Tag Archives: 3.3m

3.3.m Implement and troubleshoot loop prevention mechanisms

3.3.m [i] Route tagging, filtering

Users can define a route map to prevent OSPF routes from being added to the routing table. This filtering happens at the moment when OSPF is installing the route in the routing table. This feature has no effect on LSA flooding. In the route map, the user can match on any attribute of the OSPF route. That is, the route map could be based on the following match options:

match interface

match ip address

match ip next-hop

match ip route-source

match metric

match route-type match tag

OSPF external LSAs have a tag. The value of the tag is examined before the prefix is installed in the routing table . All OSPF external prefixes that have the tag value of 999 are filtered (prevented from being installed in the routing table). The permit statement with sequence number 20 has no match conditions, and there are no other route-map statements after sequence number 20, so all other conditions are permitted.

route-map tag-filter deny 10

match tag 999

route-map tag-filter permit 20

router ospf 1



network area 0

distribute-list route-map tag-filter in

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 2844-2848).  . Kindle Edition.


3.3.m Implement and troubleshoot loop prevention mechanisms

3.3.m [iii] Route poisoning


Route poisoning is a method to prevent a router from sending packets through a route that has become invalid within computer networks. Distance-vector routing protocols in computer networks use route poisoning to indicate to other routers that a route is no longer reachable and should not be considered from their routing tables. Unlike the split horizon with poison reverse, route poisoning provides for sending updates with unreachable hop counts immediately to all the nodes in the network.

3.3.m Implement and troubleshoot loop prevention mechanisms

Loops occur when routers act on the basis of inaccurate or old information. Link-state protocols like OSPF use reliable flooding mechanisms to ensure that all routers are acting on the basis of the same information. That is what link-state protocols avoid. All routers in a link-state database have the same view of the network.

Distance vector protocols are susceptible to routing loops. Split horizon is one of the features of distance vector routing protocols that prevents them. This feature prevents a router from advertising a route back onto the interface from which it was learned. Route poisoning is another method for preventing routing loops employed by distance vector routing protocols. When a router detects that one of its directly connected routes has failed, it sends the advertisement for that route with an infinite metric (” poisoning the route”). A router that receives the update knows that the route has failed and doesn’t use it anymore. Holddown is also a loop-prevention mechanism employed by distance vector routing protocol. This feature prevents a router from learning new information about a failed route.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Location 2832).  . Kindle Edition.


3.3.m Split horizon…

Split Horizon

loop prevention

rule: a routing interface cannot send routing information out the same interface upon which

it was received

Route Summarization

Routing information can be condensed (summarized) from a longer prefix into a shorter prefix; the routing table is more efficient, reduced, and routing updates are fewer.




Classful routing protocols do not include vlsm in their updates. they are delineated by classful masking.

ex. (rfc 1918) class A class B class C

IP Classless

The ip classless command was introduced into default behavior for ios 12.0. Classful routing protocols, ie. Rip and IGRP discard traffic bound for any unknown subnet of a major network, even with a default route configured. IP classless allows for the use of a default route in this case.