OSPF Questions

Note: If you are not sure about OSPF, please read our OSPF Tutorial.

Quick OSPF review – OSPF is a link-state routing protocol, which runs Dijkstra algorithm to calculate the shortest path. – Link-state routing protocol like OSPF has a common view of entire topology – OSPF only uses bandwidth to calculate metric. – Area 0 is called the backbone area In order to become OSPF neighbor, the following values must be matched on both routers: + Area ID + Authentication + Hello and Dead Intervals + Stub area Flag + MTU Size – Two options for OSPF authentication: + Plain text authentication + MD5 authentication – Hello packets of OSPF are multicast to 224.0.0.5 – “Broadcast” is the default network type for an OSPF enabled Ethernet interface (while Point-to-Point is the default OSPF network type for Serial interface with HDLC and PPP encapsulation). By default, the timers on a broadcast network (Ethernet, point-to-point and point-to-multipoint) are 10 seconds hello and 40 seconds dead. The timers on a non-broadcast network are 30 seconds hello 120 seconds dead. OSPF uses the following criteria to select the router ID: 1. Manual configuration of the router ID (via the “router-id x.x.x.x” command under OSPF router configuration mode). 2. Highest IP address on a loopback interface. 3. Highest IP address on a non-loopback and active (no shutdown) interface. The formula to calculate OSPF cost is: Cost = 108 / Bandwidth

Question 1

Question 2

Question 3

Explanation

The Broadcast network type is the default for an OSPF enabled ethernet interface (while Point-to-Point is the default OSPF network type for Serial interface with HDLC and PPP encapsulation).

Reference: https://www.oreilly.com/library/view/cisco-ios-cookbook/0596527225/ch08s15.html

Question 4

Question 5

Question 6

Explanation

Router2 does not have an entry for the subnet 10.10.13.128/25. It only has an entry for 10.10.13.0/25, which ranges from 10.10.13.0 to 10.10.13.127.

Question 7

Explanation

The default OSPF network type for HDLC and PPP on Serial link is point-to-point (while the default OSPF network type for Ethernet link is Broadcast).

Question 8

Explanation

OSPF uses the following criteria to select the router ID:
1. Manual configuration of the router ID (via the “router-id x.x.x.x” command under OSPF router configuration mode).
2. Highest IP address on a loopback interface.
3. Highest IP address on a non-loopback and active (no shutdown) interface.

Question 9

Explanation

From the output we can see there are Designated Router & Backup Designated Router for this OSPF domain so this is a broadcast network (point-to-point and point-to-multipoint networks do not elect DR & BDR) -> Answer B is not correct.

By default, the timers on a broadcast network (Ethernet, point-to-point and point-to-multipoint) are 10 seconds hello and 40 seconds dead (therefore answer C is correct). The timers on a non-broadcast network are 30 seconds hello 120 seconds dead.

From the line “Neighbor Count is 3”, we learn there are four OSPF routers in this OSPF domain -> Answer D is not correct.

Question 10

Explanation

With the same route (prefix), the router will choose the routing protocol with lowest Administrative Distance (AD) to install into the routing table. The AD of Internal EIGRP (90) is lowest so it would be chosen. The table below lists the ADs of popular routing protocols.

Note: The AD of IS-IS is 115. The “EIGRP” in the table above is “Internal EIGRP”. The AD of “External EIGRP” is 170. An EIGRP external route is a route that was redistributed into EIGRP.

Question 11

Explanation

The default-information originate command advertises a default route into a normal area, provided the advertising router already has a default route. But in the routing table of R1 we don’t see such a default route. Therefore we have to configure one.

We cannot only add the “always” keyword to the command “default-information originate” on R1 because R2 still does not know how to forward traffic received from R1.

Question 12

Explanation

From the output we can deduce interface Gi0/0 on R1 is the interface connecting to Router2 so interface Gi0/1 is the interface connecting to the new MPLS AS 1000.

The AD of eBGP (20) is smaller than that of OSPF (110) so the route to 10.10.13.0/25 will be updated as being learned from the new BGP path.

Question 13

Explanation

The destination of 10.10.13.165 matches the entry “O 10.10.13.160/29 [110/2]…” because of the longest prefix length rule so the packet will be forwarded to 10.10.10.5, which is Router3.