A small enterprise connects its office to two ISPs, using separate T1 links.

————T1————[ISP1]——>To Internet
|
[R1]
|
————T1————[ISP2]——>To Internet

A small enterprise connects its office to two ISPs, using separate T1 links.
A static route is used for the default route, pointing to both interfaces with a different administrative distance, so that one of the default routes is preferred.
Recently the primary link has been upgraded to a new 10 Mb/s Ethernet link. After a few weeks, they experienced a failure. The link did not pass traffic, but the primary static route remained active. They lost their Internet connectivity, even though the backup link was operating.
Which two possible solutions can be implemented to avoid this situation in the future? (Choose two.)

A. Implement HSRP link tracking on the branch router R1.
B. Use a track object with an IP SLA probe for the static route on R1.
C. Track the link state of the Ethernet link using a track object on R1.
D. Use a routing protocol between R1 and the upstream ISP.

A long description tells us many many things but the only things we need to think about are”multihome environment to the internet”, “need to failover when primary link is down”.

It says primary static route, which means they use two static routes with priority value.

Static routes will disappear only when there is no nexthop reachability or object tracking is explicitly take away a certain static route.

We sometimes use A and C at the same time. In this case, they failed to failover the links to the ISP because they could not detect the failures of ISP equipments. A and C says they still use a local link failover protocol.

B is talking about IP SLA probe. IP SLA probe checks and detects an icmp reachability to the previously implemented target. When it detects the reachability failure, they send the notification to the object tracking, then static route would be disappear.

With B technique, we can detect the failure over local link, even in ISP equipments. We have to care about the responder of an icmp echo. It could be a PE router or Provider Core Router depends on if ISP allows.

D is talking about to stop using static route between R1 and ISP, but to use dynamic routing protocol such as BGP.

Pocket

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