Posted on Thursday, 25th March 2004 by sean
I don’t want to spend much more time on IS-IS so that I can get to other things. From what I can tell, the level of knowledge required is not as deep as it is for OSPF, EIGRP, and BGP.
As mentioned earlier, a router configured as L1-L2 will form an adjacency twice, once for level 1, once for level 2. The router type is configured at the routing protocol level with the is-type level-whatever command, but can be overridden at the interface level with isis circuit-type command:
r0(config-if)#isis circuit-type ? level-1 Level-1 only adjacencies are formed level-1-2 Level-1-2 adjacencies are formed level-2-only Level-2 only adjacencies are formed
isis metric at the interface level lets you assign a metric to an interface.
The hard part about ISIS seems to be choosing the NET address in order to properly define areas. Choosing the level of the router such that there is a continuous level 2 backbone is also important.
Remembering that ISIS adjacencies are established over OSI and CLNS rather than IP in the case of OSPF, you can find your neighbours with show clns neighbors:
r0#show clns neighbors System Id Interface SNPA State Holdtime Type Protocol r1 Et0 0000.0c35.042a Up 29 L1 IS-IS
Interestingly enough, if you misconfigure IP such that the two neighbours can’t talk, they’ll still form an IS-IS adjacency, but the next hop address won’t be of any help!
Some other fun commands:
r0#show clns protocol
IS-IS Router:
System Id: 0100.5000.0001.00 IS-Type: level-1-2
Manual area address(es):
49.0001
Routing for area address(es):
49.0001
Interfaces supported by IS-IS:
Loopback99 - IP
Ethernet0 - IP
Redistribute:
static (on by default)
Distance for L2 CLNS routes: 110
RRR level: none
Generate narrow metrics: level-1-2
Accept narrow metrics: level-1-2
Generate wide metrics: none
Accept wide metrics: none
r0#show clns interface e0
Ethernet0 is up, line protocol is up
Checksums enabled, MTU 1497, Encapsulation SAP
ERPDUs enabled, min. interval 10 msec.
CLNS fast switching enabled
CLNS SSE switching disabled
DEC compatibility mode OFF for this interface
Next ESH/ISH in 40 seconds
Routing Protocol: IS-IS
Circuit Type: level-1
Interface number 0x0, local circuit ID 0x1
Level-1 Metric: 10, Priority: 64, Circuit ID: r0.01
Number of active level-1 adjacencies: 1
Next IS-IS LAN Level-1 Hello in 2 seconds
I wanted to get into IS-IS over Frame-Relay and some multiarea stuff, but it seems I’ve messed up some cabling and will have to wait until I get home to fix it up.
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