ISIS Packet type hello is used to discover neighbors and after the adjacency is established used as a keep-alive to maintain the adjacencies.
The first part is the isis hello packet header which is common to all the isis packet.
There are 2 types of hello packets lan hello packets and point to point hello packets.
Lan hello packets are further subdivided to L1 hello packets and l2 hello packets.
Circuit Type:- This indicates the type of router
01 :- L1 router
10:- L2 router
11:- L1/L2 router.
System ID:- This is the source through which the hello packet is received from
Holding Timer:- If the hello packet is not received till the holding timer expires the neighbor is brought down. In ALU the hellos' are sent every 9 seconds and the holding timer is 27 seconds.
PDU length:- This is the length of the hello packets.
Priority:- This is the priority of the Designated IS by default the priority is 64 and the highest priority that can be set is 127.
System ID :- This is the system id plus the pseudonode id
There are also other TLV's present in the hello packet
Area address:- This indicates the area address configured on the router in this case the area address configured is 49.0001 a maximum of 3 area address can be configured, this is usually required in the case of migration purposes.
IS neighbors TLV:- Present only in lan hellos l1 hellos contains the mac address of the l1 neighbors and l2 hellos contains the mac address of l2 neighbors this generally contains the mac address of the neighbors.
Protocols Supported:- This contains the protocols which are supported, for IP it is 81.
IP interface address:- This is the ip address through which the hello PDU is sent.
Padding:- Padding is done to increase the PDU to a minimum PDU.
Three way adjacency are down, init and up
There are 2 types of hello packets lan hello packets and point to point hello packets.
Lan hello packets are further subdivided to L1 hello packets and l2 hello packets.
Circuit Type:- This indicates the type of router
01 :- L1 router
10:- L2 router
11:- L1/L2 router.
System ID:- This is the source through which the hello packet is received from
Holding Timer:- If the hello packet is not received till the holding timer expires the neighbor is brought down. In ALU the hellos' are sent every 9 seconds and the holding timer is 27 seconds.
PDU length:- This is the length of the hello packets.
Priority:- This is the priority of the Designated IS by default the priority is 64 and the highest priority that can be set is 127.
System ID :- This is the system id plus the pseudonode id
There are also other TLV's present in the hello packet
Area address:- This indicates the area address configured on the router in this case the area address configured is 49.0001 a maximum of 3 area address can be configured, this is usually required in the case of migration purposes.
IS neighbors TLV:- Present only in lan hellos l1 hellos contains the mac address of the l1 neighbors and l2 hellos contains the mac address of l2 neighbors this generally contains the mac address of the neighbors.
Protocols Supported:- This contains the protocols which are supported, for IP it is 81.
IP interface address:- This is the ip address through which the hello PDU is sent.
Padding:- Padding is done to increase the PDU to a minimum PDU.
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