ISIS topology overview
Introduction to CPAM ISIS topology
The ISIS topology map displays all of the routers and ISIS links that are detected by the CPAAs. All of the NFM-P-managed and -manageable routers are displayed.
You can configure multiple ISIS instances on a CPAA. A CPAA can monitor up to 32 ISIS L1 routing domains and the ISIS backbone (L2 domain). The L2 adjacency can be established only in the instance 0. By default, all of the other instances are L1-capable only. Each ISIS L1 domain should have an adjacency to only one CPAA. You can add each ISIS interface to one instance.
The ISIS area ID for each CPAA ISIS instance must be unique in the IGP administrative domain.
If the network architecture includes Area IDs that are the same, a new Area ID that is unused in the network should be created between the CPAA instance and the connected router. If Area IDs are not unique, the CPAA configuration should be changed prior to an upgrade.
The CPAM uses the area ID to identify different L1 domains on the map with different colours. Although the routers in the L1 domain can have more than one area ID, or another area ID from the CPAA, the entire L1 domain is identified by the CPAA area ID on the map.
Instance numbers and Area IDs should be identical for a redundant CPAA pair.
The following figure shows a discovered ISIS network with multiple level 1 instances and one level 2 instance.
Figure 4-5: ISIS view
Routers
The following table lists the icons used by the CPAM to identify the roles of the routers in a routing area.
Table 4-6: Router icons
|
Icon |
ISIS Router role |
|---|---|
|
L1-L2 |
|
Network (subnet) |
|
Unmanaged L1 |
|
Managed L1 |
|
Unmanaged L1-L2 |
|
Unmanaged L2 |
|
L2 |
Broadcast multi-access networks, such as Ethernet networks, use a DR to prevent each router from forming a link with every router in an Ethernet network.
The CPAM uses the router ID as the identifier of a router. Routing protocol instances, such as IGP, must share the same router ID.
Note: In a standard ISIS, because the protocol operates using system IDs, there is no method to easily determine the router ID. The TE router ID is available when you configure the traffic engineering extension.
When assigning a Router ID to an ISIS router with no TE router ID, the assigned Router ID should be a reachable IP address (via ISIS) on the node.
With 7450 ESS and 7750 SR service routers, the router ID is always available in the LSP regardless of the traffic engineering configuration. Although the system ID, system IP address, and router ID may differ, the router ID is the system IP address by default.
The following restrictions apply to ISIS SPF calculations, next hop configurations, and topology map highlighting:
-
Full support for SPF and managed routes is available when the TE router ID is included in the protocol.
-
ISIS systems without a TE router ID can be detected by the CPAM but will not be shown on the topology map until a TE router ID is assigned.
You can navigate directly to a router if the router is currently managed by the NFM-P. You can also use the following contextual menu options by right-clicking on the router icon:
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The Highlight Next Hop menu option opens the Next Hop form for the selected object (managed and unmanaged devices).
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The NE Sessions option opens a Telnet session, an SSH session, an FTP file browser session, or an SSH file browser session with the selected device (managed devices only).
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The Properties option opens the router property form for the selected object. This form displays read-only information and configurable parameters.
Subnet objects
The subnet object represents a pseudonode in the ISIS network and appears as a small circle icon on the map. The subnet object displays the system ID and pseudonode ID of the subnet, for example, 0100.0118.2213-04.
Broadcast multi-access networks, such as Ethernet networks, use a DR to prevent each router on the network from forming a link with all of the routers on the same broadcast network. There is no specific identifier for a DR on the map.
The DR information for a subnet is available on the property form for the pseudonode subnet object.
Links
The CPAM supports the following ISIS links:
Point-to-point links directly connect two adjacent routers. Broadcast links are used when the routers are connected using an Ethernet or token ring network (hub or switch). The CPAM uses a circle on the topology map to identify a subnet, or pseudonode. A broadcast link always has one endpoint as a circle and one endpoint as an ISIS interface of a router. When you create a link from a router to a network, the CPAM creates a duplicate link in the reverse direction. The duplicate link always has a metric and bandwidth of 0.
The CPAM does not support non-broadcast multi-access links, such as frame relay or X.25.
An ISIS link under the CPAM can be designated as a Level 1 or Level 2 link, but not both at the same time. The CPAM creates two different links if there is a Level 1 and Level 2 adjacency between two ISIS interfaces. The figure below shows an example of the ISIS adjacency configuration. The different links support the unique metrics that you can configure for each level. For SPF calculations, if there are both L1 and L2 links, the determination of the active link is dependent on whether the destination address is local or external to the L1 area.
Figure 4-6: ISIS Level 1 and Level 2 links
An ISIS link is a level-specific (L1 or L2) connection from an ISIS interface to the following network components:
The map uses an arrow to display the direction of a unidirectional ISIS. Two links between two interfaces are grouped, by default, as one link with no defined direction. By default, all of the links between two routers are grouped into one link group.
Router support for ISIS-TE Neighbor TLV
Router support for ISIS-TE Neighbor TLV determines the level of support the routing analyzer provides for the parallel links between interfaces. In a standard ISIS, only one neighbor is advertised even if there are parallel links between the neighbors. The TE Neighbor TLV contains all of the parallel links and the IP addresses configured on each terminating interface. This configuration allows the routing analyzer to create the parallel links, and in the case of managed routers, provide navigation to the NFM-P ISIS interface.
In the 7750 SR amd 7450 ESS, the ISIS protocol advertises the ISIS-TE Neighbor TLV even if ISIS-TE is not enabled. Bandwidth-related TLVs are advertised to the network when you enable ISIS-TE. This functionality is not common to all of the devices. You must enable ISIS-TE for devices that do not support unconditional advertising of the TE Neighbor TLV.
ISIS L1 routing domain
The L1 routing domain is equivalent to a non-backbone OSPF routing area. An L1 routing domain can contain routers with one or more defined area IDs. In addition, two routers belonging to two different L1 routing domains can use the same routing area. A CPAA can be connected to only one L1 routing domain.
LSDB updates
Each Link State PDU (LSP) received by a CPAA is forwarded to the CPAM. In ISIS there is no distinction between router and network LSPs, with the following exceptions:
You can retrieve the LSDB from the CPAA to the database. See To update the LSDB for information.
If you enable the CPAA ISIS protocol events flag, the CPAM ensures that the LSDB is up to date by using the following rules.
-
When the CPAA becomes operationally up and ISIS events are currently enabled, the CPAM automatically retrieves the LSDB. This includes the start-up time of the CPAM.
-
When the CPAA is already operationally up and ISIS events become enabled, the CPAM automatically retrieves the LSDB.
SRLG
A set of links can be considered a shared risk link group if they share a resource whose failure may affect all of the links in the set. A link can belong to multiple SRLGs. An SRLG is identified by a 32-bit number that is unique within an IGP domain.
ISIS TE extensions for SRLG
The SRLG values are carried in sub-TLV 16 of the Link TLV in ISIS TE extensions. You can view the value of the SRLGs to which the link belongs on the Shared Link Risk Group tab of ISIS link.
When the IGP topology is checkpointed, the SRLG values are also recorded. When two checkpointed links are compared, the SRLG values are also compared between the two instances and changes are reported in the result.
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