3Com MSR 50 Series, Configuration Manual

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882 C
HAPTER
57: IS-IS C
ONFIGURATION
routing information of other areas, the Level-1 router can make a better routing
choice for the packets destined outside the area.
IS-IS Network Type Network type
IS-IS supports two network types:
■
Broadcast network, such as Ethernet, Token-Ring.
■
Point-to-point network, such as PPP, HDLC.
n
For the Non-Broadcast Multi-Access (NBMA) network, such as ATM, you need to
configure point-to-point or broadcast network on its configured subinterfaces.
IS-IS does not run on Point to Multipoint (P2MP) links.
DIS and pseudonodes
On an IS-IS broadcast network, a router has to be selected as the Designated
Intermediate System (DIS).
The Level-1 and Level-2 DISs are selected respectively. You can assign different
priorities for different level DIS selections. The higher a router’s priority is, the more
likelihood the router becomes the DIS. If there are multiple routers with the same
highest DIS priority, the one with the highest SNPA (Subnetwork Point of
Attachment) address (which is the MAC address on a broadcast network) will be
selected. A router can be the DIS for different levels.
As shown in Figure 256 , the same level routers and non-DIS routers on the same
network segment can establish adjacencies. This is different from OSPF.
Figure 256 DIS in the IS-IS broadcast network
The DIS creates and updates pseudonodes as well as their LSP to describe all
routers on the network.
The pseudonode emulates a virtual node on the broadcast network. It is not a real
router. In IS-IS, it is identified by the system ID and one byte Circuit ID (a non zero
value) of the DIS.
Using pseudonodes can reduce LSPs, the resources used by SPF and simplify the
network topology.
n
On IS-IS broadcast networks, all routers are adjacent with each other. The DIS is
responsible for the synchronization of their LSDBs.
L1
L2
L1/L2 L1/L2
DIS DIS
L1 adjacencies
L2 adjacencies