Isis packet type:- LSP

This is similar to  LSA used in ospf, and is used to exchange routing information.

PDU Length:- This is the length of the lsp in bytes

Remaining Lifetime:- The life time of the LSP in seconds, by default it is 1200 seconds and starts from the highest to the lowest.

LSP ID:- This is the system id+Pseudonode id +number of LSP

Sequence number:- Sequence number of the LSP.

ATT bit:- This is Set when the router is connected to two areas this also known as attachment bit

Checksum:- This sets the checksum.

Partition bit:- This is not used

Overload bit:- If set the receiving router will not use this router for transit this means the originating router has some memory issues

Type of IS:- This indicates the type for the originating router Level 1 or L2.

The Tlv makes isis extendable

Type contains the type present in the value field

Length contains the length of the value field

Value is the data portion of the packet

Area address Tlv

IS neighbor tlv

Interface address tlv :- The ip interfaces of the originating router.

Authentication tlv are the common tlv

IP internal reachability TLV:-  This contains the ip address/masks which are directly connected with this router this also contained the metrics associated with the prefix.

IS reachability:- These contains the neighbors for the originating router and the metrics associated with these neighbors.

A L2 LSP packet also contains the following TLVs-

•  IP External reachability: This TLV lists the IP addresses/ masks external to the routing domain, which can be reached via one of the originating router's interface.

• Inter-domain Routing Protocol Information: This TLV allows L2 LSP to transparently carry information from external routing protocols through IS-IS domain.