Configuration Guide User guide

BigIron RX Series Configuration Guide 947
53-1002484-04
IS-IS overview
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FIGURE 127 An IS-IS network contains Intermediate Systems (ISs) and host systems
NOTE
Since the Brocade implementation of IS-IS does not route OSI traffic but instead routes IP traffic, IP
hosts are shown instead of ESs.
The other basic IS-IS concepts illustrated in this figure are explained in the following sections.
Domain and areas
IS-IS is an IGP, and thus applies only to routes within a single routing domain. However, you can
configure multiple areas within a domain. A device can be a member of one area for each Network
Entity Title (NET) you configure on the device. The NET contains the area ID for the area the NET is
in.
In Figure 127, Routers A, B, and C are in area 1. Routers D and E are in area 2. All the routers are in
the same domain.
Level-1 routing and Level-2 routing
You can configure an IS-IS router such as a device to perform one or both of the following levels of
IS-IS routing
1
:
• Level-1 – A Level-1 router routes traffic only within the area the router is in. To forward traffic to
another area, the Level-1 router sends the traffic to its nearest Level-2 router.
• Level-2 – A Level-2 router routes traffic between areas within a domain.
In Figure 127 on page 947, Routers A and B are Level-1s only. Routers C and D are Level-1 and
Level-2 ISs. Router E is a Level-1 IS only.
1. The ISO/IEC specifications use the spelling “routeing”, but this document uses the spelling
“routing” to remain consistent with other Brocade documentation.
Router A Router B
Router C
Router D
Router E
BGP4
IS-IS Area 1
IS-IS Area 2
IS-IS Routing Domain
IP
Host
IP
Host
IP
Host
An IS-IS routing domain can
contain multiple areas.
IS-IS routers route within an
area at Level-1.
IS-IS routers route between
areas at Level-2.