OSPF Areas, Part 2, Normal and Stub Areas

Welcome back to our series on OSPF areas. Click here for Part 1 of the series. It is time to focus on normal areas and stub areas in this post. Recall our topology:


We have gone to R1 and created a prefix (11.11.11.0/24) using a loopback interface. We run RIP version 2 on this interface and redistribute this into OSPF Area 0. What should this create on R3 in Area 11 (a normal OSPF area)? That’s right  – a Type 5 LSA for an External prefix. Let us examine the OSPF database on R3 now and the accompanying IP routing table:

R3#show ip ospf database

            OSPF Router with ID (3.3.3.3) (Process ID 1)

  Router Link States (Area 11)

Link ID         ADV Router      Age         Seq#       Checksum Link count
2.2.2.2         2.2.2.2         1216        0x80000002 0x00023C 1
3.3.3.3         3.3.3.3         1215        0x80000002 0x00C075 1

  Net Link States (Area 11)

Link ID         ADV Router      Age         Seq#       Checksum
192.168.1.3     3.3.3.3         1215        0x80000001 0x003577

  Summary Net Link States (Area 11)

Link ID         ADV Router      Age         Seq#       Checksum
10.10.10.0      2.2.2.2         1281        0x80000001 0x0048C4
172.16.10.0     2.2.2.2         1241        0x80000001 0x00C79B

  Summary ASB Link States (Area 11)

Link ID         ADV Router      Age         Seq#       Checksum
1.1.1.1         2.2.2.2         449         0x80000001 0x0075B0

  Type-5 AS External Link States

Link ID         ADV Router      Age         Seq#       Checksum Tag
11.11.11.0      1.1.1.1         456         0x80000001 0x0075AB 0
R3#

R3#show ip route

Gateway of last resort is not set

     172.16.0.0/24 is subnetted, 1 subnets
O IA    172.16.10.0 [110/21] via 192.168.1.2, 00:24:41, FastEthernet0/0
     10.0.0.0/24 is subnetted, 1 subnets
O IA    10.10.10.0 [110/20] via 192.168.1.2, 00:24:41, FastEthernet0/0
     11.0.0.0/24 is subnetted, 1 subnets
O E2    11.11.11.0 [110/20000] via 192.168.1.2, 00:11:53, FastEthernet0/0
C    192.168.1.0/24 is directly connected, FastEthernet0/0
R3#

Sure enough, there is the Type 5 prefix in the normal area. And we cannot forget about the LSA Type 4 (Summary ASB Link State). This informs the OSPF domain of the location of the Autonomous System Boundary Router (ASBR). I am sure you have been noticing how some of the LSAs in the database do not translate directly into routing table entries. For example, the LSA Type 4. This is reminiscent of the EIGRP topology table. That protocol sure tries to act link state as well!
OK, well let us see what happens when we convert Area 11 into a STUB AREA. Remember, this is a simple configuration. All we need to do is go to ALL of the routers in the stub area (there can be many), and issue the router configuration command area 11 stub. Now that we have done that, let us examine the databases on R3.

R3#show ip ospf database

            OSPF Router with ID (3.3.3.3) (Process ID 1)

  Router Link States (Area 11)

Link ID         ADV Router      Age         Seq#       Checksum Link count
2.2.2.2         2.2.2.2         7           0x80000005 0x001A23 1
3.3.3.3         3.3.3.3         6           0x80000005 0x00D85C 1

  Net Link States (Area 11)

Link ID         ADV Router      Age         Seq#       Checksum
192.168.1.3     3.3.3.3         6           0x80000004 0x004D5E

  Summary Net Link States (Area 11)

Link ID         ADV Router      Age         Seq#       Checksum
0.0.0.0         2.2.2.2         33          0x80000001 0x0075C0
10.10.10.0      2.2.2.2         33          0x80000003 0x0062AA
172.16.10.0     2.2.2.2         33          0x80000003 0x00E181
R3#

R3#show ip route

Gateway of last resort is 192.168.1.2 to network 0.0.0.0

     172.16.0.0/24 is subnetted, 1 subnets
O IA    172.16.10.0 [110/21] via 192.168.1.2, 00:01:23, FastEthernet0/0
     10.0.0.0/24 is subnetted, 1 subnets
O IA    10.10.10.0 [110/20] via 192.168.1.2, 00:01:23, FastEthernet0/0
C    192.168.1.0/24 is directly connected, FastEthernet0/0
O*IA 0.0.0.0/0 [110/11] via 192.168.1.2, 00:01:23, FastEthernet0/0
R3#

Wow, things really changed here. Notice the Stub Area effect worked just as advertised in our Core Knowledge studies. The Type 4 and 5 LSAs were removed from the OSPF database! They were replaced with a “special” LSA Type 3. It is special because it is an automatically generated default route by the Area Border Router (ABR).
Join us in the next part of this blog series where we examine the next OSPF area type, the Totally Stubby Area.

TSHOOT: Troubleshooting EIGRP, Part 2

In the first part of this series, we subdivided the processes of EIGRP into four discrete steps, and detailed troubleshooting the first two. This is taken from the 5-Day CCNP bootcamp:

• Discovery of neighbors
• Exchange of topology information
• Best path selection
• Neighbor and topology table maintenance

Let us now discuss path selection and maintenance troubleshooting.
We should all remember that we can view the topology table of EIGRP with the command show ip eigrp topology. Here we can see the successor routes (these are the best routes that are placed in the routing table) and we can see the second best routes, the feasible successor routes. These feasible successor routes are the key to the lightening fast convergence that EIGRP can offer us. When a speaker loses its successor, it can quickly install a feasible successor route in its place.
We need to remember the important rule of feasible successors. The advertised distance of the proposed feasible successor must be less than the feasible distance of the current successor route. This is actually a loop prevention mechanism.

Another big gotcha when it comes to path selection in EIGRP is the configuration of variance to unequal cost load balance. I can remember fighting with this in an INE practice lab long ago when I was preparing for the exam. Something I had no idea of back then…in order to be considered for the unequal load balancing, the alternate paths must be feasible successors! Older editions of CCNP courses never thought to tell us that little nugget!
We should be careful when modifying bandwidth to effect path selection. Cisco gave us delay for this purpose. Modifying the bandwidth can starve EIGRP updates of bandwidth to use. Remember, by default, EIGRP will only use 50% of an interface’s bandwidth. We can control this with the command ip bandwidth percent eigrp.
For table maintenance, show ip eigrp topology is critical. Note that in this table, passive is what we want to see. Active indicates there is not a feasible successor and neighbors are being queried for an alternative path. SIA log messages indicate a Stuck in Active issue. Here the router is not receiving a reply to queries. The most common reasons this can occur:

• Bad link
• Congested link
• The query scope if too big (too many routers involved)
• Excessive redundancy is built into the network
• The router CPU is overloaded
• There is a shortage of memory on the router
• There are software defects

When it comes to table maintenance, another excellent troubleshooting command is show ip eigrp topology summary. This command displays the total number of routes in the topology table and the total number of queries the router is waiting on responses for. It also shows a quiescent interface field that shows which interface have no outstanding packets to be sent or acknowledged.
Some of our favorite EIGRP verification commands:

• show ip route eigrp
• show ip protocol
• show ip eigrp neighbor
• show ip eigrp topology
• show ip eigrp topology all-links
• show ip eigrp topology summary
• debug eigrp packet hello
• debug eigrp packet query reply