Like other routing and signaling protocols IS-IS has built-in automatic neighbor discovery mechanism which is known by IIHs (IS-IS Hello PDUs), because IS-IS is not IP based protocol the IIH PDUs and all other IS-IS PDUs are directly encapsulated on the data-link layer.
IS-IS has two hierarchical levels (L1 and L2) and two network types (Point-to-Point and Broadcast) so it’s essential to have different types of hello messages or PDUs for serve these networks since the data-link layer for broadcast and P2P links have different style of addressing so they need different treatment, due to this differences there are three IIH PDUs and they are identified in the IS-IS PDU Type field in the IS-IS 8-bit common header that is attached to all the IS-IS messages as the following:
Level-1 LAN hello (PDU type 15)
Level-2 LAN hello (PDU type 16)
P2P hello (PDU type 17)
Each of which acting slightly different, Level1 and Level-2 LAN hellos or PDUs are used in broadcast network, whereas P2P hello is used in P2P links (P2P hellos can be used on Ethernet links in some circumstance).
once the IS-IS demon get enabled each router builds its IIH for the supported levels and send them out all the IS-IS capable interfaces (ISO enabled Interfaces in JunOS and CLNS enabled interfaces in IOS), these hellos or PDUs are carrying the desired information to negotiate the protocol parameters, these parameters include:
PDU Type field
Maximum area address field
Circuit type field
Holding time field
Source ID field
DIS LAN-ID field
Other information are also carried in the IIH PDU but they are irrelevant to our discussion here.
1-PDU Type field is 1-byte length field that identifies the type of PDU that the IS-IS message is carrying.
2-Maximum area address field carries the configured areas (maximum 3 areas)
3-Circuit type field carries the circuit type configured on the interface, 1 for L1 2 for L2 or 3 for L1/L2, this field is also important for the P2P circuits because there is only one P2P IIH type, so the circuit type is used to identify which Level holding time to reset.
4-Source ID field carries the System-ID of the origin router.
5-Hold time field, this field used by the origin router to instructs the receiving router after how many seconds the receiver can declare that the adjacency with the origin router is down, this timer is also used to calculate the interval of the hello PDUs (usually holding time/3=hello interval)
6-Priority field, value 0-127 used in DIS election (default on IOS and JunOS to 64)
7-DIS LAN-ID, initially this field carries the origin system-id but once the adjacency is established this field will carry the DIS elected on the LAN.
IIH on Broadcast network.
When you enable IS-IS on a LAN interface the router builds the appropriate hello PDU (according to the Levels enabled on that interface L1, L2 or L1/2) and send it in regular interval, most implementations have L1/L2 enabled by default, however Level-1 LAN hellos send to multicast destination MAC address 01:80:c2:00:00:14, whereas Level-2 LAN hellos are send to multicast destination MAC address 01:80:c2:00:00:15, all routers running IS-IS on that segment including the local router are listing to this address, all IS-IS messages are send to these two multicast addresses, unicast communication will never happen between IS-IS routers, so once the router sends out the IIH all routers on the segment will receive the message, the tree-way hand-shake procedures will then followed to verify the communication between each two devices is working properly.
The IIH PDUs also used to verify MTU of the link by adding another TLV called Padding TLV, more details about the padding TLV and it’s function can be found in the IS-IS MTU Mismatch Post.
As example imagine that we have 3 routers JR1, JR2 and JR3 and they are running IS-IS on a GE interface which is connected to the same Ethernet segment, JR1 will build two IS-IS hellos (Level1 and Level2) and send them out each to the appropriate multicast destination MAC address and mark the interface with adjacency state NEW, R2 and R3 received JR1’s hellos and they will copy JR1’s MAC address from the layer 2 header of the received message and add it thier own hello message in a TLV called IS-Neighbor TLV (TLV#2) then they will send their own hellos out the GE interfaces, JR1 will receive this mesagge and will see his MAC address in the IS-Neighbor TLV (SeenSelf) so he will move the adjacency state from New to Initializing and generating a new IIH which will carry JR2 and JR3 MAC address in the IS-Neighbor TLV and change the adjacency state again to UP, JR1 and JR2 on the other hand will doing the same (SeenSelf then generates new IIH) and get the adjacency up.
JUNOS Level-1 and Level-2 LAN IIHs
mounir@JR1.core1> show log isis Apr 7 09:32:18 trace_on: Tracing to "/var/log/isis" started Apr 7 09:32:18.748497 Sending L1 LAN IIH on ge-0/0/0.0 Apr 7 09:32:18.748949 max area 0, circuit type l1l2 Apr 7 09:32:18.748974 hold time 27, priority 64, circuit id JR3.core3.02 Apr 7 09:32:18.748993 neighbor b0:c6:9a:23:7c:0 Apr 7 09:32:18.749009 neighbor b0:c6:9a:23:80:0 Apr 7 09:32:18.749025 speaks IP Apr 7 09:32:18.749039 speaks IPv6 Apr 7 09:32:18.749070 IP address 184.108.40.206 Apr 7 09:32:18.749096 area address 49.0001 (3) Apr 7 09:32:18.749115 restart RR reset RA reset holdtime 0 Apr 7 09:32:18.749135 packet length 62 Apr 7 09:32:18.749178 Sending L2 LAN IIH on ge-0/0/0.0 Apr 7 09:32:18.749196 max area 0, circuit type l1l2 Apr 7 09:32:18.749213 hold time 27, priority 64, circuit id JR3.core3.02 Apr 7 09:32:18.749229 neighbor b0:c6:9a:23:7c:0 Apr 7 09:32:18.749244 neighbor b0:c6:9a:23:80:0 Apr 7 09:32:18.749258 speaks IP Apr 7 09:32:18.749272 speaks IPv6 Apr 7 09:32:18.749293 IP address 220.127.116.11 Apr 7 09:32:18.749315 area address 49.0001 (3) Apr 7 09:32:18.749332 restart RR reset RA reset holdtime 0 Apr 7 09:32:18.749349 packet length 62 Apr 7 09:32:21.029220 Received L1 LAN IIH, source id JR3.core3 on ge-0/0/0.0 Apr 7 09:32:21.029338 intf index 68, snpa b0:c6:9a:23:80:0 Apr 7 09:32:21.029357 max area 0, circuit type l1l2, packet length 62 Apr 7 09:32:21.029375 hold time 9, priority 64, circuit id JR3.core3.02 Apr 7 09:32:21.029397 neighbor b0:c6:9a:23:78:0 (ourselves) Apr 7 09:32:21.029413 neighbor b0:c6:9a:23:7c:0 Apr 7 09:32:21.029428 speaks IP Apr 7 09:32:21.029446 speaks IPV6 Apr 7 09:32:21.029472 IP address 18.104.22.168 Apr 7 09:32:21.029496 area address 49.0001 (3 bytes) Apr 7 09:32:21.029513 restart flags  Apr 7 09:32:21.029566 updating neighbor JR3.core3 Apr 7 09:32:21.937635 ISIS L2 hello from JR3.core3 interface ge-0/0/0.0 absorbed Apr 7 09:32:22.696952 Received L2 LAN IIH, source id JR2.core2 on ge-0/0/0.0 Apr 7 09:32:22.697199 intf index 68, snpa b0:c6:9a:23:7c:0 Apr 7 09:32:22.697221 max area 0, circuit type l1l2, packet length 62 Apr 7 09:32:22.697240 hold time 27, priority 64, circuit id JR3.core3.02 Apr 7 09:32:22.697261 neighbor b0:c6:9a:23:78:0 (ourselves) Apr 7 09:32:22.697278 neighbor b0:c6:9a:23:80:0 Apr 7 09:32:22.697293 speaks IP Apr 7 09:32:22.697311 speaks IPV6 Apr 7 09:32:22.697337 IP address 22.214.171.124 Apr 7 09:32:22.697361 area address 49.0001 (3 bytes) Apr 7 09:32:22.697378 restart flags  Apr 7 09:32:22.697432 updating neighbor JR2.core2
IOS Level-1 and Level-2 LAN IIHs
Apr 10 05:25:04.551: ISIS-Adj: Sending L1 LAN IIH on GigabitEthernet0/1, length 1497 Apr 10 05:25:07.851: ISIS-Adj: Sending L2 LAN IIH on GigabitEthernet0/1, length 1497 Apr 10 05:25:07.851: ISIS-Adj: Sending L1 LAN IIH on GigabitEthernet0/1, length 1497 Apr 10 05:25:09.083: ISIS-Adj: Rec L2 IIH from 0002.fc3b.f400 (GigabitEthernet0/1), cir type L1L2, cir id 0000.0000.0005.01, length 1497 Apr 10 05:25:09.739: ISIS-Adj: Rec L1 IIH from 0005.3206.63c2 (GigabitEthernet0/1), cir type L1, cir id 0000.0000.0004.01, length 1497 Apr 10 05:25:11.031: ISIS-Adj: Sending L1 LAN IIH on GigabitEthernet0/1, length 1497 Apr 10 05:25:13.655: ISIS-Adj: Rec L1 IIH from 0002.fc3b.f400 (GigabitEthernet0/1), cir type L1L2, cir id 0000.0000.0005.01, length 1497
Once the three-way hand-shake process is done, the DIS election process will take place which will be discussed in the next post IS-IS DIS in Practice.