JN0-351: Enterprise Routing and Switching, Specialist (JNCIS-ENT)

Junos OS supports both narrow and wide IS-IS metric encodings and sends both TLV pairs by default. In the normal metric mode, the maximum supported metric value is 63; configuring wide-metrics-only enables the wider metric range.

Martian routes identify invalid host or network destination prefixes whose routing information is ignored, so they are not installed as usable routes. Junos OS also permits additional destination prefixes to be configured as martian addresses. They are not limited to publicly used prefixes and are not necessarily host addresses.

A static default route with ISP1 as its ordinary next hop and ISP2 and ISP3 as qualified next hops provides primary/backup forwarding. Junos selects the next hop with the lowest preference; the static route’s default preference is 5, so qualified next hops assigned preferences of 6 or higher are used only when the ISP1 path is no longer usable. The resulting static default route can be advertised to internal OSPF neighbors.

GRE tunnel establishment depends on IP reachability between the tunnel source and destination endpoints. Confirming that the tunnel endpoints can reach each other is the first diagnostic step.

The log action in the first term logs frames matching source MAC address 88:05:00:29:3c:de/48; with no explicit terminating action, that term has an implicit accept. The second term has no from statement, so it matches all remaining traffic and explicitly accepts it. Thus the specified source-MAC frames are accepted and logged, and all traffic is permitted. Junos makes from optional; omitting it matches all packets or frames.