Cisco 350-501 Questions Answers

In an EVPN operation, the PE (Provider Edge) router determines and advertises Ethernet segment reachability by discovering remote ESIs (Ethernet Segment Identifiers) and determining their redundancy mode (all-active or single-active). This is crucial in multihoming scenarios where the PE needs to auto-discover remote PEs and the corresponding redundancy mode. In case of segment failures, PEs withdraw the routes used at this stage to trigger fast convergence by signaling a MAC mass withdrawal on remote PEs3.

The ttl-security hops 2 command sets the accepted Time to Live (TTL) value for BGP packets received from a neighbor. It is used to protect against BGP session hijacking by ensuring that BGP packets come from an expected number of router hops away. In this case, the command specifies that BGP packets from neighbor 2.2.2.2 must have a TTL of at least 253, which means they can only be 1 hop away (255 - 2 = 253) to be accepted.

 The issue is that the R2 Serial 1/0 interface is configured with an IP address, but the R1 Serial 1/0 interface is unnumbered. In OSPF, for two routers to form a neighbor relationship, they must be able to communicate on the same subnet. If one interface is unnumbered and the other has an IP address, they won’t be able to form a neighbor relationship, leading to OSPF prefixes missing from the routing table. References: Implementing and Operating Cisco Service Provider Network Core Technologies (SPCOR) - OSPF Configuration Guide