Point-to-Point Serial-Link Configuration

The IS-IS configuration for point-to-point links is the most commonly used in real networks, for point-to-point serial interfaces, as well as to NBMA point-to-point subinterfaces. The point-to-point configuration lends itself to various advantages because it provides congruency to the underlying physical topology. Figures 9-1 and 9-2 depict a simple point-to-point setup of two routers (RT1 and RT2) connected directly by a serial link. The routers are in the same IS-IS area in Figure 9-1 but in different areas in Figure 9-2.

Figure 9-1. IS-IS point-to-point configuration (routers in same area).

Figure 9-2. IS-IS point-to-point configuration (routers in different areas).

Also shown are relevant excerpts of the IS-IS configurations on the routers. In Figure 9-1, both routers are in the same area and so share a common area prefix (49.0001). Therefore, according to the default IOS behavior, they will establish Level 1 and Level 2 adjacencies. In contrast, however, the routers in Figure 9-2 are in different areas, so they will form only a Level 2 adjacency .

The following commands are useful for verifying proper configuration and operation of IS-IS on Cisco routers:

• show clns protocol ” Provides a summary of the protocol state on the router(see Example 9-1)

• show clns neighbors [detail] ” Shows adjacency state information about known neighbors (see Example 9-2)

• show clns interface ” Shows IS-IS routing information pertaining to an interface (see Example 9-3)

• show isis topology ” Shows path information for IS-IS nodes in the Level 1 and Level 2 topology (see Example 9-4)

• show isis database ” Displays known LSPs in the level-1 and level-2 databases(see Example 9-5)

Examples 9-1 through 9-5 show outputs of these commands captured from the routers set up in Figure 9-2. Each example features an output from each router to show the stateof either side of the connection. Because the setup is basic, most of the output is self-explanatory. Some information is provided on the show isis database output in Table 9-1. More information is provided on the other commands in Chapter 10, "Troubleshooting the IS-IS Routing Protocol."

Example 9-1 show clns protocol Output

 RT1#  show clns protocol  IS-IS Router: <Null Tag>   System Id: 0000.0000.0001.00  IS-Type: level-1-2   Manual area address(es):         49.0001   Routing for area address(es):         49.0001   Interfaces supported by IS-IS:         Loopback0 - IP         Serial0/0 - IP   Redistributing:     static   Distance: 110   RRR level: none   Generate narrow metrics: level-1-2   Accept narrow metrics:   level-1-2   Generate wide metrics:   none   Accept wide metrics:     none RT2#  show clns protocol  IS-IS Router: <Null Tag>   System Id: 0000.0000.0002.00  IS-Type: level-1-2   Manual area address(es):         49.0002   Routing for area address(es):         49.0002   Interfaces supported by IS-IS:         Loopback0 - IP         Serial0/0 - IP   Redistributing:     static     Distance: 110   RRR level: none   Generate narrow metrics: level-1-2   Accept narrow metrics:   level-1-2   Generate wide metrics:   none   Accept wide metrics:     none 

Example 9-2 show clns neighbors detail Command Output

 RT1#  show clns neighbors detail  System Id   Interface   SNPA    State   Holdtime   Type   Protocol RT2            Se0/0       *HDLC*          Up     27   L2   IS-IS   Area Address(es): 49.0002   IP Address(es):  192.168.1.2*   Uptime: 00:48:46 RT2#  show clns neighbors detail  System Id     Interface   SNPA           State  Holdtime  Type   Protocol RT1            Se0/0       *HDLC*       Up      26        L2   IS-IS   Area` Address(es): 49.0001   IP Address(es):  192.168.1.1*   Uptime: 00:52:14 

Example 9-3 show clns interface Command Output

 RT1#  show clns interface ser 0/0  Serial0/0 is up, line protocol is up   Checksums enabled, MTU 1500, Encapsulation HDLC   ERPDUs enabled, min. interval 10 msec.   RDPDUs enabled, min. interval 100 msec., Addr Mask enabled   Congestion Experienced bit set at 4 packets   CLNS fast switching enabled   CLNS SSE switching disabled   DEC compatibility mode OFF for this interface   Next ESH/ISH in 3 seconds   Routing Protocol: IS-IS     Circuit Type: level-1-2     Interface number 0x0, local circuit ID 0x100     Level-1 Metric: 10, Priority: 64, Circuit ID: RT1.00     Number of active level-1 adjacencies: 0     Level-2 Metric: 10, Priority: 64, Circuit ID: RT1.00     Number of active level-2 adjacencies: 1     Next IS-IS Hello in 8 seconds RT2#  show clns interface serial0/0  Serial0/0 is up, line protocol is up   Checksums enabled, MTU 1500, Encapsulation HDLC   ERPDUs enabled, min. interval 10 msec.   RDPDUs enabled, min. interval 100 msec., Addr Mask enabled   Congestion Experienced bit set at 4 packets   clns fast switching enabled   clns SSE switching disabled   DEC compatibility mode OFF for this interface   Next ESH/ISH in 8 seconds   Routing Protocol: IS-IS     Circuit Type: level-1-2     Interface number 0x0, local circuit ID 0x100     Level-1 Metric: 10, Priority: 64, Circuit ID: RT2.00     Number of active level-1 adjacencies: 0     Level-2 Metric: 10, Priority: 64, Circuit ID: RT2.00     Number of active level-2 adjacencies: 1     Next IS-IS Hello in 2 seconds 

Example 9-4 show isis topology Command Output

 RT1#  show isis top  IS-IS paths to level-1 routers System Id       Metric  Next-Hop     Interface    SNPA RT1             -- IS-IS paths to level-2 routers System Id       Metric  Next-Hop     Interface    SNPA RT1             -- RT2             10      RT2          Se0/0        *HDLC* RT2#  show isis topology  IS-IS paths to level-1 routers System Id       Metric  Next-Hop     Interface    SNPA RT2             -- IS-IS paths to level-2 routers System Id       Metric  Next-Hop     Interface    SNPA RT1             10      RT1             Se0/0           *HDLC* RT2             -- 

Example 9-5 show isis database Command Output

 RT1#  show isis database  IS-IS Level-1 Link State Database LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL RT1.00-00           * 0x00000008   0x8B75        1126              1/0/0 RT1.01-00           * 0x00000001   0x459B        1131              0/0/0 IS-IS Level-2 Link State Database LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL RT1.00-00           * 0x0000008A   0x8FED        1126              0/0/0 RT2.00-00             0x0000001E   0xB82C        998               0/0/0 RT2#  show isis database  IS-IS Level-1 Link State Database LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL RT2.00-00           * 0x00000019   0x3DAB        883               1/0/0 RT2.01-00           * 0x0000000D   0x339F        980               0/0/0 IS-IS Level-2 Link State Database LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL RT1.00-00           0x0000008A    0x8FED         931               0/0/0 RT2.00-00        * 0x0000001E    0xB82C          808               0/0/0 

Table 9-1 describes the fields in the show isis database command output shown in Example 9-5.

Table 9-1. Explanation of the Fields in the show isis database Commands

Attribute	Comments
*	Indicates LSP is originated by the local system.
LSPID	LSP identifier. Column lists all known Level 1 and Level 2 LSPs.
LSP Seq Number	LSP sequence number for tracking the current version of LSP.
LSP Checksum	Checksum calculated at LSP origin. If it changes during storage or flooding, the LSP is corrupted and needs to be purged from the network.
LSP Holdtime	Time to expiration of LSP in seconds.
ATT	Attached bit. Set in Level 1 LSP by Level 2 routers to help Level 1 interarea traffic.
P	Partition bit. Indicates that the router supports partition repair.
OL	Overload bit. When set, it indicates resource problems, and the originating router should not be used in transit paths. Can be manually set for administrative reasons in IOS with set-overload-bit command.