Jun 052013

R4 advertises loopback 0 into RIP, which is further redistributed by R2 into OSPF. R3 then redistributes OSPF back to RIP with a metric of 1 which causes 2 problems.

First problem is that R2 has 2 paths to reach the loopback of R4 one from R4 itself and other being redistributed by R3. Second problem is that link between R1 and R2 will not be advertised to R4 because of the distance vector problem (a router will only advertise a route if it has the protocol activated on the link or if it learns the route from a remote source via the same routing protocol).

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May 312013

R4 redistributes loopback 0 in EIGRP, which is further redistributed by R2 into OSPF. R3 then redistributes OSPF back to EIGRP. This causes R2 to have 2 paths to reach the loopback of R4 and this may cause routing loops

When there are multiple points of redistribution, and the network which is being redistributed could only be reached via one path, then the initial router which is redistributing must not choose any other path other than the one from which it initially learned.


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Mar 272013
A Multihomed customer will receive probably the whole BGP table from both service providers.  He will then filter out routes inbound from the service provider. This kind of construct is flexible and scalable in the sense of less administrative burden.
The routers from the provider side have to send the complete BGP table, which must be then received and processed by the customer router, which then filters about 90% of the routes probably with a filter list. There is a lot of waste of resources which could be utilized elsewhere.
If the customer router could dynamically request a subset of routes from the provider will be the optimum solution to this problem. This situation could be solved with outbound route filtering (RFC 5291). The idea is to send the prefix list which the customer uses to filter towards the provider.  Then the provider will filter the routes outbound.


Router R2 receives 4 networks via BGP 1.1.0/24, 1.1.1/24, 1.1.2/24 and 1.1.3/24. It then filters the networks 1.1.2/24 and 1.1.3/24 networks inbound with a prefx-list. The debug ip bgp updates in displays the effect.

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