Lab 25: OSPF Multiple Area Routing, Route Redistribution and Summarization-Part II

 <  Free Open Study  >  

OSPF Summarization

OSPF allows for two forms of summarization. One form of summarization is used when summarizing routes redistributed into OSPF from another routing protocol. The other form of summarization is used when summarizing an area. With both forms of summarization, summary LSAs are created and flooded toward Area 0, or the backbone area. The backbone area, in turn , floods the link states to the other areas. Use these guidelines when configuring summarization in OSPF:

  • Use continuous address space in each OSPF area. This allows for easy summarization on the ABRs. Summarizing many networks into a single advertisement decreases the route table and improves the overall performance and scalability of OSPF.

  • Summarize on major bit boundaries or simple octet boundaries whenever possible. If you have a classful routing protocol in your network, such as RIP or IGRP, you must summarize on bit boundaries that the classful routing protocol can receive.

  • You cannot summarize Area 0 or the backbone area. All summaries are flooded into Area 0 and then are flooded out from that point. Therefore, Area 0 routes cannot be summarized.

To summarize external routes, or routes redistributed into OSPF, use the following router command on the ASBR:

 Router(config-router)  summary-address   network_address network_mask  [  tag   tag_number  ] 

The tag parameter can be used in the same manner as it is during redistribution, to mark routes with a numeric number.

To summarize routes from one OSPF area into Area 0, use the following router command:

 Router(config-router)  area   area_id range network_address network_mask  

In Figure 12-10, we have modified the network once again. We have changed some interfaces and increased the networks in the RIP domain. OSPF Area 20 now contains the subnets 100.10.1.0/24 to 100.10.3.0/24. First, you will want to summarize these networks into a single route, 100.10.0.0/16. To accomplish this, you will use the router command area 20 range 100.10.0.0 255.255.0.0 on the ABR, or the router john. The router john will advertise the summary route 100.10.0.0/16 to mark. This is referred to as interarea summarization.

Figure 12-10. Summarizing OSPF

graphics/12fig10.gif

This network must use another form of interarea summarization for full IP connectivity to the RIP domain. The RIP Ethernet interface has an IP address on a 24-bit boundary. Recall from Chapter 9, "Distance Vector Protocols: Routing Information Protocol Versions 1 and 2 (RIP-1 and RIP-2)," that for RIP to receive a route, it must be on a natural bit boundary 8, 16, or 24, or on the same bit boundary that the interface receiving the route is on ”in this case, a 24-bit boundary. This is true for IGRP as well. All of the networks in the OSPF domain reside on a 24-bit boundary except for the Frame Relay point-to-point network between the routers john and luke. To accommodate RIP, you must summarize the Frame Relay point-to-point network at a 24-bit boundary. To summarize the 172.16.2.4/30 network into 172.16.2.0/24, use the area 100 range 172.16.2.0 255.255.255.0 command on the router john. The router john will now advertise the summary route 172.16.2.0/24 to mark and finally to peter. Example 12-24 lists the route table on the mark router.

Example 12-24 Route Table of the mark Router
 mark#  show ip route  Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area        N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2        E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP        i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area        * - candidate default, U - per-user static route, o - ODR        P - periodic downloaded static route Gateway of last resort is not set      100.0.0.0/16 is subnetted, 1 subnets O IA    100.10.0.0 [110/129] via 172.16.6.5, 00:26:45, Serial0.1      172.16.0.0/24 is subnetted, 6 subnets O IA    172.16.9.0 [110/74] via 172.16.6.5, 00:26:45, Serial0.1 C       172.16.6.0 is directly connected, Serial0.1 O IA    172.16.1.0 [110/128] via 172.16.6.5, 00:26:45, Serial0.1 O IA    172.16.2.0 [110/128] via 172.16.6.5, 00:26:45, Serial0.1 O IA    172.16.3.0 [110/134] via 172.16.6.5, 00:26:45, Serial0.1 C       172.16.100.0 is directly connected, Ethernet0      128.100.0.0/24 is subnetted, 2 subnets O E2    128.100.1.0 [110/20] via 172.16.6.5, 00:26:45, Serial0.1 O E2    128.100.2.0 [110/20] via 172.16.6.5, 00:26:45, Serial0.1 R    192.168.1.0/24 [120/1] via 172.16.100.8, 00:00:06, Ethernet0 R    192.168.2.0/24 [120/1] via 172.16.100.8, 00:00:07, Ethernet0 mark# 

The final summary that you want to perform is external summarization. On the mark router, you want to summarize the two RIP routes 192.168.1.0/24 and 192.168.2.0/24 into a single OSPF route 192.168.0.0/16. To perform this task, use the OSPF router command summary-address 192.168.0.0 255.255.0.0. Example 12-25 lists the routing protocol configurations of the john and mark routers, respectively.

Example 12-25 john and mark Configurations
  hostname mark   !   router ospf 7   router-id 192.168.250.6   summary-address 192.168.0.0 255.255.255.0   redistribute rip subnets   passive-interface Ethernet0   network 172.16.6.0 0.0.0.255 area 10   default-metric 10   !   router rip   redistribute ospf 7   passive-interface Serial0   network 172.16.0.0   default-metric 3   !  ____________________________________________________________________  hostname john   !   router ospf 7   router-id 192.168.250.250   area 7 stub   area 20 range 100.10.0.0 255.255.0.0   area 100 stub no-summary   area 100 range 172.16.2.0 255.255.255.0   network 172.16.1.0 0.0.0.255 area 20   network 172.16.2.4 0.0.0.3 area 100   network 172.16.6.0 0.0.0.255 area 10   network 172.16.9.0 0.0.0.255 area 0   !  

Example 12-26 lists the route table of paul, listing all the summary routes.

Example 12-26 Route Table of paul
 paul#  show ip route  <<<text omitted>>> Gateway of last resort is not set      100.0.0.0/16 is subnetted, 1 subnets O IA    100.10.0.0 [110/75] via 172.16.9.5, 1d22h, Ethernet0/0      172.16.0.0/24 is subnetted, 6 subnets C       172.16.9.0 is directly connected, Ethernet0/0 O IA    172.16.6.0 [110/74] via 172.16.9.5, 1d22h, Ethernet0/0 O IA    172.16.1.0 [110/74] via 172.16.9.5, 1d22h, Ethernet0/0 O IA    172.16.2.0 [110/74] via 172.16.9.5, 1d22h, Ethernet0/0 O IA    172.16.3.0 [110/80] via 172.16.9.5, 1d22h, Ethernet0/0 O E2    172.16.100.0 [110/10] via 172.16.9.5, 01:19:17, Ethernet0/0      128.100.0.0/24 is subnetted, 2 subnets C       128.100.1.0 is directly connected, Loopback20 C       128.100.2.0 is directly connected, Loopback21 O E2 192.168.0.0/16 [110/10] via 172.16.9.5, 00:01:17, Ethernet0/0 paul# 
 <  Free Open Study  >  


CCIE Practical Studies, Volume I
CCIE Practical Studies, Volume I
ISBN: 1587200023
EAN: 2147483647
Year: 2001
Pages: 283
Authors: Karl Solie

flylib.com © 2008-2017.
If you may any questions please contact us: flylib@qtcs.net