Is-Is Overview; Chapter 9 Configuring I-I - Cisco Nexus 7000 Series Configuration Manual

Information about IS-IS
S e n d d o c u m e n t c o m m e n t s t o n e x u s 7 k - d o c f e e d b a c k @ c i s c o . c o m .
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IS-IS Overview

IS-IS sends a
packet contains information, such as the authentication, area, and supported protocols, which the
receiving interface uses to determine compatibility with the originating interface. The hello packets are
also padded to ensure that IS-IS establishes adjacencies only with interfaces that have matching
maximum transmission unit (MTU) settings. Compatible interfaces form adjacencies, which update
routing information in the link-state database through link-state update messages (LSPs). By default, the
router sends a periodic LSP refresh every 10 minutes and the LSPs remain in the link-state database for
20 minutes (the LSP lifetime). If the router does not receive an LSP refresh before the end of the LSP
lifetime, the router deletes the LSP from the database.
The LSP interval must be less than the LSP lifetime or the LSPs time out before they are refreshed.
IS-IS sends periodic hello packets to adjacent routers. If you configure transient mode for hello packets,
these hello packets do not include the excess padding used before IS-IS establishes adjacencies. If the
MTU value on adjacent routers changes, IS-IS can detect this change and send padded hello packets for
a period of time. IS-IS uses this feature to detect mismatched MTU values on adjacent routers. See the
"Configuring the Transient Mode for Hello Padding" section on page
IS-IS Areas
You can design IS-IS networks as a single area that includes all routers in the network or as multiple
areas that connect into a backbone or Level 2 area. Routers in a nonbackbone area are Level 1 routers
which establish adjacencies within a local area (intra-area routing). Level 2 area routers establish
adjacencies to other Level 2 routers and perform routing between Level 1 areas (inter-area routing). A
router can have both Level 1 and Level 2 areas configured. These Level 1/Level 2 routers act as area
border routers which route information from the local area to the Level 2 backbone area (see
Within a Level 1 area, routers know how to reach all other routers in that area. The Level 2 routers know
how to reach other area border routers and other Level 2 routers. Level 1/Level 2 routers straddle the
boundary between two areas, routing traffic to and from the Level 2 backbone area. Level1/Level2
routers use the attached (ATT) bit signal Level 1 routers to set a default route to this Level1/Level2 router
to connect to the Level 2 area.
In some instances, such as when you have two or more Level1/Level 2 routers in an area, you may want
to control which Level1/Level2 router that the Level 1 routers use as the default route to the Level 2 area.
You can configure which Level1/Level2 router sets the attached bit. See the
Configuration" section on page
Cisco Nexus 7000 Series NX-OS Unicast Routing Configuration Guide, Release 4.x
9-2
IS-IS Authentication, page 9-3
Mesh Groups, page 9-4
Overload Bit, page 9-4
Route Summarization, page 9-4
Route Redistribution, page 9-5
Load Balancing, page 9-5
Virtualization Support, page 9-5
High Availability and Graceful Restart, page 9-5
Multiple IS-IS Instances, page 9-5
hello packet out every configured interface to discover IS-IS neighbor routers. The hello
9-28.
Chapter 9 Configuring IS-IS
9-17.
Figure
"Verifying IS-IS
OL-20002-02
9-1).