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Routing & Switching · Advanced

Type-Length-Value (TLV)

An extensible encoding of a type code, length and value used by IS-IS, LLDP, BGP and other protocols.

Definition

IS-IS carries all routing information as TLVs, for example TLV 22 (extended IS reachability), TLV 135 (extended IP reachability) and TLV 236 (IPv6 reachability). New features are added through new TLVs or sub-TLVs, such as segment routing SIDs, without changing the base protocol.

Example

TLV 137 carries the dynamic hostname.

Exam tip

TLV extensibility is why IS-IS adapted easily to IPv6 and segment routing.

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