Datasheet

311
Release Notes for Cisco IOS Release 12.1E on the Catalyst 6500 and Cisco 7600 Supervisor Engine and MSFC
OL-2310-11
Caveats
• A Border Gateway Protocol (BGP) UPDATE contains Network Layer Reachability Information
(NLRI) and attributes that describe the path to the destination. Each path attribute is a type, length,
value (TLV) object.
The type is a two-octet field that includes the attribute flags and the type code. The fourth high-order
bit (bit 3) of the attribute flags is the Extended Length bit. It defines whether the attribute length is
one octet (if set to 0) or two octets (if set to 1). The extended length bit is used only if the length of
the attribute value is greater than 255 octets.
The AS_PATH (type code 2) is represented by a series of TLVs (or path segments). The path segment
type indicates whether the content is an AS_SET or AS_SEQUENCE. The path segment length
indicates the number of autonomous systems in the segment. The path segment value contains the
list of autonomous systems (each AS is represented by two octets).
The total length of the attribute depends on the number of path segments and the number of
autonomous systems in them. For example, if the AS_PATH contains only an AS_SEQUENCE, then
the maximum number of autonomous systems (without having to use the extended length bit) is 126
[= (255-2)/2]. If the UPDATE is propagated across an AS boundary, then the local Abstract Syntax
Notation (ASN) must be appended and the extended length bit used.
The caveat was caused by the mishandling of the operation during which the length of the attribute
was truncated to only one octet. Because of the internal operation of the code, the receiving border
router would not be affected, but its iBGP peers would detect the mismatch and issue a
NOTIFICATION message (update malformed) to reset their session.
The average maximum AS_PATH length in the Internet is between 15 and 20 autonomous systems,
so there is no need to use the extended length. The failure was discovered because of a malfunction
in the BGP implementation of another vendor. There is no workaround.
[Part of the text was taken from RFC 1771.] This problem is resolved in Release 12.1(3a)E3.
(CSCdr54230)
• When BGP sessions get reset, currently, with lob neighbor-changes, the event is errlogged. However,
to find out the reasons as to why there was a reset, one has to turn on the debugs. This fix will
automatically errlog the NOTIFICATION message when the sessions are reset. This feature will be
turned on by the same log neighbor-changes knob. (CSCdr54231)
Resolved Caveats in Release 12.1(5a)E3
Note Release 12.1(5a)E3 contains the Service Provider feature set image for Supervisor Engine 1 with
MSFC2 (c6sup12-psv-mz.121-5.E3).
• MBGP peer updates with extended attributes are a byte longer than updates without extended
attributes. The extra byte overwrites the next-hop data in the update, resulting in malformed updates.
This problem is resolved in Release 12.1(5a)E3. (CSCdt06780)
Resolved Caveats in Release 12.1(5a)E1
None.