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Configuring OSPF Routing

Introduction to OSPF

Configuring OSPF routing in the MAX

Introduction to OSPF

RIP limitations solved by OSPF

Ascend implementation of OSPF

The MAX acts as an OSPF internal router with limited border router capability. At this release, Ascend does not recommend an Area Border Router (ABR) configuration for the MAX, so the Ethernet interface and all of the MAX WAN links should be configured in the same area.

The MAX does not function as a full AS Border Router (ASBR) at this release. However, it performs ASBR calculations for external routes such as WAN links that do not support OSPF. The MAX imports external routes into its OSPF database and flags them as Autonomous System External (ASE). It redistributes those routes by means of OSPF ASE advertisements, and propagates its OSPF routes to remote WAN routers that are running RIP.

The MAX supports null and simple password authentication.

OSPF features

An Autonomous System (AS) is a group of OSPF routers exchanging information, typically under the control of one company. An AS can include a large number of networks, all of which are assigned the same AS number. All information exchanged within the AS is interior.

Exterior protocols are used to exchange routing information between Autonomous Systems. The protocols are referred to by the acronym EGP (exterior gateway protocol). Border routers can use the AS number to filter out certain EGP routing information. OSPF can make use of EGP data generated by other border routers and added into the OSPF system as ASEs, and can also use static routes configured in the MAX or RADIUS.

Security

Support for variable length subnet masks

Interior gateway protocol (IGP)

The MAX requires an AS Border Router (ASBR) to use an external gateway protocol (EGP) for communicating with other autonomous systems, as shown in Figure 8-1. An EGP acts as a shuttle service between autonomous systems.

Figure 8-1. Autonomous system border routers

ASBRs perform calculations related to external routes. The MAX imports external routes from RIP (for example, when it establishes a WAN link with a caller that does not support OSPF) and always performs the ASBR calculations.

If you must prevent the MAX from performing ASBR calculations, you can disable them in Ethernet > Mod Config > OSPF Global Options.

Exchange of routing information

Figure 8-2. Adjacency between neighboring routers

An OSPF router dynamically detects its neighboring routers by sending Hello packets to the multicast address All SPFRouters. It then attempts to form adjacencies with some of its newly acquired neighbors.

Adjacency is a relationship formed between selected neighboring routers for the purpose of exchanging routing information. Not every pair of neighboring routers becomes adjacent. Adjacencies are established during network initialization in pairs, between two neighbors. As the adjacency is established, the neighbors exchange databases and build a consistent, synchronized database between them.

When an OSPF router detects a change on one of its interfaces, it modifies its topological database and multicasts the change to its adjacent neighbor, which in turn propagates the change to its adjacent neighbor until all routers within an area have synchronized topological databases. The result is quick convergence among routers. OSPF routes can also be summarized in Link-State Advertisements (LSAs).

Designated and backup designated routers

Figure 8-3. Designated and backup designated routers

The MAX can function as a Designated Router (DR) or Backup Designated Router (BDR). However, many sites choose to assign a LAN-based router for these roles in order to dedicate the MAX to WAN processing.

To reduce the number of adjacencies each router must form, OSPF calls one of the routers the designated router. A designated router is elected as routers are forming adjacencies, and then all other routers establish adjacencies only with the designated router. This simplifies the routing table update procedure and reduces the number of link-state records in the database. The designated router also plays other important roles in reduce the overhead of a OSPF link-state procedures. For example, other routers send link-state advertisements it to the designated router only by using the all-designated-routers multicast address of All SPFRouters.

To prevent the designated router from becoming a serious liability to the network if it fails, OSPF elects a backup designated router at the same time. Other routers maintain adjacencies with both the designated router and its backup router, but the backup router leaves as many of the processing tasks as possible to the designated router. If the designated router fails, the backup immediately becomes the designated router and a new backup is elected.

The administrator chooses which router is to be the designated router on the basis of the processing power, speed, and memory of the system, and then assigns priorities to other routers on the network in case the backup designated router is also down at the same time.

Configurable metrics

You can also use the OSPF cost for preferred path selection. If two paths to a destination have equal costs, you can assign a higher cost to one of the paths, to configure it as a backup to be used only when the primary path is not available.

Figure 8-4 shows how costs direct traffic over high-speed links. For example, if Router-2 in Figure 8-4 receives packets destined for Host B, it routes them through Router-1, across two T1 links (Cost=20), rather than across one 56Kbps B-channel to Router-3 (Cost=240).

Figure 8-4. OSPF costs for different types of links

The MAX has a default cost of one for a connected route (Ethernet) and ten for a WAN link. If you have two paths to the same destination, the MAX selects the one with the lower cost. You might want to account for the bandwidth of a connection when assigning costs. For example, for a single B-channel connection, the cost would be 24 times greater than for a T1 link.

Hierarchical routing (areas)

The backbone area is special and always has the area number 0.0.0.0. Other areas are assigned area numbers that are unique within the autonomous system.

Each areas acts like its own network. All area-specific routing information stays within the area, and all routers within an area must have a synchronized topological database. To tie the areas together, some routers belong to the backbone area and to another area. These routers are Area Border Routers (ABRs). In Figure 8-5, all of the routers are ABRs. If you set up the ABRs and area boundaries correctly, link-state databases are unique to an area.

Figure 8-5. Dividing an AS into areas

Stub areas

To prevent flooding of external routes throughout the AS, you can configure an area as a stub if the area has a single exit point or if the choice of exit point need not be made on a per-external-destination basis. You might need to specify a stub area with no default cost (StubNoDefault) if the area has more than one exit point.

In a stub area, routing to AS-external destinations is based on a per-area default cost. The per-area default cost is advertised to all routers within the stub area by a border router, and is used for all external destinations.

If the MAX supports external routes across its WAN links, you should not configure it in a stub area. Because an ABR configuration is not currently recommended for the MAX, the area in which it resides should not be a stub area if any of its links are AS-external.

Not So Stubby Areas (NSSAs)

NSSAs and Type-7 LSAs

• NSSAs can originate and import Type-7 LSAs; like stub areas, NSSAs cannot originate or import type-5 LSAs.

• Type-7 LSAs can only be advertised within a single NSSA; they are not flooded throughout the AS as are type-5 LSAs.

You must also specify whether these Type-7 LSAs should be advertised outside the NSSA. If you choose to advertise a Type-7 LSA, the NSSA Area Border Router (ABR) converts it to a Type-5 LSA, which can then be flooded throughout the AS. If you choose not to advertise a Type-7 LSA, it is not advertised beyond the NSSA.

(For complete information about NSSAs, see RFC 1587.)

Configuring the MAX as an NSSA internal router

• The area-type must be the same on all MAX interfaces running OSPF.

• The area ID (configured in the Area parameter) must be the same on all MAX interfaces running OSPF.

1. Select Ethernet > Mod Config > OSPF options.

2. Set AreaType to NSSA.

3. Exit and save the Mod Config profile.

4. Select Ethernet > Static Rtes > any Static Route profile.

5. Configure a static route to the destination outside the NSSA which include the following parameters (shown with sample settings):

   Ethernet
             Static Rtes
             any Static Rtes profile
                   Name=descriptivename
                   Active=Yes
                   Dest=20.20.20.20
                   Gateway=10.10.10.10
                   ...
   

                   ...
                   NSSA-ASE7=
   

   Note: Set the NSSA-ASE7 parameter to Advertise or to DoNotAdvertise to specify whether you want to advertise this route outside the NSSA.

                Metric=
                Preference=
                Private=
                Ospf-Cost=
                LSA-type=
                ....
                ASE-tag=
              Third-Party=

1. Exit and save the Static Rtes profile.

2. Reset the MAX.

OSPF routers exchange routing information and build link-state databases. Link-state databases are synchronized between pairs of adjacent routers (as described in Exchange of routing information). In addition, each OSPF router uses its link-state database to calculate a self-rooted tree of shortest paths to all destinations, as shown in Figure 8-6.

Figure 8-6. Sample network topology

The routers then use the trees to build their routing tables, as shown in Table 8-1.

Table 8-1. Link state databases for network topology in Figure 8-6

Router-1	Router-2	Router-3
Network-1/Cost 0	Network-2/Cost0	Network-3/Cost 0
Network-2/Cost 0	Network-3/Cost0	Network-4/Cost 0
Router-2/Cost 20	Router-1/Cost 20	Router-2/Cost 30
	Router-3/Cost 30

Table 8-2, Table 8-3, and Table 8-4 show another example of self-rooted shortest-path trees calculated from link-state databases, and the resulting routing tables. Actual routing tables also contain externally derived routing data, which is advertised throughout the AS but kept separate from the link-state data. Also, each external route can be tagged by the advertising router, enabling the passing of additional information between routers on the boundary of the AS.

Table 8-2. Shortest-path tree and resulting routing table for Router-1

	Destination	Next Hop	Metric
	Network-1	Direct	0
	Network-2	Direct	0
	Network-3	Router-2	20
	Network-4	Router-2	50

Table 8-3. Shortest-path tree and resulting routing table for Router-2

	Destination	Next Hop	Metric
	Network-1	Router-1	20
	Network-2	Direct	0
	Network-3	Direct	0
	Network-4	Router-2	30

Table 8-4. Shortest-path tree and resulting routing table for Router-3

	Destination	Next Hop	Metric
	Network-1	Router-2	50
	Network-2	Router-2	30
	Network-3	Direct	0
	Network-4	Direct	0

Configuring OSPF routing in the MAX

Ethernet
      Mod Config
            OSPF options...
                  RunOSPF=Yes
                  Area=0.0.0.0
                  AreaType=Normal
                  HelloInterval=10
                  DeadInterval=40
                  Priority=5
                  AuthType=Simple
                  AuthKey=ascend0
                  Cost=1
                  ASE-type=N/A
                  ASE-tag=N/A
                  TransitDelay=1
                  RetransmitInterval=5

            OSPF global options...
                  Enable ASBR=Yes

Ethernet
      Connections
            OSPF options
                  RunOSPF=Yes
                  Area=0.0.0.0
                  AreaType=Normal
                  HelloInterval=40
                  DeadInterval=120
                  Priority=5
                  AuthType=Simple
                  AuthKey=ascend0
                  Cost=10
                  ASE-type=N/A
                  ASE-tag=N/A
                  TransitDelay=5
                  RetransmitInterval=20
Ethernet
      Static Rtes
            any Static Rtes profile
                  LSA-type=ExternalType1

Notice that the same configuration parameters appear in Ethernet > Mod Config > OSPF Options and Ethernet > Connections > OSPF Options. The parameters are the same, but some of the default values are different. For OSPF routing, you configure the following parameters:

Parameter	Description
RunOSPF	OSPF is turned off by default. To enable it on the interface, set RunOSPF to Yes.
Area	Sets the area ID for the interface. The format for this ID is dotted decimal, but it is not an IP address. (For a description of areas, see Hierarchical routing (areas).)
AreaType	Specifies the type of area: Normal, Stub, or StubNoDefault. (For descriptions, see Stub areas.)
Intervals for communicating with an adjacent router	HelloInterval specifies how frequently, in seconds, the MAX sends out Hello packets on the specified interface. OSPF routers use Hello packets to dynamically detect neighboring routers in order to form adjacencies.
DeadInterval	Specifies how many seconds the MAX waits before declaring its neighboring routers down after it stops receiving their Hello packets. (For background information, see Exchange of routing information.)
Priority	Specifies a value the routers in the network use to elect a Designated Router (DR) and Backup Designated Router (BDR). Assigning a priority of 1 would place the MAX near the top of the list of possible designated routers. (Currently, you should assign a larger number.) Acting as a DR or BDR significantly increases the amount of OSPF overhead for the router. (For a discussion of the functions of DRs and BDRs, see Designated and backup designated routers.)
AuthType	Type of authentication supported. The Normal setting specifies that the MAX supports OSPF router authentication.
Auth Key	Specifies the key the MAX looks for in packets to support OSPF router authentication. (For more information, see Security.)
Cost	Specifies the link-state or output cost of a route. Assign realistic costs for each interface that supports OSPF. The lower the cost, the higher the likelihood of using that route to forward traffic. (For more information, see Configurable metrics.)
Autonomous System External (ASE) and their LSAs are used. ASE-Type ASE-Tag	Autonomous System External routes only when OSPF is turned off on a particular interface. When OSPF is enabled, these parameters are not applicable. ASE-Type specifies the type of metric that the MAX advertises for external routes. A Type-1 external metric is expressed in the same units as the link-state metric (the same units as interface cost). A Type-2 external metric is considered larger than any link- state path. Use of Type-2 external metrics assumes that routing between autonomous systems is the major cost of routing a packet, and eliminates the need for conversion of external costs to internal link-state metrics. ASE-Tag is a hexadecimal number used to tag external routes for filtering by other routers.
LSA-Type	Use LSAType to specify the OSPF ASE type of this link-state advertisement (LSA). Specify one of the following values: ExternalType-1-A type-1 external metric is expressed in the same units as the link-state metric (the same units as interface cost). The default is Type-1. ExternalType-2-Considered larger than any link state path. Use of type-2 external metrics assumes that routing between autonomous systems is the major cost of routing a packet, and eliminates the need for conversion of external costs to internal link-state metrics. Internal-Indicates that this static route should be advertised in an internal LSA. The MAX advertises the static route only if the Static Route gateway has a corresponding entry in a Connection profile. When you set LSA-type to Internal, the internal LSA static route appears as a stub area to external OSPF routers.
TransitDelay	Specifies the estimated number of seconds it takes to transmit a Link State Update Packet over this interface, taking into account transmission and propagation delays. On a connected route, you can leave the default of 1.
RetransmitInterval	Specifies the number of seconds between retransmissions of Link-State Advertisements, Database Description, and Link State Request Packets.
Enable ASBR	In the OSPF Global Options submenu, you set this parameter to enable or disable Autonomous System Border Routers (ASBRs) perform calculations related to external routes. The MAX imports external routes from RIP (for example, when it establishes a WAN link with a caller that does not support OSPF) and performs the ASBR calculations. If you must prevent the MAX from performing ASBR calculations, set Ethernet > Mod Config > OSPF Global Options > Enable ASBR to No.

Examples of configurations for adding the MAX to an OSPF network

Figure 8-7. Example of an OSPF setup

In Figure 8-7, all OSPF routers are in the same area (the backbone area), so the units all form adjacencies and synchronize their databases together.

Configuring OSPF on the Ethernet interface

Another way to configure the same units would be to create a second area (such as 0.0.0.1) in one of the existing OSPF routers, and add MAX-1 to that area. You could then assign the same area number (0.0.0.1) to all OSPF routers reached through the MAX across a WAN link.

After you configure MAX-1 as an IP host on that interface, you can configure it in the Ethernet profile as an OSPF router in the backbone area. To configure MAX-1 as an OSPF router on Ethernet:

1. Open Ethernet > Mod Config > Ether Options, and make sure the MAX is configured as an IP host. For example:

   Ethernet
         Mod Config
               Ether options...
                     IP Adrs=10.168.8.17/24
                     2nd Adrs=0.0.0.0
                     RIP=Off
                     Ignore Def Rt=Yes
                     Proxy Mode=Always
                     Filter=0
                     IPX Frame=N/A
   

1. Open Ethernet > Mod Config > OSPF Options and turn on RunOSPF:

               OSPF options...
                     RunOSPF=Yes
   

2. Specify the area number and area type for the Ethernet:

                     Area=0.0.0.0
                     AreaType=Normal
   

   In this case, the Ethernet is in the backbone area. (The backbone area number is always 0.0.0.0.) The backbone area is not a stub area, so leave the setting at its default. (For background information, see Stub areas.)

3. Leave the HelloInterval, DeadInterval, and Priority values set to their defaults:

                     HelloInterval=10
                     DeadInterval=40
                     Priority=5
   

4. If access to the backbone area requires authentication, specify the password. For example:

                     AuthType=Simple
                     AuthKey=ascend0
   

   If authentication is not required, set AuthType=None.

5. Configure the cost for the MAX to route into the backbone area. For example:

                     Cost=1
   

   Then type a number greater than zero and less than 16777215. By default the cost of an Ethernet-connected route is 1.

6. Set the expected transit delay for Link State Update packets. For example:

                     TransitDelay=1
   

7. Specify the retransmit interval for OSPF packets. For example:

                     RetransmitInterval=5
   

   This parameter specifies the number of seconds between retransmissions of Link-State Advertisements, Database Description and Link State Request Packets.

8. Close the Ethernet profile.

Configuring OSPF across the WAN

An OSPF WAN link has a default cost of ten. You can assign a higher cost to reflect a slower connection or a lower cost to set up a preferred route to a certain destination. If the cost of one route is lower than that of another to the same destination, the MAX does not select the higher-cost route unless route preferences change the equation.

OSPF on the WAN link is configured in a Connection profile. In this example, the MAX is connecting to another MAX unit across a T1 link (as in Figure 8-7 on page 8-13). To configure this interface:

1. Open the Connection profile for the remote MAX unit.

2. Turn on Route IP and configure the IP routing connection. For example:

   Ethernet
         Connections
               IP options
                     LAN Adrs=10.2.3.4/24
                     WAN Alias=0.0.0.0
                     IF Adrs=0.0.0.0
                     Metric=7
                     Preference=N/A
                     Private=No
                     RIP=Off
                   Pool=0
   

   (For detailed information, see Chapter 7, Configuring IP Routing.)

3. Open Connections > OSPF Options and turn on RunOSPF.

               OSPF options
                     RunOSPF=Yes
   

4. Specify the area number for the remote device and the area type.

   The area number must always be specified in dotted-quad format similar to an IP address. For example:

                     Area=0.0.0.0
                     AreaType=Normal
   

   You should use the same area number for the Ethernet interface of the MAX and each of its WAN links. In this example, the Ethernet interface is in the backbone area (0.0.0.0). You can use any area numbering scheme that is consistent throughout the AS and that uses this format.

5. Leave the HelloInterval, DeadInterval, and Priority values set to their defaults.

                     HelloInterval=40
                     DeadInterval=120
                     Priority=5
   

   Use the Priority value to configure the MAX as a DR or BDR.

6. If you require authentication to get into the backbone area, specify the password. For example:

                     AuthType=Simple
                     AuthKey=ascend0
   

   If you do not require authentication, set AuthType=None.

7. Configure the cost for the route to MAX-2.

   For example, for a T1 link the cost should be at least ten.

                     Cost=10
   

8. Close the Connection profile.

Configuring a WAN link that does not support OSPF

To enable OSPF to add the RIP-v2 routes to its routing table, configure RIP-v2 normally in this Connection profile. OSPF imports all RIP routes as Type-2 ASEs.

In this example, RIP is turned off on the link and ASE information is configured explicitly.

1. Open the Connection profile for the remote Pipeline unit.

2. Turn on Route IP and configure the IP routing connection. For example:

   Ethernet
         Connections
             any Connection profile
                 IP options
                     LAN Adrs=10.2.3.4/24
                     WAN Alias=0.0.0.0
                     IF Adrs=0.0.0.0
                     Metric=7
                     Preference=N/A
                     Private=No
                     RIP=Off
                     Pool=0
   

   (For detailed information, see Chapter 7, Configuring IP Routing.) Note that in a Connection profile, the OSPF Options subprofile includes two ASE parameters that are active only when OSPF is not running on a link. If you configure these parameters, the route configured in the Connection profile is advertised whenever the MAX is up.

3. Open the OSPF Options subprofile.

4. Leave RunOSPF set to No.

               OSPF options
                     RunOSPF=No
   

5. Configure the cost for the route to the remote Pipeline.

   For example, a single-channel BRI link could have a cost approximately 24 times the cost of a dedicated T1 link:

                     Cost=240
   

6. Specify the ASE type for this route.

                     ASE-type=Type 2
   

   This parameter specifies the type of metric to be advertised for an external route.

   A Type-1 external metric is expressed in the same units as the link state metric (the same units as interface cost). Type-1 is the default.

   A Type-2 external metric is considered larger than any link-state path. Use of Type- 2 external metrics assumes that routing outside the AS is the major cost of routing a packet, and eliminates the need for conversion of external costs to internal link-state metrics.

7. Enter an ASE tag for this route.

   The ASE tag is a hexadecimal number that shows up in management utilities and flags this route as external. It can also be used by border routers to filter this record. For example:

                     ASE-tag=cfff8000
   

8. Close the Connection profile.

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Copyright © 1998, Ascend Communications, Inc. All rights reserved.