Integrated IS-IS: Intermediate System to Intermediate System RFC 1195
 Generally used in large ISP networks because of its simplicity and stability
 ISO routing protocol
 Capable of routing IPv4 and IPv6 by using type field
 Less common than OSPF because OSPF is has native IP support
 Uses CLNS addresses to maintain LSDB
Terminology:
 CLNS – connectionless network service
o CLNP – connectionless network protocol – the layer 3 protocol used by CLNS
 CSNP – complete sequence number packet
 PSNP – partial sequence number packet
 IS – intermediate system (router)
 ES – end system (host)
 DIS – designated intermediate system (similar role to DR in OSPF)
 Pseudonode – virtual router created by DIS
 SNPA – subnetwork point of attachment (this can be the MAC of an Ethernet interface or DLCI
on a frame link)
Comparisons to OSPF
 Similarities
o Link state protocols
o Dijkstra’s algorithm
o VLSM support
o Quick convergence
o Open standards
o 2 level hierarchy
 Differences
o Area design (OSPF requires Area 0 backbone, IS-IS does not)
o Area membership
 In OSPF, links are only in one area
 In IS-IS, routers are only in one area
o IS-IS combines many LSA’s into fewer LSP’s
 More efficient due to fewer LSP types
o IS-IS detects failures faster than OSPF
o Easy to extend type, length, value mechanism
 This makes supporting new protocols(IPv6) very easy
o CLNS addresses are assigned to the entire host, instead of just an interface
IS-IS Addressing
 Router addresses are called NSAPs. (network service access point)
 NSEL (NSAP Selector)
o When NSEL is set to 00, the NSAP is called a NET (network entity title)
o This must be two digits!
 Cisco’s NSAP structure:
ISO 10589
o Three fields
o Area ID/Address
 AFI/IDP/IDI
 Authority and Format Identifier
 Initial Domain Part
 Initial Domain Identifier
 39=country code, 49=local/private (similar to 1918)
 High Order DSP (domain specific part)
 Area ID: same as OSPF area ID
o System ID
 6 bytes long (generally the MAC address of any interface on the router
o NSEL
Routing levels and Router Types
 L1 routing – intra area
o L1 routers route to ES
o System IDs need to be locally significant
o L1 routers need to be in same area to form adjacencies
 L2 routing – inter area
o L2 routers route between areas
o System IDs need to be globally significant
o L2 routers do not need to be in the same area to form adjacencies
o Exchange prefix information between areas
o Summarization occurs at L2 routers
o L2 only routers are similar to Backbone routers in OSPF
 L1/L2 routers
o Equivalent to an ABR in OSPF
 L0 routing routes between ES-IS
 L1 routing intra area
 L2 routing inter area
 L3 routing inter domain
o OSI supports IDRP to accomplish L3 routing. Cisco does not support IDRP or L3 IS-IS
routing.
PDU Types
 Hello
 LSP
o LSPs are specific to the level of routing. L1/L2 routers will send L1 and L2 LSPs
separately, and keep separate L1/L2 LSDBs
 PSNP: partial sequence number packet
o PSNP is a request for an LSP or an ACK for the LSP being received.
 CSNP: complete sequence number packet
o Contains complete list of LSPs in a router’s database. These help to keep other routers
up to date
o CSNPs are used across point to point links
Hello Packets
 IIH – IS to IS hello packets used to communicate between routers
 ISH – IS Hello packets are hello packets sent to hosts from Routers
 ESH – End System Hello are hello packets sent by hosts
 IP hosts do not use ISH/ESH/IIH
 Hello/Dead timers on broadcast networks are 10/40
 DIS sends hellos every 3.3 seconds and CSNP every 10 seconds
Network Types
 Broadcast – default for LAN and multipoint WAN
o BCAST mode for multipoint WANs can cause some serious issues. Better to treat
Multipoint WANs as a series of PTP links
 Point to Point
Pseudonode/DIS
 DIS creates virtual router called pseudonode
 All routers on segment form adjacency with the pseudonode
 Only one DIS per segment. No backup
 If router comes online with a higher priority, it overrides current DIS
o Default priority 64
o Ties are broken with highest DLCI or MAC
Metrics
 Narrow Metrics = 6 bit interface, 10 bit path
 Wide Metrics = 24 bit interface, 32 bit path
 Cisco IOS 12.0+ uses wide metrics
o Older routers and other vendor equipment may have issues working with wide metrics
 Interface metric is always set to 10 on Cisco routers. This can result in sub-optimal routing
o Changed with isis metric # interface command.
o Verify with show clns interface command
 Seed metric for ISIS is 0
Configuration examples
! entering routing process. No AS or process ID needed
R1(config)# router isis
! configuring a CLNS address for the host. 49 creates private domain
! 0001 is the area ID (area 1)
! 1111.1111.1111 is the system ID. This will generally be a MAC address of an interface on the router
! the trailing 00 is the NSEL making this a NET address. This must be 2 digits!
R1(config-router)# net 49.0001.1111.1111.1111.00
! We are going to tell the router to act as a L1 router globally. We can change the interfaces on an
individual basis
! Cisco defaults to L1/L2 globally
R1(config-router)# isis-type level-1
! If this is an ABR, we can summarize an IP network here
R1(config-router)# summary-address network mask
! we will now enable ISIS on an interface and change its level
R1(config-router)#int s0/0
R1(config-if)#ip router isis ! interface must be configured with IP or for IP unnumbered before this takes.
R1(config-if)#isis circuit-type level-2
! to change priority or metric on interface, use the following
R1(config-if)# isis metric # ! maximum can be used in place of #. This will cause other routers to
exclude this interface from their SPF
R1(config-if)# isis priority # ! this can be from 0 to 127