OSPF Questions

Quick OSPF Overview OSPF router ID selection: 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. OSPF forms neighbor relationship with other OSPF routers on the same segment by exchanging hello packets. The hello packets contain various parameters. Some of them should match between neighboring routers. These include: + Hello and Dead intervals + Area ID + Authentication type and password + Stub Area flag + Subnet ID and Subnet mask When OSPF neighbor relationship is formed, a router goes through several state changes before it becomes fully adjacent with its neighbor. The states are Down -> Attempt (optional) -> Init -> 2-Way -> Exstart -> Exchange -> Loading -> Full. Short descriptions about these states are listed below: Down: no information (hellos) has been received from this neighbor Attempt: only valid for manually configured neighbors in an NBMA environment. In Attempt state, the router sends unicast hello packets every poll interval to the neighbor, from which hellos have not been received within the dead interval Init: specifies that the router has received a hello packet from its neighbor, but the receiving router’s ID was not included in the hello packet 2-Way: indicates bi-directional communication has been established between two routers Exstart: Once the DR and BDR are elected, the actual process of exchanging link state information can start between the routers and their DR and BDR Exchange: OSPF routers exchange and compare database descriptor (DBD) packets Loading: In this state, the actual exchange of link state information occurs. Outdated or missing entries are also requested to be resent Full: routers are fully adjacent with each other When OSPF is run on a network, two important events happen before routing information is exchanged: + Neighbors are discovered using multicast hello packets. + DR and BDR are elected for every multi-access network to optimize the adjacency building process. All the routers in that segment should be able to communicate directly with the DR and BDR for proper adjacency (in the case of a point-to-point network, DR and BDR are not necessary since there are only two routers in the segment, and hence the election does not take place). For a successful neighbor discovery on a segment, the network must allow broadcasts or multicast packets to be sent. In an NBMA network topology, which is inherently nonbroadcast, neighbors are not discovered automatically. OSPF tries to elect a DR and a BDR due to the multi-access nature of the network, but the election fails since neighbors are not discovered. Neighbors must be configured manually to overcome these problems Each OSPF area only allows some specific LSAs to pass through. Below is a summarization of which LSAs are allowed in each OSPF area: Area Restriction Normal None Stub No Type 5 AS-external LSA allowed Totally Stub No Type 3, 4 or 5 LSAs allowed except the default summary route NSSA No Type 5 AS-external LSAs allowed, but Type 7 LSAs that convert to Type 5 at the NSSA ABR can traverse NSSA Totally Stub No Type 3, 4 or 5 LSAs except the default summary route, but Type 7 LSAs that convert to Type 5 at the NSSA ABR are allowed OSPF Summarization OSPF offers two methods of route summarization: 1) Summarization of internal routes performed on the ABRs 2) Summarization of external routes performed on the ASBRs 1) To summarize routes at the area boundary (ABRs), use the command: area area-id range ip-address mask [advertise | not-advertise] [cost cost] An internal summary route is generated if at least one subnet within the area falls in the summary address range and the summarized route metric is equal to the lowest cost of all the subnets within the summary address range. Interarea summarization can only be done for the intra-area routes of connected areas, and the ABR creates a route to Null0 to avoid loops in the absence of more specific routes. 2) To summarize external routes on the domain boundary (ASBRs), use the command: summary-address {{ip-address mask} | {prefix mask}} [not-advertise] [tag tag] The ASBR will summarize external routes before injecting them into the OSPF domain as type 5 external LSAs. Note: An exception of using the “summary-address” is at the boundary of a NSSA area. In both methods of route summarization described above, a summarized route is only generated if at least one subnet in the routing table falls in the summary address range.

Question 1

Explanation

LSA Type 7 is generated by an ASBR inside a Not So Stubby Area (NSSA) to describe routes redistributed into the NSSA. LSA 7 is translated into LSA 5 as it leaves the NSSA. These routes appear as N1 or N2 in the routing table inside the NSSA. Much like LSA 5, N2 is a static cost while N1 is a cumulative cost that includes the cost upto the ASBR -> LSA Type 7 only exists in an NSSA area.

Question 2

Question 3

Explanation

Answer B is not correct because using “passive-interface” command on ASW1 & ASW2 does not prevent DSW1 & DSW2 from sending routing updates to two access layer switches.

Question 4

Explanation

From the output above, we see the following LSAs:

+ Router Link States (Area 0): LSA Type 1 (Area 0)
+ Net Link States (Area 0): LSA Type 2 (Area 0)
+ Summary Net Link States (Area 0): LSA Type 3 (Area 0)
+ Router link States (Area 4): LSA Type 1 (Area 4)
+ Net Link States (Area 4): LSA Type 2 (Area 4)
+ Summary Net Link States (Area 4): LSA Type 3 (Area 4)

There are two areas represented on this router, which are Area 0 & Area 4. So we conclude this is an ABR router.

Just for your information, from the Router Link States (Area 0) part, we only see one entry 15.15.15.33. It is both the Link ID and ADV Router so we can conclude this is an IP address of one of the interfaces on the local router.

Question 5

Question 6

Questions 7

Explanation

When OSPF is run on a network, two important events happen before routing information is exchanged:
+ Neighbors are discovered using multicast hello packets.
+ DR and BDR are elected for every multi-access network to optimize the adjacency building process. All the routers in that segment should be able to communicate directly with the DR and BDR for proper adjacency (in the case of a point-to-point network, DR and BDR are not necessary since there are only two routers in the segment, and hence the election does not take place).
For a successful neighbor discovery on a segment, the network must allow broadcasts or multicast packets to be sent.

In an NBMA network topology, which is inherently nonbroadcast, neighbors are not discovered automatically. OSPF tries to elect a DR and a BDR due to the multi-access nature of the network, but the election fails since neighbors are not discovered. Neighbors must be configured manually to overcome these problems -> C is not correct while D is correct.

In Point-to-Multipoint network: This is a collection of point-to-point links between various devices on a segment. These networks also allow broadcast or multicast packets to be sent over the network. These networks can represent the multi-access segment as multiple point-to-point links that connect all the devices on the segment. -> A is correct.

Question 8

Explanation

OSPF forms neighbor relationship with other OSPF routers on the same segment by exchanging hello packets. The hello packets contain various parameters. Some of them should match between neighboring routers. These include:

+ Hello and Dead intervals
+ Area ID
+ Authentication type and password
+ Stub Area flag
+ Subnet ID and Subnet mask

So there are three correct answers in this question. Maybe in the exam you will see only two correct answers.

Question 9

Explanation

Let’s have a quick review of LSAs Type 4 & 5:

Summary ASBR LSA (Type 4) – Generated by the ABR to describe an ASBR to routers in other areas so that routers in other areas know how to get to external routes through that ASBR. For example, suppose R8 is redistributing external route (EIGRP, RIP…) to R3. This makes R3 an Autonomous System Boundary Router (ASBR). When R2 (which is an ABR) receives this LSA Type 1 update, R2 will create LSA Type 4 and flood into Area 0 to inform them how to reach R3. When R5 receives this LSA it also floods into Area 2.

In the above example, the only ASBR belongs to area 1 so the two ABRs send LSA Type 4 to area 0 & area 2 (not vice versa). This is an indication of the existence of the ASBR in area 1.

Note:
+ Type 4 LSAs contain the router ID of the ASBR.
+ There are no LSA Type 4 injected into Area 1 because every router inside area 1 knows how to reach R3. R3 only uses LSA Type 1 to inform R2 about R8 and inform R2 that R3 is an ASBR.

External Link LSA (LSA 5) – Generated by ASBR to describe routes redistributed into the area and point the destination for these external routes to the ASBR. These routes appear as O E1 or O E2 in the routing table. In the topology below, R3 generates LSAs Type 5 to describe the external routes redistributed from R8 and floods them to all other routers and tell them “hey, if you want to reach these external routes, send your packets to me!”. But other routers will ask “how can I reach you? You didn’t tell me where you are in your LSA Type 5!”. And that is what LSA Type 4 do – tell other routers in other areas where the ASBR is!

Each OSPF area only allows some specific LSAs to pass through. Below is a summarization of which LSAs are allowed in each OSPF area:

Area	Restriction
Normal	None
Stub	No Type 5 AS-external LSA allowed
Totally Stub	No Type 3, 4 or 5 LSAs allowed except the default summary route
NSSA	No Type 5 AS-external LSAs allowed, but Type 7 LSAs that convert to Type 5 at the NSSA ABR can traverse
NSSA Totally Stub	No Type 3, 4 or 5 LSAs except the default summary route, but Type 7 LSAs that convert to Type 5 at the NSSA ABR are allowed

Reference: http://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/13703-8.html

Therefore there are two OSPF areas that prevent LSAs Type 4 & 5: Totally Stub & NSSA Totally Stub areas