Topic: tech juniper jir prev
tech juniper jir > 16: Introduction to IS-IS
A link state IGP using SPF. Originally developed for routing ISO Connectionless Network Protocol (CLNP) packets; support for IP was added later. Defined in ISO/IEC and RFC documents.
The ISO developed IS-IS at the same time the Internet Advisory Board (IAB) was working on OSPF. At the time, ISO pushed for adoption of IS-IS on the basis that TCP/IP was an interim protocol suite, and that eventually the OSI suite would be adopted. ‘Integrated IS-IS’, also called ‘Dual IS-IS’, routes IP in addition to CLNP. As a link state protocol, IS-IS requires that all routers run the same routing algorithm.
Similar to OSPF LSA and LSU. IS-IS PDUs are the Layer 2 protocol, not IP. IP reachability and protocol support information is included in routing updates. Not all Junos products support CLNP.
An IS-IS network is a single autonomous system. Within the network, end systems send and receive packets. Intermediate systems send and receive packets, and forward packets for other systems.
As in OSPF, a single AS can be divided into smaller groups referred to as areas. ISs are arranged hierarchically; L1 ISs route within an area only. L2 ISs route between areas. A single IS can be both an L1 and an L2 IS simultaneously.
An L1 router is analogous to an interior router in OSPF. An L1/L2 router is analogous to an area border router in OSPF. An L2 router is analogous to a backbone router in OSPF.
The collection of all L2 routers is the backbone in IS-IS, whereas in OSPF, Area 0 is the backbone.
Both protocols:
PDUs are sent with tag-length-value (or type-length-value) (TLV) encoding.
Types of PDU:
Used to build and maintain adjdacencies. Similar to the hello packets in OSPF. There are separate types of PDU for LAN and PTP links. For broadcast networks, well known multicast MAC addresses 01-80-C2-00-00-14 and 01-80-C2-00-00-15 are used to address L1 and L2 routers respectively.
IIH PDUs are transmitted at regular intervals of three seconds by default for designated routers and nine seconds for non designated routers.
IIH Fields
Used to build the link state database. They are analogous to LSAs in OSPF. There are separate link state PDUs for L1 and L2 systems. Link state PDUs are sent as a result of network changes, adjacency formations, and in response to a sequence number PDU (request for missing LSPs).
Link state PDUs describe the state of an IS’s adjacencies and its reachable prefixes. Once an IS has received all required LSPs, it uses the Dijkstra algorithm to compute optimal paths to every end system.
Used to maintain the database synchronisation. PSNPs are sent when a route detects that it is missing a link state PDU, or when its database is out of date. CSNPs contain a complete description of all link state PDUs in the database. CSNPs are sent periodically by all routers on point to point links and by DIS routers on broadcast networks.
All values in IS-IS are sent as TLV (type-length-value) fields. Unknown types are ignored in IS-IS, making the protocol easily extensible. Common TLVs include those for addresses, authentication and protocol support.
IS-IS adjacency rules:
The router priority is announced in IIHs. The router with the higher priority becomes the DIS. In Junos, the default priority is 64. In the case of a tie, the router with the highest subnet point of attachment or MAC address wins the election.
IS-IS elects a DIS for the same reason that OSPF elects a DR. The IS-IS network itself is considered a pseudonode and considered as a single router. The DIS acts as a representative of the pseudonode.
Unlike OSPF, there is no backup designated router. Also unlike OSPF, if a new router joins with a higher preference to become the DIS, it takes over as DIS.
IS-IS has a single, required, default metric, with a maximum path cost of 1023. The metric is assigned by the network administrator. Any single link in a path can have a maximum value of 63. The total path cost is calculated by summing the values on the links.
There are three optional metrics; the delay cost, expense cost, and error cost. These metrics are used in conjunction with QoS options.
Wide metrics are always sent. To ensure compatibility with routers which do
not support wide metrics, their value is limited to 63 unless the standard
metrics are disabled on a per level basis. Wide metrics are more suitabled for
networks with vastly differing link speeds. The maximum wide metric value for
a link is 2^24. The maximum network diameter is 256, so the maximum path
value is 2^32.
How to configure and monitor IS-IS.
By default, in Junos, all interfaces configured for IS-IS become both Level 1 and Level 2 interfaces. Levels can be disabled individually.
protocols {
isis {
interface ge-0/0/0.0 {
level 1 disable;
}
interface ge-0/0/1.0 {
level 2 disable;
}
}
}
The ISO family must be included on all interfaces on which IS-IS should run. A
network entity title must be configured on one of the router’s interfaces,
usually lo0. Junos supports the assignment of multiple ISO NETs to a
loopback interface. This can be helpful for merging networks.
interfaces {
ge-0/0/0 {
unit 0 {
family iso;
family inet {
address ...;
}
}
}
lo0 {
unit 0 {
family inet {
address 192.168.2.1/24;
}
family iso {
address 49.001.0192.0168.0201.00;
}
}
}
}
show isis interface, optionally brief or detail.
show isis database, optionally brief or detail. The detail output
includes the IP prefix advertised in each LSP along with its advertising
neighbour.
show isis adjacency, optionally brief or detail, to show immediate
adjacencies by local interface.
The detail output includes whether the router is L1, L2 or both, and which
protocols are spoken by the neighbour.
Adjacencies can be cleared or restarted with the clear isis adjacency
<SYSTEM> command.
show isis spf log shows the time taken by SPF calculations, and the reason
for the SPF algorithm being triggered.
show isis statistics shows the number of IS-IS packets received, processed,
dropped, transmitted and retransmitted.
SNP and LSP queues are almost always zero.
If the SPF runs value increases rapidly, it is likely that the network is unstable.
‘Purges initiated’ counts the number of times that the router decided to remove an LSP from the network.
The show route protocol isis command lists routes learned by IS-IS. show
isis route shows more information specific to IS-IS.
protocols {
isis {
traceoptions {
file isis-trace;
flag error detail;
flag hello detail;
flag lsp detail;
}
}
}
Unlike OSPF, IS-IS does not require or rely on IP configuration. It simply carries IP information in its TLVs. Therefore, an IS-IS adjacency can be formed between two routers, even if they do not agree on the IP subnet to which they belong. As a result, it is possible that an adjacency can form, yet a router can be unable to ping its neighbours.
Troubleshooting checklist:
lo0 as an IS-IS interface