[Top][Contents][Prev][Next][Last]Search

Configuring OSPF Routing

This chapter covers these topics:
Introduction to OSPF
Configuring OSPF routing in the MAX
Administering OSPF

Introduction to OSPF

OSPF (Open Shortest Path First) is the next generation Internet routing protocol. The Open in its name refers to the fact that OSPF was developed in the public domain as an open specification. The Shortest Path First refers to an algorithm developed by Dijkstra in 1978 for building a self-rooted shortest-path tree from which routing tables can be derived. This algorithm is described in The link-state routing algorithm.

Note: If you have a MAX running Multiband Simulation, ospf is disabled.

RIP limitations solved by OSPF

The rapid growth of the Internet has pushed RIP (Routing Information Protocol) beyond its capabilities, especially because of the following problems:

Problem

Description and solution
Distance-vector metrics

 RIP is a distance-vector protocol, which uses a hop count to select the shortest route to a destination network. RIP always uses the lowest hop count, regardless of the speed or reliability of a link.

OSPF is a link-state protocol, which means that OSPF can take into account a variety of link conditions, such as the reliability or speed of the link, when determining the best path to a destination network.

Note: You can configure the DownMetric and DownPreference parameters to assign different metrics or preferences to routes on the basis of whether the route is in use or is down. You might want to direct the MAX to use active routes, if available, rather than choose routes that are down.

15-hop limitation

 A destination that requires more than 15 consecutive hops is considered unreachable, which inhibits the maximum size of a network.

OSPF has no hop limitation. You can add as many routers to a network as you want.

Excessive routing traffic and slow convergence

 RIP creates a routing table and then propagates it throughout the internet of routers, hop by hop. The time it takes for all routers to receive information about a topology change is called convergence. A slow convergence can result in routing loops and errors.

A RIP router broadcasts its entire routing table every 30 seconds. On a 15-hop network, convergence can be as high as 7.5 minutes. In addition, a large table can require multiple broadcasts for each update, which consumes a lot of bandwidth.

OSPF uses a topological database of the network and propagates only changes to the database (as described in Exchange of routing information).

Ascend implementation of OSPF

The primary goal of OSPF at this release is to allow the MAX to communicate with other routers within a single autonomous system (AS).

The MAX acts as an OSPF internal router with limited border router capability. At this release, we do 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, ASBR calculations are performed 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 ASE (autonomous system external). It redistributes those routes via OSPF ASE advertisements, and propagates its OSPF routes to remote WAN routers running RIP.

The MAX supports null and simple password authentication.

OSPF features

This section provides a brief overview of OSPF routing to help you configure the MAX properly. For full details about how OSPF works, see RFC 1583, "OSPF Version 2", 03/23/1994, J. Moy.

An AS (autonomous system) 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. They are referred to by the acronym EGP (exterior gateway protocol). The AS number may be used by border routers 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, as well as static routes configured in the MAX or RADIUS.

Security

All OSPF protocol exchanges are authenticated. This means that only trusted routers can participate in the AS's routing. A variety of authentication schemes can be used; in fact, different authentication types can be configured for each area. In addition, authentication provides added security for the routers that are on the network. Routers that do not have the password will not be able to gain access to the routing information, because authentication failure prevents a router from forming adjacencies.

Support for variable length subnet masks

OSPF enables the flexible configuration of IP subnets. Each route distributed by OSPF has a destination and mask. Two different subnets of the same IP network number may have different sizes (different masks). This is commonly referred to as variable length subnet masks (VLSM), or Classless Inter-Domain Routing (CIDR). A packet is routed to the best (longest or most specific) match. Host routes are considered to be subnets whose masks are "all ones" (0XFFFFFFFF).

Note: Although OSPF is very useful for networks that use VLSM, we recommend that you attempt to assign subnets that are as contiguous as possible in order to prevent excessive link-state calculations by all OSPF routers on the network.

Interior gateway protocol (IGP)

OSPF keeps all AS-internal routing information within that AS. All information exchanged within the AS is interior.

An AS border router (ASBR) is required to communicate with other autonomous systems by using an external gateway protocol (EGP), as shown in Figure 11-1. An EGP acts as a shuttle service between autonomous systems.

Figure 11-1. 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 the ASBR calculations are always performed.

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

OSPF uses a topological database of the network and propagates only changes to the database. Part of the SPF algorithm involves acquiring neighbors, and then forming an adjacency with one neighbor, as shown in Figure 11-2.

Figure 11-2. Adjacency between neighboring routers

An OSPF router dynamically detects its neighboring routers by sending its Hello packets to the multicast address All SPFRouters. It 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 become 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. This results in quick convergence among routers. OSPF routes can also be summarized in link-state advertisements (LSAs).

Designated and backup designated routers

In OSPF terminology, a broadcast network is any network that has more than two OSPF routers attached and supports the capability to address a single physical message to all of the attached routers.

Figure 11-3. 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. The administrator chooses a DR and BDR based on the device's processing power and reliability.

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 plays other important roles as well to 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 224.0.0.6.

To prevent the designated router from becoming a serious liability to the network if it fails, OSPF also 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 based on 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

The administrator assigns a cost to the output side of each router interface. The lower the cost, the more likely the interface is to be used to forward data traffic. Costs can also be associated with the externally derived routing data.

The OSPF cost can also be used 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 11-4 shows how costs are used to direct traffic over high-speed links. For example, if Router-2 in Figure 11-4 receives packets destined for Host B, it will route them through Router-1 across two T1 links (Cost=20) rather than across one 56kbps B-channel to Router-3 (Cost=240).

Figure 11-4. OSPF costs for different types of links

The MAX has a default cost of 1 for a connected route (Ethernet) and 10 for a WAN link. If you have two paths to the same destination, the one with the lower cost will be used. You may want to reflect the bandwidth of a connection when assigning costs; for example, for a single B-channel connection, the cost would be 24 times greater than a T1 link.

Note: Be careful when assigning costs. Incorrect cost metrics can cause delays and congestion on the network.

Hierarchical routing (areas)

If a network is large, the size of the database, time required for route computation, and related network traffic become excessive. An administrator can partition an AS into areas to provide hierarchical routing connected by a backbone.

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 an area and to the backbone area. These routers are area border routers (ABRs). In Figure 11-5, all of the routers are ABRs.If the ABRs and area boundaries are set up correctly, link-state databases are unique to an area.

Figure 11-5. Dividing an AS into areas

Note: At this release, we recommend that you do not configure the MAX as an ABR. We currently recommend that you use the same area number for the Ethernet interface of the MAX and each of its WAN links. That are number does not have to be the backbone; the MAX can reside in any OSPF area.

Stub areas

To reduce the cost of routing, OSPF supports stub areas, in which a default route summarizes all external routes. For areas that are connected to the backbone by only one ABR (that is, the area has one exit point), there is no need to maintain information about external routes. Stub areas are similar to regular areas except that the routers do not enter external routes in the area's databases.

To prevent flooding of external routes throughout the AS, you can configure an area as a stub when there is a single exit point from the area, or when 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)

The MAX supports OSPF Not So Stubby Areas (NSSAs) as described in RRC 1587. NSSAs allow you to treat complex networks similar to stub areas. This can simplify your networks topology and reduce OSPF-related traffic.

NSSAs and type-7 LSAs
NSSAs are similar to stub areas, except that they allow limited importing of Autonomous System (AS) external routes. NSSAs use type-7 LSAs to import external route information into an NSSA. Type-7 LSAs are similar to type-5 LSAs except that:

When you configure the MAX as an NSSA internal router, you define the type-7 LSAs you want to advertise throughout the NSSA as static routes.

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.

Refer to RFC 1587 for complete information on NSSAs.

Configuring the MAX as an NSSA internal router
Because the MAX cannot be an area border router, when you configure OSPF on the MAX keep in mind that:

Refer to the documentation that came with your MAX for complete information on configuring OSPF on the MAX.

To configure the MAX as NSSA:

  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.

  6. Refer to the documentation that came with your MAX.

  7. In this static route profile, specify whether you want to advertise this route outside the NSSA:

  8. Exit and save the Static Rtes profile.

  9. Reset the MAX.

The link-state routing algorithm

 .Link-state routing algorithms require that all routers within a domain maintain synchronized (identical) topological databases, and that the databases describe the complete topology of the domain. An OSPF router's domain may be an AS or an area within an AS.

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 11-6.

Figure 11-6. Sample network topology

The routers then use the trees to build their routing tables, as shown in Table 11-1.

Table 11-1. Link state databases for network topology in Figure 11-6

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 11-2, Table 11-3, and Table 11-4 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 11-2. 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 11-3. 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 11-4. 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

These are the parameters related to OSPF routing in the MAX:

For more information on each parameter, see the MAX Reference Guide.

Understanding the OSPF routing parameters

This section provides some background information about the OSPF parameters. 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 settings:

Setting

Description
Enabling OSPF on an interface

 OSPF is turned off by default. To enable it on an interface, set RunOSPF to Yes.

Specifying an area number and type

 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

 The routers in the network use the Priority value 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.)

Authentication type and key

 You can specify that the MAX supports OSPF router authentication, and the key it looks for in packets to support that authentication. See Security.

Cost of the route on this interface

 This parameter 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. See Configurable metrics.

Autonomous System External route (ASE) type and tag

 ASEs are used only when OSPF is turned off on a particular interface. When OSPF is enabled, the ASE parameters are not applicable.
ASE-type specifies he 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

 LSAType is used only when OSPF is turned off on a particular interface. When OSPF is enabled, the LSA type is not applicable.

LSA 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. You can also select Internal, which indicates that the static route be advertised in an internal LSA.

Transit delay

 Specify 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.

Retransmit interval

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

OSPF global option for disabling ASBR calculations

 Autonomous ASBRs (autonomous system border routers) 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 the ASBR calculations are always performed. If you must prevent the MAX from performing ASBR calculations, you can disable them in Ethernet > Mod Config > OSPF Global Options.

Example configurations adding the MAX to an OSPF network

This section describes how to add a MAX to your OSPF network. It assumes that you know how to configure the MAX with an appropriate IP address as described in Chapter 10, Configuring IP Routing. The procedures in this section are examples based on Figure 11-7. Configuring the unit labeled MAX-1 in Figure 11-7. To apply one or more of the procedures to your network, enter the appropriate settings instead of the ones shown.

Figure 11-7. An example OSPF setup

In Figure 11-7, all OSPF routers are in the same area (the backbone area), so the units will all form adjacencies and synchronize their databases together.

Note: All OSPF routers in Figure 11-7 have RIP turned off. OSPF can learn routes from RIP without the added overhead of running RIP.

Configuring OSPF on the Ethernet interface

The MAX Ethernet interface in the example network diagram is in the OSPF backbone area. Although there is no limitation stated in the RFC about the number of routers in the backbone area, it is recommended that you keep the number of routers relatively small, because changes that occur in area zero are propagated throughout the AS.

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

    Note that RIP is turned off. It is not necessary to run both RIP and OSPF, and it reduces processor overhead to turn RIP off. OSPF can learn routes from RIP, incorporate them in the routing table, assign them an external metric, and tag them as external routes. See Chapter 10, Configuring IP Routing.

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

  3. Specify the area number and area type for the Ethernet.

    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. See Stub areas for background information.

  4. Leave the Hello interval, Dead interval, and Priority values set to their defaults.

  5. If authentication is required to get into the backbone area, specify the password.

    For example:

    If authentication is not required, set AuthType=None.

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

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

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

  8. Specify the retransmit interval for OSPF packets.

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

  9. Close the Ethernet profile.

When you close the Ethernet profile, the MAX comes up as an OSPF router on that interface. It forms adjacencies and begins building its routing table.

Configuring OSPF across the WAN

The WAN interface of the MAX is a point-to-point network. A point-to-point network is any network that joins a single pair of routers. These networks typically do not provide a broadcasting or multicasting service, so all advertisements are sent point to point.

An OSPF WAN link has a default cost of 10. You can assign higher costs to reflect a slower connection or lower costs to set up a preferred route to a certain destination. If the cost of one route is lower than another to the same destination, the higher-cost route will not be used unless route preferences change that equation (see Route preferences).

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 (see Figure 11-7 on page 11-13). 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:

    See Chapter 10, Configuring IP Routing.

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

  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:

    At this release, we recommend that you 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 uses this format.

  5. Leave the Hello interval, Dead interval, and Priority values set to their defaults.

    The Priority value is used to configure the MAX as a DR or BDR.

  6. If authentication is required to get into the backbone area, specify the password.

    For example:

    If authentication is not required, 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 10.

  8. Close the Connection profile.

Note: Of course, the remote MAX unit must also have a comparable Connection profile to connect to MAX-1.

Configuring a WAN link that does not support OSPF

In this example, the MAX has a Connection profile to a remote Pipeline unit across a BRI link (see Figure 11-7 on page 11-13). The remote Pipeline is an IP router that transmits routes using RIP-v2. The route to this network, as well as any routes the MAX learns about from the remote Pipeline, are ASEs (external to the OSPF system).

To enable OSPF to add the RIP-v2 routes to its routing table, configure RIP-v2 normally in this Connection profile. OSPF will import 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:

    See Chapter 10, Configuring IP Routing. Note that Connections > OSPF Options includes two ASE parameters that are active only when OSPF is not running on a link. When you configure these parameters, the Connection profile route will be advertised whenever the MAX is up.

  3. Open the OSPF Options subprofile.

  4. Leave RunOSPF set to No.

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

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

  6. Specify the LSA type for this route.

    This 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 may also be used by border routers to filter this record. For example:

  8. Close the Connection profile.

Note: Of course, the remote MAX unit must also have a comparable Connection profile to connect to MAX-1.

Administering OSPF

This section describes how to work with OSPF information in the routing table and how to monitor OSPF activity in the terminal server command-line interface.

To invoke the terminal-server interface, select System > Sys Diag > Term Serv and press Enter.

Working with the routing table

The OSPF routing table includes routes built from the router's link-state database as well as those added by external routing protocols such as RIP. You can also add routes statically, for example, to direct traffic destined for a remote site through one of several possible border routers. For details on adding static routes, for example, if you want to force the use of one route over those learned from OSPF, see Chapter 10, Configuring IP Routing..

To view the IP routing table with added OSPF information, invoke the terminal-server (System > Sys Diag > Term Serv) and use the Iproute Show command with the -l option:

In addition to the standard routing-table fields, which are described in Chapter 10, Configuring IP Routing, the following three columns are specific to OSPF and are displayed only when you use the -l option. These OSPF-specific columns are displayed on the far right of each entry in the routing table:

Multipath routing

 A MAX running OSPF can alternate between two equal cost gateways. When OSPF detects more than one equally good gateway, in terms of routing costs, each equal-cost gateway is put on an equal-cost list. The router will alternate between all the gateways on the list. This is called equal-cost multipath routing.

For example, if a router A has two equal-cost routes to example.com, one via router B and the other via router C, the routing table could look like this:

Destination        Gateway         IF       Flg   Pref Met     Use     
Age

10.174.88.0/25    10.174.88.12    wan2     OGM     10  10      52      19

10.174.88.0/25    10.174.88.13    wan3     OGM     10  10      52      19

10.174.88.12/32   10.174.88.12    wan2     OG      10  10       0      28

10.174.88.13/32   10.174.88.13    wan3     OG      10  10       0      28

192.168.253.0/24   -              ie0      C        0   0       1      49

192.168.253.6/32   -              lo0      CP       0   0      53      49

223.1.1.0/24       10.174.88.12   wan2     OG      10  10       0      19

223.5.1.0/24       10.174.88.12   wan2     OG      10  10       0      19

223.12.9.0/24      10.174.88.12   wan2     OG      10  10       0      19

255.255.255.255/32 -              ie0      CP       0   0       0      49
Note that the "M" in the Flags column indicates an equal-cost multipath. A Traceroute from A to example.com would look like this:

Note: Notice the alternating replies. The replies are statistically dispatched to B and C, with roughly 50% of the packets sent through each gateway. For background information on the routing table and on the Traceroute command, see Chapter 10, Configuring IP Routing.

Third-party routing

A MAX running OSPF can advertise routes to external destinations on behalf of another gateway (a "third-party"). This is commonly known as advertising a forwarding address. Depending on the exact topology of the network, it may be possible for other routers to use this type of LSA and route directly to the forwarding address without involving the advertising MAX, increasing the total network throughput.

Third-party routing requires that all OSPF routers know how to route to the forwarding address. This will usually mean that the forwarding address must be on an Ethernet that has an OSPF router acting as the forwarding router, or that designated router is sending LSAs for that Ethernet to any area that sees the static route's forwarding address LSAs. To configure a static route for OSPF to advertise a third-party gateway:

  1. Open a static route in Ethernet > Static Rtes.

  2. Set Third-Party to Yes.

  3. Set the Gateway to the forwarding address.

  4. Close the static route.

How OSPF adds RIP routes

 When the MAX establishes an IP routing connection with a caller that does not support OSPF, it imports the AS-external route from the Connection profile and adds it to the routing table. The MAX does not have to run RIP to learn these routes. RIP should be turned off when the MAX is running OSPF.

To enable OSPF to add the RIP-v2 routes to its routing table, configure RIP-v2 normally in this Connection profile. OSPF will import all RIP routes as Type-2 ASEs. The reason why RIP routes are imported with Type-2 metrics by default is that RIP metrics are not directly comparable to OSPF metrics. To prevent OSPF from interpreting RIP metrics, we assign the imported ASE route a Type-2 metric, which means that it is so large compared to OSPF costs that the metric can be ignored.

Route preferences

Route preferences provide additional control over which types of routes take precedence over others. They are necessary in a router which speaks multiple routing protocols, largely because RIP metrics are not comparable with OSPF metrics.

For each IP address and netmask pair, the routing table holds one route per protocol, where the protocols are defined as follows:

When choosing which routes should be put in the routing table, the router first compares the Preference value, preferring the lower number. If the Preference values are equal, the router then compares the Metric field, using the route with the lower Metric.

If multiple routes exist for a given address and netmask pair, the route with the lower Preference is better. If two routes have the same Preference, then the lower Metric is better. The best route by these criteria is actually used by the router. The others remain latent or hidden, and are used in case the best route was removed.

To assign a WAN link the same preference as a route learned from OSPF:

  1. Open Connections > IP Options.

  2. Specify a preference value of 10 (the default value for OSPF routes). For example:

  3. Close the Connection profile.

On Ethernet, the route preferences also include ASE type and ASE tag information for routes learned from RIP. These values affect all RIP information learned across the Ethernet. To change the route preferences on Ethernet:

  1. Open Ethernet > Mod Config > Route Pref.

  2. Modify the parameters to adjust preference values. For example, to assign static routes the same preference value as those learned from OSPF:

    Or, to change RIP metrics to Type 1:

  3. Close the Ethernet profile.

Monitoring OSPF

 The terminal server command-line interface provides commands for monitoring OSPF in the MAX. To see the options, invoke the terminal server interface (System > Sys Diag > Term Serv) and use the Show OSPF command; for example:

Viewing OSPF errors

 To see OSPF errors, type:

ERRORS from:                       boot

   0: IP: Bad OSPF pkt type             0: IP: Bad IP Dest                 

   0: IP: Bad IP proto id               1: IP: Pkt src = my IP addr        

   0: OSPF: Bad OSPF version            0: OSPF: Bad OSPF checksum         

   0: OSPF: Bad intf area id            0: OSPF: Area mismatch             

   0: OSPF: Bad virt link info          0: OSPF: Auth type != area type    

   0: OSPF: Auth key != area key        0: OSPF: Packet is too small       

   0: OSPF: Packet size > IP length     0: OSPF: Transmit bad              

   0: OSPF: Received on down IF         0: Hello: IF mask mismatch         

   0: Hello: IF hello timer mismatch    0: Hello: IF dead timer mismatch


   0: Hello: Extern option mismatch     0: Hello: Nbr Id/IP addr confusion


   0: Hello: Unknown Virt nbr           0: Hello: Unknown NBMA nbr         

   0: DD: Unknown nbr                   0: DD: Nbr state low               

   0: DD: Nbr's rtr = my rtrid          0: DD: Extern option mismatch      

   0: Ack: Unknown nbr                  0: Ack: Nbr state low              

   0: Ls Req: Nbr state low             0: Ls Req: Unknown nbr             

   0: Ls Req: Empty request             0: LS Req: Bad pkt                 

   0: LS Update: Nbr state low          0: Ls Update: Unknown nbr          

   0: Ls Update: Newer self-gen LSA     0: Ls Update: Bad LS chksum 

   0: Ls Update: less recent rx         0: Ls Update: Unknown type
The output lists all error messages related to OSPF, with each message preceded by the number of times it has been generated since the MAX powered up. Immediately following the number is a field indicating the packet type:

Viewing OSPF areas

 To view information about OSPF areas, type:

Viewing OSPF general info

 To see general information about OSPF, type:

To display the OSPF interfaces, type:

Area      IP Address   Type   State   Cost   Pri  DR           BDR

----------------------------------------------------------------------
-----

0.0.0.0   10.5.2.154   Bcast  BackupDR   1    5   10.5.2.155   
10.5.2.154

0.0.0.0   10.5.2.154   PtoP   P To P     10   5   None         None

0.0.0.0   10.5.2.154   PtoP   P To P     10   5   None         None

Viewing the OSPF link-state database

 To view the router's link-state database, type:

Note: You can expand each entry in the link-state database to view additional information about a particular LSA. See Viewing OSPF link-state advertisements. 

LS Data Base:

Area         LS Type Link ID       Adv Rtr       Age  Len Seq #    Metric

----------------------------------------------------------------------
-

0.0.0.0      STUB    10.5.2.146    10.5.2.146    3600 24  0        0     

0.0.0.0      STUB    10.5.2.154    10.5.2.154    3600 24  0        0     

0.0.0.0      STUB    10.5.2.155    10.5.2.155    3600 24  0        0     

0.0.0.0      STUB    10.5.2.162    10.5.2.162    3600 24  0        0     

0.0.0.0      STUB    10.5.2.163    10.5.2.163    3600 24  0        0     

0.0.0.0      RTR     10.5.2.146    10.5.2.146    659  72  8000003e 0     

0.0.0.0      RTR     10.5.2.154    10.5.2.154    950  84  8000000a 0     

0.0.0.0      RTR     10.5.2.155    10.5.2.155    940  60  80000005 0     

0.0.0.0      RTR     10.5.2.162    10.5.2.162    980  84  8000003b 0     

0.0.0.0      RTR     10.5.2.163    10.5.2.163    961  60  80000005 0     

0.0.0.0      NET     10.5.2.155    10.5.2.155    940  32  80000003 0     

0.0.0.0      NET     10.5.2.163    10.5.2.163    961  32  80000003 0     

0.0.0.0      ASE     10.5.2.16     10.5.2.163    18   36  80000098 3     

0.0.0.0      ASE     10.5.2.18     10.5.2.163    546  36  80000004 10    

0.0.0.0      ASE     10.5.2.144    10.5.2.146    245  36  80000037 1     

0.0.0.0      ASE     10.5.2.152    10.5.2.154    536  36  80000006 1     

0.0.0.0      ASE     10.5.2.152    10.5.2.155    526  36  80000004 1     

0.0.0.0      ASE     10.5.2.152    10.5.2.163    18   36  80000097 9     

0.0.0.0      ASE     10.5.2.155    10.5.2.163    17   36  80000097 9     

0.0.0.0      ASE     10.5.2.160    10.5.2.162    568  36  80000037 1

Viewing OSPF link-state advertisements

 To view additional information about an LSA in the link-state database, first display the database as described in the preceding section. You can specify an LSA to expand using this format:

This command requires that you include the first four fields of the LSA as listed in the database. You can select the first four fields and paste them in after typing the command, for example, to see an expanded view of the last entry in the link-state database shown in the previous section:

Viewing OSPF neighbors

 To view adjacencies, type:

Area       Interface     Router Id     Nbr IP Addr   State      Mode   
Pri

----------------------------------------------------------------------
-----

0.0.0.0    10.5.2.154    10.5.2.155    10.5.2.155    Full       Slave  5   

0.0.0.0    10.5.2.154    10.5.2.146    10.5.2.146    Full       Master 5   

0.0.0.0    10.5.2.154    10.5.2.162    10.5.2.162    Full       Slave  5

Viewing the OSPF routing table

 To view the OSPF routing table, type:

SPF algorithm run 24 times since                       boot

Dest          D_mask          Area    Cost E Path Nexthop AdvRtr      L

----------------------------------------------------------------------
-----

Nets:

10.5.2.163   255.255.255.248  0.0.0.0  10  3 EXT  10.5.2.163 10.5.2.163 
0

10.5.2.163   255.255.255.255  0.0.0.0  20  0 EXT  10.5.2.163 10.5.2.163 
0

10.5.2.146   255.255.255.248  0.0.0.0  20  1 EXT  10.5.2.154 10.5.2.146 
0

10.5.2.146   255.255.255.255  0.0.0.0  20  0 STUB 10.5.2.154 10.5.2.146 
0

10.5.2.155   255.255.255.248  0.0.0.0  10  0 INT  10.5.2.154 10.5.2.155 
1

10.5.2.154   255.255.255.255  0.0.0.0  21  0 STUB 10.5.2.163 10.5.2.154 
0

10.5.2.155   255.255.255.255  0.0.0.0  20  9 STUB 10.5.2.155 10.5.2.155 
1

10.5.2.163   255.255.255.248  0.0.0.0  11  1 INT  10.5.2.163 10.5.2.163 
0

10.5.2.162   255.255.255.255  0.0.0.0  20  0 STUB 10.5.2.163 10.5.2.162 
0

10.5.2.163   255.255.255.255  0.0.0.0  10  0 STUB 10.5.2.163 10.5.2.163 
0

Viewing OSPF protocol i/o

 To display information about packets sent and received by the OSPF protocol, type:


[Top][Contents][Prev][Next][Last]Search

techpubs@eng.ascend.com

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