Company XYZ runs OSPF in their network. A design engineer decides to implement hot-potato routing architecture. How can this implementation be achieved?
A.
Enable iBGP and apply prepend to ensure all prefixes will have the same length of the AS path attribute value.
B.
Redistribute the external prefixes onto OSPF and ensure that the total metric calculation includes external and internal values.
C.
Enable OSPF load-balancing over unequal cost path.
D.
Redistribute the external prefixes onto OSPF and ensure the total metric calculation includes only the external value and the value is the same in all ASBRs.
D is using BGP to OSPF redistribution metric-type 2. If we use the same metric at every redistribution point, then for OSPF the only tie-breaker for these routes is the internal cost to the ASBR.
Answer: B
"hot potato" routing is the model where the exit point selection is performed based on the local metric to the exit point associated with the prefix. In other words, "hot potato" model tries to push packets out of the local system as quick as possible, optimizing internal link utilization.
https://ine.com/blog/2011-04-04-understanding-ospf-external-route-path-selection
"Now within the scope of OSPF, think of the E2 route selection process: OSPF chooses the best exit point based on the external metric and uses the internal cost to ASBR as a tie breaker. In other words, OSPF performs "cold potato" routing with respect to E2 prefixes. It is easy to turn this process into "hot potato" by ensuring that every exit point uses the same E2 metric value. It is also possible to perform other sorts of traffic engineering by selectively manipulating the external metric associated with the E2 route, allowing for full flexibility of exit point selection."
https://ine.com/blog/2011-04-04-understanding-ospf-external-route-path-selection
Answer is D:
To achieve a hot-potato routing architecture in an OSPF network, the goal is to route traffic to the closest exit point from the network to minimize the distance that the traffic travels within the network. This approach reduces the internal transit of traffic and pushes it out to external networks as quickly as possible. In the context of OSPF, this can be influenced by how external prefixes are redistributed into the OSPF network and how their metrics are calculated.
The most appropriate way to achieve this is:
D. Redistribute the external prefixes onto OSPF and ensure the total metric calculation includes only the external value and the value is the same in all ASBRs.
Option D is likely the preferable choice for implementing hot-potato routing in an OSPF environment. This option suggests equalizing external metrics for redistributed prefixes across all ASBRs, making the internal OSPF cost the primary factor in exit selection, aligning with the principle of hot-potato routing—exiting the internal network as quickly as possible, even if it's not the shortest end-to-end path.
If external value is all the same in all ASBRs, routers will select the closest ASBR to exit. Weather OSPF uses E1 (compares external+internal) or E2 (compares external, then internal). Not sure why D insists on E2 but that's the only option where external metric is the same in all ASBRs.
Hot-potato routing is to follow the shortest exit path based on metrics
https://www.packetmischief.ca/2017/02/28/five-functional-facts-about-ospf/#:~:text=OSPF%20allows%20traffic%20engineering%20using,carry%20a%20type%201%20metric.
Why not D ? hot potato is to send out of the local AS as soon as possible without considering external metric. By all ASBR having the same metric for external route, internal router will select a 'nearest' ASBR to get out, this looks more like hot-potato then.
Think of a routing domain learning external routes. Typically those prefixes have some "external" metric associated with them - for example, E2 external metric or the BGP MED attribute value. If the routers in the local domain select the exit point based on the external metric they are said to perform "cold potato" routing. This means that the exit point is selected based on the external metric preference, e.g. distances to the prefix in the bordering routing system. This optimizes link utilization in the external system but may lead to suboptimal path selection in the local domain. Conversely, "hot potato" routing is the model where the exit point selection is performed based on the local metric to the exit point associated with the prefix. In other words, "hot potato" model tries to push packets out of the local system as quick as possible, optimizing internal link utilization.
https://ine.com/blog/2011-04-04-understanding-ospf-external-route-path-selection
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