Cisco OSPF Multi-Area Configuration & Summary Routes
This article is a complete step by step guide for OSPF Multi-Area Configuration & Summary Routes on Cisco Router. Also topology examples of multi-area OSPF v2 and OSPF v3 configuration are explained here. Following are some OSPF related articles:- OSPF Area
- OSPF Authentication Types
- OSPF DR BDR Election
- Advanced Single-Area OSPF Configurations On Cisco Router
Configuration of OSPF Multi-Area
The OSPF implementation can be single or multi-area. The type of OSPF implementation chosen depends on the specific requirements and the existing topology.To implement multi-area OSPF, four steps must be followed.
Steps 1 and 2 are part of the planning process.
- Step 1. Collect network parameters and requirements : this includes determining the number of host and network devices, the IP addressing scheme (if already implemented), the size of the domain and routing tables, the risk of changes in the topology and other characteristics of the network.
- Step 2. Define the OSPF parameters : Based on the information you collected in step 1, the network administrator must determine if the preferred implementation is single-area or multi-area OSPF. If multi-area OSPF is selected, the network administrator must consider the following:
- IP addressing plan : this governs the way in which OSPF can be implemented and how well the OSPF implementation could be scaled.
- OSPF Areas : The division of an OSPF network into areas decreases the size of the LSDB and limits the propagation of link state updates when the topology is modified. The routers that are going to fulfill the function of ABR and ASBR must be identified, as well as those that will carry out the summarization or redistribution.
- Network topology: this consists of links that connect the network equipment and that belong to different OSPF areas in a multi-area OSPF design. The network topology is important to determine the main and backup links.
- Step 3 . Configure the multi-area OSPF implementation according to the parameters.
- Step 4 . Verify the implementation of multi-area OSPF according to the parameters.
OSPF Configuration of Different Areas
In Figure 1, the multi-area OSPF reference topology is shown. In this example:
- R1 is an ABR because it has interfaces in area 1 and an interface in area 0.
- R2 is an internal backup router because all its interfaces are in area 0.
- R3 is an ABR because it has interfaces in area 2 and an interface in area 0.
No special commands are required to implement this OSPF network from various areas. A router simply becomes ABR when it has two network instructions in different areas .
OSPF v2 Multi-Area Configuration Example
As shown below, router ID 1.1.1.1 was assigned to R1. This example activates OSPF on the two LAN interfaces in area 1.The serial interface is configured as part of OSPF area 0. Since R2 has interfaces connected to two different areas, it is an ABR.
R1 (config) # router ospf 10
R1 (config-router) # router-id 1.1.1.1
R1 (config-router) # network 10.1.1.1 0.0.0.0 area 1
R1 (config-router) # network 10.1.2.1 0.0.0.0 area 1
R1 (config-router) # network 192.168.10.1 0.0.0.0 area 0
R1 (config-router) # end
R1 #
Multi-area OSPF is configured on R2 and the wildcard mask of the interface network address is used:
R2 (config) # router ospf 10
R2 (config-router) # router-id 2.2.2.2
R2 (config-router) # network 192.168.10.0 0.0.0.3 area 0
R2 (config-router) # network 192.168.10.4 0.0.0.3 area 0
R2 (config-router) # network 10.2.1.0 0.0.0.255 area 0
R2 (config-router) # end
R2 #
* Apr 19 18: 11: 04.029:% OSPF-5-ADJCHG: Process 10, Nbr 1.1.1.1 on
Serial0 / 0/0 from LOADING to FULL, Loading Done
R2 #
Then multi-area OSPF is configured on R3. Here the wildcard mask 0.0.0.0 is used for all networks.R3 (config) # router ospf 10
R3 (config-router) # router-id 3.3.3.3
R3 (config-router) # network 192.168.10.6 0.0.0.0 area 0
R3 (config-router) # network 192.168.1.1 0.0.0.0 area 2
R3 (config-router) # network 192.168.2.1 0.0.0.0 area 2
R3 (config-router) # end
Apr 19 18: 12: 55,881:% OSPF-5-ADJCHG: Process 10, Nbr 2.2.2.2 on
Serial0 / 0/1 from LOADING to FULL, Loading Done
OSPFv3 Configuration of various areas
As in OSPFv2, the implementation of the multi-area OSPFv3 topology of Image 2 is simple.
No special commands are required. A router becomes ABR when it has two interfaces in two different areas.
OSPFv3 Multi-Area Configuration Example
For example, in the scheme below, router ID 1.1.1.1 was assigned to R1. The example also enables OSPF on the two LAN interfaces in area 1 and on the serial interface in area 0. Since R1 has interfaces connected to two different areas, it is an ABR.R1 (config) # ipv6 router ospf 10 R1 (config-rtr) # router-id 1.1.1.1 R1 (config-rtr) # exit R1 (config) # R1 (config) # interface GigabitEthernet 0/0 R1 (config-if) # ipv6 ospf 10 area 1 R1 (config-if) # R1 (config-if) # interface Serial0 / 0/0 R1 (config-if) # ipv6 ospf 10 area 0 R1 (config-if) # end R1 #
For R2, we enter the OSPFv3 router configuration mode with process ID 10 and configure router ID 2.2.2.2.
R2 (config) # ipv6 router ospf 10
* Apr 24 14: 18: 10,463:% OSPFv3-4-NORTRID: Process OSPFv3-10-IPv6
could not pick a router-id, please configure manually
R2 (config-rtr) # router-id 2.2.2.2
R2 (config-rtr) # exit
Now, OSPFv3 is configured for process ID 10 on each of the interfaces:
R2 (config) # interface g0 / 0 R2 (config-if) # ipv6 ospf 10 area 0 R2 (config-if) # interface S0 / 0/0 R2 (config-if) # ipv6 ospf 10 area 0 R2 (config-if) # interface S0 / 0/1 R2 (config-if) # ipv6 ospf 10 area 0 R2 (config-if) # end * Apr 24 14: 18: 35.135:% OSPFv3-5-ADJCHG: Process 10, Nbr 1.1.1.1 on Serial0 / 0/0 from LOADING to FULL, Loading Done
For R3, we enter the OSPFv3 router configuration mode with process ID 10 and configure router ID 3.3.3.3.
R3 (config) # ipv6 router ospf 10 * Apr 24 14: 20: 42.463:% OSPFv3-4-NORTRID: Process OSPFv3-10-IPv6 could not pick a router-id, please configure manually R3 (config-rtr) # router-id 3.3.3.3 R3 (config-rtr) # exit
And finally, OSPFv3 is configured for process ID 10 in each of the interfaces:
R3 (config) # interface g0 / 0 R3 (config-if) # ipv6 ospf 10 area 2 R3 (config-if) # interface S0 / 0/1 R3 (config-if) # ipv6 ospf 10 area 0 R3 (config-if) # end * Apr 24 14: 21: 01.439:% OSPFv3-5-ADJCHG: Process 10, Nbr 2.2.2.2 on Serial0 / 0/1 from LOADING to FULL, Loading Done R3 #
OSPF Summary Routes
The summary helps to make the routing tables shorter. It involves consolidating several routes into a single ad, which then spreads to the backbone area.Typically, type 1 and type 2 LSAs are generated within each area , translated into type 3 LSAs and sent to other areas. If area 1 had thirty networks to announce, thirty Type 3 LSAs would be forwarded to the backbone network. With the route summary, the ABR consolidates the 30 networks into one of two advertisements.
In Image 3, R1 consolidates all network announcements in a summary LSA.
Instead of resending LSA individually for each route in area 1, R1 forwards a summary LSA to core router C1. C1, in turn, forwards the summary LSA to R2 and R3. R2 and R3 then forward it to their respective internal routers.
The summary also helps increase network stability, because it reduces unnecessary flooding of LSA.
This situation directly affects the amount of memory, CPU and bandwidth resources used by the OSPF routing process. Without a route summary, all specific link LSAs propagate on the OSPF backbone network and beyond, which generates unnecessary network traffic and router reload.
In Image, a network link on R1 fails. R1a sends an LSA to R1. However, R1 does not propagate the update, since it has a summary route configured. The saturation of specific links outside the area with LSA does not occur.
Summarization of external and inter-area routes
In OSPF, summarization can be configured only in ABRs or ASBRs. Instead of announcing many specific networks, ABR and ASBR routers announce a summary route. The ABR summary LSA type 3 routers and the ASBR summary LSA type 5 routers.By default, summary LSAs (type 3 LSAs) and external LSAs (type 5) do not contain summary (aggregate) routes; that is, by default, summary LSAs are not summarized.
As shown in above Images , route summarization can be configured as follows:
- Inter-area route summarization : inter-area route summarization occurs in the ABRs and is applied to routes within each area. It does not apply to external routes introduced in OSPF through redistribution.
To effectively summarize inter-area routes, network addresses must be assigned contiguously, so that these addresses can be summarized in a minimum number of summary addresses.
- Summarization of external routes : the summarization of external routes is specific to the external routes that are introduced in OSPF through the redistribution of routes. Again, it is important to ensure the contiguity of the external address ranges that are summarized.
Usually, only ASBRs summarize external routes. As shown in Image 6, ASBR R2 summarizes the external EIGRP routes in a single LSA and sends them to R1 and R3.
Note : In ASBRs, the summarization of external routes is configured using the summary-address address mask command of the router configuration mode.
Inter-area route summarization
OSPF does not perform automatic summarization. The inter-area summarization must be manually configured in the ABRs.Only ABRs can summarize internal routes. When summarization is enabled in an ABR, a single type 3 LSA describing the summary route is introduced into the backbone. This unique LSA summarizes several routes within the area.
A summary route is generated if at least one subnet within the area is within the range of summary addresses. The summary route metric equals the lowest cost of all subnets within the range of summary addresses.
In Image, a multi-area OSPF topology is shown. The routing tables of R1 and R3 are analyzed to see the effect of summarization.
In Image below, the routing table of R1 is shown before summarization is configured, while in next Image , the routing table of R3 is shown.
Calculation of the summary route
The figure shows that the summary of networks in a single address and mask can be done in three steps:
- Step 1 . List the networks in binary format. In the example, the two networks in area 1 (10.1.1.0/24 and 10.1.2.0/24) are indicated in binary format.
- Step 2 . Count the number of bits that match on the far left to determine the summary path mask. As highlighted, the first 22 digits of the far left match. This produces the prefix / 22 or subnet mask 255.255.252.0.
- Step 3 . Copy the matching bits and then add the zero bits to determine the summarized network address. In this example, the bits coinciding with leading zeros show us as a result the network address 10.1.0.0/22. This summary address brings together four networks: 10.1.0.0/24, 10.1.1.0/24, 10.1.2.0/24 and 10.1.3.0/24.
In the example, the summary address matches four networks although there are only two networks.
Inter-route summary configuration
In Image, to demonstrate the effect of route summarization, R1 is configured to summarize the internal routes of area 1.To manually configure inter-route route summarization in an ABR, use the area area-id range address mask command in router configuration mode. This instructs the ABR to summarize the routes for a specific area before introducing them into another area in the form of LSA type 3 summary through the backbone.
Note : In OSPFv3, the command is identical, except for the IPv6 network address. The command syntax for OSPFv3 is area area id range prefix / prefix-length .
The two internal routes of area 1 are summarized below in an OSPF inter-area summary route on R1. The summary route 10.1.0.0/22 summarizes four network addresses: from 10.1.0.0/24 to 10.1.3.0/24.
R1 (config) # router ospf 10
R1 (config-router) # area 1 range 10.1.0.0 255.255.252.0
R1 (config-router) #
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