Problem: OSPF Not Installing External Routes in the Routing Table

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When OSPF redistributes any routes, whether connected, static, or from a different routing protocol, it generates a Type 5 LSA for those external routes. These Type 5 LSAs are flooded into every OSPF router, with the exception of those in stub and NSSA areas. Sometimes, the problem is that the external routes are in the OSPF database but are not being installed in the routing table.

The most common causes of this problem are as follows :

  • The forwarding address is not known through the intra-area or interarea route.

  • The ABR is not generating Type 4 summary LSAs.

OSPF Not Installing External Routes in the Routing Table ‚ Cause: Forwarding Address Is Not Known Through the Intra-Area or Interarea Route

When OSPF learns an external LSA, it makes sure that the forwarding address is known through an OSPF intra-area or interarea route before it installs it into the routing table. If the forwarding address is not know through an intra-area or interarea route, OSPF will not install the route in the routing table. This is in accordance with the RFC 2328 standard.

Figure 9-76 shows a network with the following specifications:

  • R3 is an ASBR that is redistributing RIP routes into OSPF.

  • R4 is running RIP with R3.

  • R4 is learning 200.200.200.0/24 through RIP.

  • R2 is running OSPF with R3.

  • R2 is the ABR.

Figure 9-76. OSPF Network Experiencing a Problem of External Routes Not Getting Installed in the Routing Table

graphics/09fig76.gif

Example 9-207 shows the output of show ip route for 200.200.200.0. This network resides in a RIP domain. Because RIP is being redistributed into OSPF on R3, all OSPF routers should see this router as OSPF external. However, R1 is not seeing this route in its routing table.

Example 9-207 R1 Is Missing RIP Route of 200.200.200.0 in Its Routing Table
 R1#  show ip route 200.200.200.0   % Network not in table  R1# 

Figure 9-77 shows the flowchart to follow to solve this problem.

Figure 9-77. Problem-Resolution Flowchart

graphics/09fig77.gif

Debugs and Verification

Example 9-208 shows the external LSA for router 200.200.200.0. The external LSA exists in the OSPF database, but the route still is not being installed in the routing table. Also, there is a forwarding address involved in this external LSA.

Example 9-208 External LSA Database for RIP Route 200.200.200.0
 R1#  show ip ospf database external 200.200.200.0  OSPF Router with ID (131.108.1.1) (Process ID 1)  Type-5 AS External Link States  LS age: 14   Options: (No TOS-capability, DC)   LS Type: AS External Link  Link State ID: 200.200.200.0 (External Network Number )  Advertising Router: 131.108.0.129   LS Seq Number: 80000001   Checksum: 0x88BE   Length: 36   Network Mask: /24         Metric Type: 2 (Larger than any link state path)         TOS: 0         Metric: 20  Forward Address: 131.108.0.4  External Route Tag: 0 

Example 9-209 shows that the route to the forwarding address is known through an OSPF external route.

Example 9-209 Route to the Forwarding Address Is Known Through an OSPF External Route
 R1#  show ip route 131.108.0.4   Routing entry for 131.108.0.0/26  Known via "ospf 1", distance 110, metric 20,  type extern 2  , forward metric 70   Redistributing via ospf 1   Last update from 131.108.1.2 on Serial0, 00: 00: 40 ago   Routing Descriptor Blocks:   * 131.108.1.2, from 131.108.0.129, 00: 00: 40 ago, via Serial0       Route metric is 20, traffic share count is 1 

Example 9-210 shows that the ABR is summarizing 131.108.0.0/24 with an area range com-mand, so the more specific intra-area routes are summarized into one route. This range summarizes all routes under the 131.108.0.0/16 range.

Example 9-210 R2 Summarizes the Intra-area Routes as 131.108.0.0/24
 R2# router ospf 1  network 131.108.1.0 0.0.0.255 area 0  network 131.108.0.0 0.0.0.255 area 2  area 2 range 131.108.0.0 255.255.255.0  

Example 9-211 shows that the ASBR is doing the redistribution from RIP into OSPF. It also shows that the connected networks in the range of 131.108.0.0/26 are being redistributed into OSPF because RIP owns those connected routes. This configuration redistributes 131.108.0.4/26, which is a connected interface on R3. This subnet covers the forwarding address that appeared in Example 9-209.

Example 9-211 R3 Redistributes RIP Routes in the 131.108.0.0/26 Range into OSPF
 R3#  router ospf 1    redistribute rip subnets    network 0.0.0.0 255.255.255.255 area 2   !   router rip   network 131.108.0.0  
Solution

R1 is seeing the forwarding address learned through OSPF external because R3 has redistribute connected under router ospf. This leaks a more specific route for the connected interfaces of R3. This also includes the forwarding address subnet, which is 131.108.0.4/26. Also, the intra-area route for this subnet is suppressed by R2 because R2 is summarizing 131.108.0.0/16. Because the more specific route always is preferred, R1 prefers the more specific external route of 131.108.0.4/26 over the less specific summarized route of 131.108.0.0/16.

This problem can be solved in two ways:

  • Do not summarize at the ABR.

  • Filter the connected subnet from being redistributed into OSPF at the ASBR.

To implement the first solution, go to the ABR and remove the area range command.

Example 9-212 shows the new configuration on ABR that solves this problem.

Example 9-212 Removing Summarization Capabilities at the ABR
 R2#  router ospf 1   network 131.108.1.0 0.0.0.255 area 0   network 131.108.0.0 0.0.0.255 area 2   no area 2 range 131.108.0.0 255.255.255.0  

To implement the second solution, go to the ASBR and add a filter to control redistributed routes.

Example 9-213 shows the new configurations that fix this problem. The filter actually prevents the route 131.108.0.0/26 from getting redistributed into OSPF as an external route because only 200.200.200.0 is permitted; all other routes are denied .

Example 9-213 Configuring the ASBR to Filter Connected Routes
 R3#  router ospf 1    redistribute rip subnets route-map no_connected    network 0.0.0.0 255.255.255.255 area 2   !   router rip   network 131.108.0.0   !    route-map no_connected permit 10     match ip address 1    !   access-list 1 permit 200.200.200.0 0.0.0.255  

Example 9-214 shows that after fixing this problem, R1 starts installing the routes in the routing table because the forwarding address is now known through an OSPF interarea route.

Example 9-214 Confirming That External Routes Are Being Installed Again on R1
 R1#  show ip route 200.200.200.0   Routing entry for 200.200.200.0/24   Known via "ospf 2", distance 110, metric 20, type extern 2, forward metric 128  Redistributing via ospf 2   Last update from 131.108.1.2 on Serial0.1, 00:47:24 ago   Routing Descriptor Blocks:   * 131.108.1.2, from 131.108.0.29, 00:47:24 ago, via Serial0.1       Route metric is 20, traffic share count is 1 

Also, Example 9-215 shows that the forwarding address is now known through OSPF interarea instead of OSPF external.

Example 9-215 Forwarding Address Is Now Known Through OSPF Interarea
 R1#  show ip route 131.108.0.4   Routing entry for 131.108.0.4/26   Known via "ospf 2", distance 110, metric 64, type inter area  Redistributing via ospf 2   Last update from 131.108.1.2 on Serial0.1, 00:50:25 ago   Routing Descriptor Blocks:   * 131.108.1.2, from 131.108.0.193, 00:50:25 ago, via Serial0.1      Route metric is 64, traffic share count is 1 

OSPF Not Installing External Routes in the Routing Table ‚ Cause: ABR Not Generating Type 4 Summary LSA

One of the functions of a Type 4 summary LSA is to announce the reachability of an ASBR to the other areas. This Type 4 LSA is not required if the ASBR exists in the same area.

The ASBR doesn't generate the Type 4 summary LSA if it's not connected to area 0. To generate a summary LSA of Type 3 or Type 4, a router must have a connection into area 0. As a result, the external routes will not be installed in the network. Chapter 8 covers Type 3 and Type 4 LSAs in greater detail.

Figure 9-78 shows a network in which R3 redistributes RIP routes into OSPF.

Figure 9-78. OSPF Network Experiencing External Route Installation Problem Because of Missing Type 4 Summary LSAs

graphics/09fig78.gif

Example 9-216 shows that R1 is not installing the external route 200.200.200.0/24 into the routing table.

Example 9-216 R1 Not Installing External Route 200.200.200.0
 R1#  show ip route 200.200.200.0   % Network not in table  R1# 

Figure 9-79 shows the flowchart to follow to solve this problem.

Figure 9-79. Problem-Resolution Flowchart

graphics/09fig79.gif

Debugs and Verification

Example 9-217 shows that the route exists in the external database.

Example 9-217 Confirming That Route 200.200.200.0 Exists in the External OSPF Database
 R1#  show ip ospf database external 200.200.200.0  OSPF Router with ID (131.108.2.1) (Process ID 1)  Type-5 AS External Link States  LS age: 199   Options: (No TOS-capability, DC)   LS Type: AS External Link  Link State ID: 200.200.200.0 (External Network Number )  Advertising Router: 131.108.3.3   LS Seq Number: 80000001   Checksum: 0x4B3A   Length: 36   Network Mask: /24         Metric Type: 2 (Larger than any link state path)         TOS: 0         Metric: 20         Forward Address: 0.0.0.0         External Route Tag: 0 

Example 9-218 shows the there is no Type 4 LSA for this route.

Example 9-218 No Type 4 LSA Exists for the External Route
 R1#  show ip ospf database asbr-summary 131.108.3.3  OSPF Router with ID (131.108.2.1) (Process ID 1) 

The next logical step is to go on the ABR and see if it is indeed an ABR. If it does not consider itself an ABR, it will not generate any summary LSA of Type 3 or Type 4. Example 9-219 shows that R2 is not identifying itself as an ABR in the output of show ip ospf. If R2 were an ABR, the output would display "It's an area border router."

Example 9-219 R2 Doesn't Acknowledge Itself as an ABR
 R2#  show ip ospf  Routing Process "ospf 1" with ID 131.108.2.2 Supports only single TOS(TOS0) routes SPF schedule delay 5 secs, Hold time between two SPFs 10 secs 
Solution

In this example, R2 doesn't generate the Type 4 summary LSAs because it is not connected to area 0. To generate a summary LSA of Type 3 or Type 4, a router must have a connection into area 0.

To solve this problem, connect R2 to area 0 either physically or virtually by creating a virtual link, as shown in Example 9-220. To read more about virtual links, refer to Chapter 8.

Example 9-220 Configuring Virtual Link Between R2 and R1
 R1#  router ospf 1    area 2 virtual-link 131.108.2.2   _____________________________________________________________________________________ R2#  router ospf 1   area 2 virtual-link 131.108.2.1  

By configuring a virtual link on R2, the router is now virtually connected to area 0; therefore, it now considers itself an ABR. Example 9-221 shows that after connecting R2 to area 0, the output of show ip ospf shows that it is an ABR. Compare this output to Example 9-219, in which the router doesn't consider itself as an ABR.

Example 9-221 Confirming That R2 Is Now Aware of Its ABR Status
 R2#  show ip ospf  Routing Process "ospf 1" with ID 131.108.2.2 Supports only single TOS(TOS0) routes  It is an area border router  SPF schedule delay 5 secs, Hold time between two SPFs 10 secs 

Now, get on R1 again and see if the Type 4 LSA is being received. Example 9-222 shows that after the configuration changes on R2, it has started generating Type 4 summary LSAs into area 3.

Example 9-222 R2 Now Generates Type 4 LSAs into Area 2, and R1 Receives It
 R1#  show ip ospf database asbr-summary  OSPF Router with ID (131.108.2.1) (Process ID 1)  Summary ASB Link States (Area 2)  LS age: 17   Options: (No TOS-capability, DC)   LS Type: Summary Links(AS Boundary Router)  Link State ID: 131.108.3.3 (AS Boundary Router address)   Advertising Router: 131.108.2.2  LS Seq Number: 80000001   Checksum: 0xE269   Length: 28   Network Mask: /0         TOS: 0  Metric: 64 

Because the Type 4 LSA now is being generated, R1 installs the external routes in its routing table. Example 9-223 shows that after fixing this problem, the external route 200.200.200.0/24 is in the routing table of R1.

Example 9-223 External Route 200.200.200.0 Is Now in R1's Routing Table
 R1#  show ip route 200.200.200.0   Routing entry for 200.200.200.0/24   Known via "ospf 2", distance 110, metric 20, type extern 2, forward metric 128  Redistributing via ospf 2   Last update from 131.108.2.2 on Serial0.1, 00:47:24 ago   Routing Descriptor Blocks:   * 131.108.2.2, from 131.108.3.3, 00:47:24 ago, via Serial0.1                   Route metric is 20, traffic share count is 1 
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Troubleshooting IP Routing Protocols
Troubleshooting IP Routing Protocols (CCIE Professional Development Series)
ISBN: 1587050196
EAN: 2147483647
Year: 2002
Pages: 260

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