The Update Message and BGP Attributes

The UPDATE message is used to transfer routing intelligence. Its format is shown in Figure 116. The UPDATE message may advertise routes, withdraw routes, or both.

Figure 11-6. The UPDATE Message

Unieasibte Routes Length {£ octets)

Withdrawn Routes (variable)

Total Path Attribute Length (2 octets)

Path Attributes (variable)

Network Layer Reachability Information [variable)

The UPDATE message begins with the withdrawn-routes length, which may be zero, in which case no routes are withdrawn. Otherwise, the withdrawn-routes length contains a number of <length,prefix> triples, with length being one octet, and indicates the number of octets in the prefix field. A length of zero matches all IP addresses, in which case the prefix's field is of zero length. In all other cases, the prefix field contains an IP address prefix, padded with trailing bits so that the field ends on an octet boundary.


Most network protocols pad related fields so that they are located on an octet or byte boundary. This allows for more efficient processing by modern microprocessors, which have instruction sets optimized for operating on single or multiple byte-size chunks.

The Total Path Attribute Length field sizes the path attributes that will follow. As shown in Figure 11-7, each path attribute consists of an Attribute Flag's octet, followed by an Attribute Type Code octet, and finally the attribute information itself.

Figure 11-7. The Format of the AS-PATH Attribute

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