGP as PE-CE with back door link and it's simulated results.

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Static are recursive in nature

Static routes are recursive in nature. Recursive means to check the route for the next hop address which is used for static route like bgp does recursive lookip for finding the next hop address. A good example of recursive lookup is MPLSVPN SP network.In SP network always PE loopback is used for customer VPNv4 routes and bgp does the recursive lookup to reach to that loopback. It means when you are going to place the static route in the routing table and the interface which is directly connected to it get down at that time the route should flush from the routing table. It usually happens but if the next hop is reachable by some other path in that case the route is not going to flush from the routing table because recursive lookup will take for finding the next hop address. This type or problem usually comes in service provider network when customer requires the redundancy. In those type scenarios try to use the static route with interface and next hop address. The main advantage of using the command is that the next hop is reachable only if the interface state is up else it will not reachable and route will be flushed from the routing table. If you configure a simple static route pointing the next hop address in that case the route is not going to flush from the table and floating route will never come in picture consequence traffic will be blackholed.


regards
shivlu jain
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