Designated Router

Designated Router
For broadcast multi-access networks, IS-IS, like OSPF, supports the election of a designated
router. The difference is that in IS-IS this designated router is known as a
designated IS (DIS)
.
The DIS reduces the amount of traffic required for all routers to advertise their links over broadcast
multi-access networks and the amount of traffic required to flood LSPs. The DIS advertises
a pseudonode. The pseudonode is a representation of the network all the routers are connected
to. The DIS appears in the link-state database as another router. Each router on that network
will then form one adjacency with the pseudonode.
The DIS assigns a one-octet pseudonode ID to the broadcast multi-access network, which
is then added to the system ID of the DIS to create the LAN ID. The LAN ID will be the source
of the LSPs for the pseudonode in the link-state database. Even though the DIS has its own
set of LSPs, it still generates the LSPs for the pseudonode.
One of the key differences between the OSPF DR and the IS-IS DIS is how they handle the
flooding of the LSPs. In OSPF, a router will form an adjacency only with a DR and a BDR,
which are then responsible for making sure all of the routers get the LSAs from the DR. The
BDR is there in case this goal cannot be achieved.
This is not true with the IS-IS DIS. Routers in an area will form an adjacency with the DIS,
but the routers will still form adjacencies with each other. Each router will multicast LSPs to its
neighbors. The main function of the DIS is to make sure the routers receive all the LSPs. This
is accomplished through the use of sequence number PDUs (SNPs). The use of SNPs will be covered
in detail in the upcoming section, “LSP Flooding.”
Another important concept for you to understand about the DIS’s use in IS-IS is the fact that there
can be more than one DIS. Remember how there are Level 1 and Level 2 adjacencies? Well, there are
Level 1 and Level 2 DISs also. If Level 1 and Level 2 areas exist in the same broadcast multi-access
network, a DIS will be elected for each level. A DIS is not elected if it is a point-to-point network.
To make it even more interesting, the same router could end up filling the role as both the L1 DIS
and the L2 DIS. Each of the pseudonodes created would be independent of each other.
Like OSPF, IS-IS has a process for electing the DIS. The first item taken into consideration
when electing the DIS is the router priority. The router priority can be manually set to any value
between 0 and 127. A router with the priority of 0 will never be elected. The default priority for
Cisco devices is 64. The router with the highest router priority will win. Also, if a router is an
L1/L2, you can manually set the router priority for the L1 portion to a value different than the
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Chapter 7 
Integrated IS-IS
router’s L2 priority. If more than one router is found with the highest router ID, the highest system
ID will be used to break the tie. In order to manually set the router priority for Level 1, you
need to enter the following command in interface configuration mode:
isis priority value level-1
If you wanted to set the router priority for Level 2, you would use the same command but
replace the Level-1 keyword with the Level-2 keyword.
In order to see what the current router priority setting is for an IS-IS interface, use the show
clns interface command. Here’s a sample output:
plano#show clns interface
FastEthernet2/0 is up, line protocol is up
CLNS protocol processing disabled
Serial2/0 is up, line protocol is down
CLNS protocol processing disabled
Serial2/0.1 is down, line protocol is down
Checksums enabled, MTU 1500, Encapsulation FRAME-RELAY
ERPDUs enabled, min. interval 10 msec.
CLNS fast switching disabled
CLNS SSE switching disabled
DEC compatibility mode OFF for this interface
Next ESH/ISH in 33 seconds
Routing Protocol: IS-IS
Circuit Type: level-1-2
Interface number 0x0, local circuit ID 0x100
Level-1 Metric: 10, Priority: 127, Circuit ID: plano.00
Number of active level-1 adjacencies: 0
Level-2 Metric: 10, Priority: 0, Circuit ID: plano.00
Number of active level-2 adjacencies: 0
Next IS-IS Hello in 5 seconds
Serial2/0.16 is down, line protocol is down
CLNS protocol processing disabled
Serial2/0.17 is down, line protocol is down
CLNS protocol processing disabled
FastEthernet2/1 is up, line protocol is up
CLNS protocol processing disabled
Serial2/1 is administratively down, line protocol is down
CLNS protocol processing disabled
Loopback0 is up, line protocol is up
CLNS protocol processing disabled
Loopback1 is up, line protocol is up
CLNS protocol processing disabled
Integrated IS-IS Operation 215
As you can deduce from the preceding output, Plano has only one interface configured for
IS-IS. The Level 1 priority is set to 127. This means that more than likely this interface will
become the DIS for the Level 1 area. The Level 2 priority is set to 0, which means this interface
will never become the DIS for the Level 2 area.
There are a couple of notes you should make about the differences between OSPF DRs and
IS-IS DISs. The first is the fact that IS-IS does not elect a backup designated router like OSPF
does. The second, and in my opinion the most interesting, is the difference in the way IS-IS and
OSPF handle the situation when a router comes online with a higher router priority than the
current designated router. If this occurs in OSPF, nothing will happen. However, IS-IS will
allow for pre-empting the DIS. This means that if a router comes online that has a higher router
priority than the current DIS, the new router will become the DIS. A new LAN ID will be created,
and new LSPs reflecting the new DIS will be generated.