Tag Archives: ISIS

3.8 ISIS

isis cfg

from: http://www.cisco.com/c/en/us/support/docs/ip/integrated-intermediate-system-to-intermediate-system-is-is/13795-is-is-ip-config.html#t1

R1#sh clns neigh
System Id      Interface   SNPA                State  Holdtime  Type Protocol
R2                 Fa0/0       ca01.d6c8.0008  Up     9               L1L2 IS-IS

Since R1 and R2 are configured with default configurations, they send and receive both L1 and L2 hellos, and form an adjacency type L1L2.

R2#sh clns neigh
System Id  Interface   SNPA                State     Holdtime  Type Protocol
R1             Fa0/0       ca00.d6c8.0008      Up     26            L1L2 IS-IS
R3             Se1/0       *HDLC*                   Up     23            L1L2 IS-IS

The adjacency explanation holds true for the serial interface also.

R2#sh run int f0/0 | b inter
interface FastEthernet0/0
ip address 172.16.12.2 255.255.255.0
ip router isis

R2#sh run int s1/0 | b S
interface Serial1/0
ip address 172.16.23.2 255.255.255.0
ip router isis

R2

router isis
net 49.0001.1720.1600.2002.00
passive-interface Loopback0

R1#sh clns int f0/0
FastEthernet0/0 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 9 seconds  !!!!ISs discover ESs by listening to ESHs, and ISs send ISHs to ESs
Routing Protocol: IS-IS
Circuit Type: level-1-2
Interface number 0x0, local circuit ID 0x1
Level-1 Metric: 10, Priority: 64, Circuit ID: R2.01
DR ID: R2.01
Level-1 IPv6 Metric: 10
Number of active level-1 adjacencies: 1
Level-2 Metric: 10, Priority: 64, Circuit ID: R2.01
DR ID: R2.01
Level-2 IPv6 Metric: 10
Number of active level-2 adjacencies: 1
Next IS-IS LAN Level-1 Hello in 8 seconds
Next IS-IS LAN Level-2 Hello in 9 seconds

DIS designated intermediate system

Since the Metric/Priority are the same for both routers in L1/L2, the tiebreaker for the DIS is the highest Subnetwork Points of Attachment (SNPA) address on the LAN segment. The SNPA address refers to the data link address, and in this case is the MAC address.

R1#sh int f0/0
FastEthernet0/0 is up, line protocol is up
Hardware is i82543 (Livengood), address is ca00.d6c8.0008 (bia ca00.d6c8.0008)

R2#sh int f0/0
FastEthernet0/0 is up, line protocol is up
Hardware is i82543 (Livengood), address is ca01.d6c8.0008 (bia ca01.d6c8.0008)

Notice that the DIS is elected for both levels, and that no backup DIS exists, as with Open Shortest Path First (OSPF), which has a backup Designated Router (DR).

notice:

  • Circuit Type: L1L2
  • L1 and L2 metrics and priorities are at default values: 10 and 64
  • L1 and L2 adjacencies: 1 (from R1 perspective on the Ethernet interface – it is R2 only)
  • IS-IS LAN hellos for L1 and L2 (Next ESH/ISH)
  • Maximum Transmission Unit (MTU): 1497. This is because the Open Systems Interconnection (OSI) IS-IS header is encapsulated inside a 3 byte 802.2 header.

R2#sh isis database

IS-IS Level-1 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R1.00-00              0x00000006   0xAFBA        829               0/0/0
R2.00-00            * 0x00000008   0x10A0        638               0/0/0
R2.01-00            * 0x00000005   0x32E2        806               0/0/0
R3.00-00              0x00000006   0x4CC9        731               0/0/0
IS-IS Level-2 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R1.00-00              0x00000009   0x51E8        931               0/0/0
R2.00-00            * 0x0000000A   0x0101        985               0/0/0
R2.01-00            * 0x00000005   0xC1DB        521               0/0/0
R3.00-00              0x00000007   0x1FD5        755               0/0/0

The LSP-ID, R1.00-00, can be broken down into three sections: R1/00/00

  • R1 = system ID
  • 00 = non-zero value for the pseudonode. Notice R2.01-00 is the pseudonode LSP.
  • 00 = fragment number. In this case, there are only fragment numbers of 00, which indicates that all the data fit into this LSP fragment, and there was no need to create more fragments. If there had been information that did not fit into the first LSP, IS-IS would have created more LSP fragments, such as 01, 02, and so on.

The * denotes the LSPs that were generated by this router, the router that the show command was issued on. Also, since this router is an L1 and L2 router, it contains an L1 and L2 database.

R2#sh isis database R2.00-00 detail

look at the generated lsp:
IS-IS Level-1 LSP R2.00-00
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R2.00-00            * 0x00000009   0x0EA1        1199              0/0/0
Area Address: 49.0001
NLPID:        0xCC
Hostname: R2
IP Address:   172.16.2.2
Metric: 10         IP 172.16.12.0 255.255.255.0
Metric: 10         IP 172.16.23.0 255.255.255.0
Metric: 0          IP 172.16.2.0 255.255.255.0
Metric: 10         IS R2.01
Metric: 10         IS R3.00

IS-IS Level-2 LSP R2.00-00
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R2.00-00            * 0x0000000A   0x0101        763               0/0/0
Area Address: 49.0001
NLPID:        0xCC
Hostname: R2
IP Address:   172.16.2.2
Metric: 10         IS R2.01
Metric: 10         IS R3.00
Metric: 10         IP 172.16.1.0 255.255.255.0
Metric: 0          IP 172.16.2.0 255.255.255.0
Metric: 10         IP 172.16.3.0 255.255.255.0
Metric: 10         IP 172.16.12.0 255.255.255.0
Metric: 10         IP 172.16.23.0 255.255.255.0

output shows that the loopback address of this router is advertised with a value of 0. This is because the loopback is advertised with a passive-interface command under the router IS-IS process, and the loopback interface by itself is not enabled for IS-IS. All other IP prefixes have a value of 10, which is the default cost on the interfaces running IS-IS.

 

3.8.c Describe network types, levels and router types

■ Level 0 routing: Routing between two ES nodes on the same link, or between an ES
node and its nearest IS
■ Level 1 routing: Routing between ES nodes in a single area of a domain
■ Level 2 routing: Routing between ES nodes in different areas of a domain
■ Level 3 routing: Routing between ES nodes in different domains

is-is_levels

3.8.e Describe optimization features

To minimize the number of adjacencies, LSDBs, and related SPF and PRC computations that are performed, it is recommended that you have configured all Level 1 routers as Level 1 by using the is-type command . We recommend that you use the metric-style wide command because some features, such as setting prefix tags and MPLS traffic engineering, require that routers that are running IS-IS generate the new-style TLVs that have wider metric fields. If you use the default narrow metric style for IS-IS, the router generates and accepts old-style type, length, and value objects (TLVs).

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 4305-4309).  . Kindle Edition.

http://www.itcertnotes.com/2012/03/tuning-and-optimizing-is-is.html

3.8.e Describe optimization features

3.8.e [i] Metrics, wide metric

Cisco IOS Software allows for wide metrics with the support of a 24-bit metric field. Using the new metric style, link metrics now have a maximum value of 16777215 (2 24-1) with a total path metric of 4261412864 (254 x 2 24). Deploying IS-IS in the IP network with wide metrics is recommended to enable finer granularity and to support future applications such as Traffic Engineering.

Running different metric styles within one network poses a serious problem: Link-state protocols calculate loop-free routes because all routers (within one area) calculate their routing table based on the same link- state database. This principle is violated if some routers look at old-style (narrow), and some at new-style (wider) TLVs. However, if the same interface cost is used for both the old- and new-style metrics, then the SPF will compute a loop-free topology.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 4317-4318).  . Kindle Edition.

http://www.cisco.com/en/US/products/ps6599/products_white_paper09186a00800a3e6f.shtml

3.8.d Describe operations

From a high level, IS-IS operates as follows:

● Routers running IS-IS will send hello packets out all IS-IS-enabled interfaces to discover neighbors and establish adjacencies.

● Routers sharing a common data link will become IS-IS neighbors if their hello packets contain information that meets the criteria for forming an adjacency. The criteria differ slightly depending on the type of media being used (p2p or broadcast). The main criteria are matching authentication, IS-type and MTU size.

● Routers may build a link-state packet (LSP) based upon their local interfaces that are configured for IS-IS and prefixes learned from other adjacent routers.

● Generally, routers flood LSPs to all adjacent neighbors except the neighbor from which they received the same LSP. However, there are different forms of flooding and also a number of scenarios in which the flooding operation may differ.

● All routers will construct their link-state database from these LSPs.

● A shortest -path tree (SPT) is calculated by each IS, and from this SPT the routing table is built.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 4288-4303).  . Kindle Edition.

http://www.cisco.com/c/en/us/td/docs/ios/12_2/ip/configuration/guide/1cfisis.html