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Chapter 7
BGP's Basic Components
Network Layer Reachability Information
T
he Network Layer Reachability Information (NLRI) lists the pre-
fixes that must be updated. One thing to understand is that all the prefixes
listed in this field must match all the attributes listed in the Path Attributes
field.
This means that more than one route can be withdrawn in the same
UPDATE message, but if you want to add a route, you must do so in
another UPDATE message. As opposed to the length of the overall With-
drawn Routes field, prefix lengths apply to specific routes. A length of zero
here implies the default route.
Each prefix in the NLRI field contains a one-octet prefix length and a
variable-length prefix, which does not necessarily contain an IP address.
NOTIFICATION Message
I
f an error occurs during a BGP session, a BGP NOTIFICATION mes-
sage is generated. As soon as the BGP speaker sends the NOTIFICATION
message, it immediately terminates its BGP connection. The administrator can
use this message to help troubleshoot why the connection was terminated.
There are two types of error codes in NOTIFICATION message fields to
watch for. These are the Error Code and Error Subcode, which are shown in
Figure 7.10.
F I G U R E 7 . 1 0
The NOTIFICATION message fields added to the BGP common header
Common
Header
Data
(length varies)
Error
Subcode
(1 octet)
Error
Code
(1 octet)
+
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