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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OSPF Fast Convergence Tools - Updating RIB

RIB (Routing Information Base) update is the last post of OSPF fast convergence series whereas earlier posts have already covered OSPF Event Detection, OSPF Event Propagation and OSPF Event Processing.

Whenever there is change of interface state arises, OSPF detect that change and according to that Dijkstra runs the algorithm and update the RIB table according to that. This process is time consuming and requires lot of CPU and memory cycles.

Reducing convergence time depends how fast the OSPF detects the state change and notifies the RIB. In OSPF we normally perform the summarization of the prefixes on ABR and ASBR, this is done only to save the CPU cycles so that every state change information is not flooded across the OSPF routing domain.

Secondly, interface purge command can be used to reduce convergence time. This optimization allows interface link state changes to directly impact RIB rather than waiting for routing protocol updates to RIB to take place.

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OSPF Fast Convergence Tools - Event Processing

OSPF Event Processing is the third post about OSPF fast convergence tool, earlier posts have already covered OSPF Event Detection and OSPF Event Propagation.

OSPF event processing is all about how to throttle SPF calculations and increase the OSPF convergence time. Event processing is having three different tools as below listed:-
• OSPF SPF Calculations
• OSPF SPF Throttling
• OSPF Incremental SPF

OSPF SPF Calculations:- OSPF full calculations are only triggered by LSA1 and LSA2 where as partial SPF is triggered by LSA 3, LSA 4, LSA 5 and LSA 7.

OSPF SPF Throttling:- OSPF Event propagation deals with the LSA throttling whereas OSPF Event processing deals with SPF throttling. SPF and iSPF(increment SPF) cab be rate limited to delay calculations and save lot of network resources during when the network is not stable. We can throttle only full SPF and iSPF not the partial SPF. Below is the command line to configure SPF throttling:-
router ospf 10
timers throttle spf (start) (hold) (max)

start:[default = 5000 ms] Initial SPF schedule delay in milliseconds.
Recommendation = 10 ms

hold:[default=10000 ms] Minimum hold time between two consecutive SPF calculations. Range is 1 to 600000 milliseconds.
Recommendation = 100 ms

max:[default = 10000 ms] Default ‘timers spf spf-holdtime’ value was 10s. Setting max-interval to 10000 ms provides response equivalent to pre SPF throttle.
Recommendation = 5000 ms


OSPF Incremental SPF:- Before the introduction of iSPF, Dijkstra Algorithm is run by all the routers whenever there is any state change occurs in the OSPF routing area. This requires lot of CPU and memory and wastes lot of resources.

So iSPF helps Dijkstra to run the algorithm only at the change portion of the OSPF tree for LSA 1 and LSA 2 only. This process saves lot of CPU and memory resources consequences reduce OSPF network convergence.

router ospf 10
network 0.0.0.0 255.255.255.255 area 0
ispf

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