Which of the following are true of both OSPFv2 and OSPFv3? (Select 2 choices.)
A. Both support IPv4.
B. Both support IPv6.
C. Both elect the BDR before electing the DR.
D. Both use MD5 for secure communications.
E. Both allow multiple OSPF instances to run on a link.
Explanation:
Open Shortest Path First version 2 (OSPFv2) and OSPFv3 both support IPv4, and both elect the backup designated router (BDR) before electing the designated router (DR). OSPFv3, which is described in Request for Comments (RFC) 2740, was developed as an enhancement to OSPFv2, which supports only IPv4. An OSPFv3 instance can support either IPv4 or IPv6, but it cannot support both. However, you can run multiple OSPFv3 instances on a single link. You can issue the ospfv3 processid area areaid {ipv4 | ipv6} [instance instanceid] command to enable OSPFv3 on an interface for a particular address family.
The DR and BDR election process for OSPFv3 multiaccess segments ishandled the same way as it is handled in OSPFv2: the BDR is elected first, and then the DR is elected. The router with the highest priority, as long as it has not already declared itself as the DR, becomes the BDR. Of those routers that have declared themselves as the DR, the router with the highest priority is elected to become the DR. If priority values are equal, the router with the highest router ID is elected. To change the OSPF priority of a router, you should issue the ip ospf priority value command, where value is an integer from 0 through 255. The default OSPF priority is 1, and a router with an OSPF priority of 0 will never be elected the DR or BDR.
Unlike OSPFv2, OSPFv3 allows multiple OSPF instances to run on a link. To keep track of each instance, OSPFv3 includes an instance ID field in the packet header. If no instance ID is specified on a link, the default value of 0 is used. When a router receives an OSPFv3 packet, it checks the instance ID in the packet header. If the instance ID in the header does not match the instance ID on the receiving interface, the router discards the packet even if the packet has a matching area ID.
OSPFv2 uses Message Digest 5 (MD5) for secure communications. OSPFv3 cannot use MD5; instead, OSPFv3 uses IP Security (IPSec) for secure communications.
Reference:
Cisco: Implementing OSPF for IPv6
IETF: RFC 2328: OSPF Version 2
IETF: RFC 2740: OSPF for IPv6Cisco: OSPFv3 IPSec ESP Encryption and Authentication