D-Link DGS-3324SR Product Manual - Page 188
Link State Advertisement Header
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xStack DGS/DXS-3300 Series Layer 3 Stackable Gigabit Ethernet Switch User Manual Link State Advertisement Header All link state advertisements begin with a common 20-byte header. This header contains enough information to uniquely identify the advertisements (Link State Type, Link State ID, and Advertising Router). Multiple instances of the link state advertisement may exist in the routing domain at the same time. It is then necessary to determine which instance is more recent. This is accomplished by examining the link state age, link state sequence number and link state checksum fields that are also contained in the link state advertisement header. The format of the Link State Advertisement Header is shown below: Link-State Advertisement Header Link-State Age Options Link-State Type Link-State ID Advertising Router Link-State Sequence Number Link-State Checksum Length Figure 6- 135. Link State Advertisement Header Field Description Link State Age The time is seconds since the link state advertisement was originated. Options The optional capabilities supported by the described portion of the routing domain. Link State Type The type of the link state advertisement. Each link state type has a separate advertisement format. The link state type are as follows: Router Links, Network Links, Summary Link (IP Network), Summary Link (ASBR), AS External Link. Link State ID This field identifies the portion of the internet environment that is being described by the advertisement. The contents of this field depend on the advertisement's Link State Type. Advertising Router The Router ID of the router that originated the Link State Advertisement. For example, in network links advertisements this field is set to the Router ID of the network's Designated Router. Link State Sequence Number Detects old or duplicate link state advertisements. Successive instances of a link state advertisement are given successive Link State Sequence numbers. Link State Checksum The Fletcher checksum of the complete contents of the link state advertisement, including the link state advertisement header by accepting the Link State Age field. Length The length in bytes of the link state advertisement. This includes the 20byte link state advertisement header. 173