Specifications

10
Caveats for Cisco IOS Release 12.0
78-6455-12
IP Routing Protocols
• CSCdp71893
A (S,G) entry created by receipt of data on an interface will have a NULL outgoing interface list.
Unless the X-flag is set for this entry, the entry expiration time will not be updated and X-flag will
time out and then be recreated by the next data packet. This condition may increase the CPU usage
on routers that see multicast packets for a large number of sources. There is no workaround.
• CSCdr02975
The Cisco router has a CPU hog when there are a large number of link-state advertisement (LSA)
deletions. Two autonomous systems (AS) were accidentally merged. When they were restored, the
CPU hogged while removing 60,000 link-state advertisements (LSA). There is no workaround.
• CSCdr54230
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 (ASes) in the segment. The path segment value
contains the list of ASes (each AS is represented by two octets).
The total length of the attribute depends on the number of path segments and the number of ASes
in them. For example, if the AS_PATH contains only an AS_SEQUENCE, then the maximum
number of ASes (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 internal Border Gateway Protocol (BGP) 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 ASes, 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.
• CSCdr73140
After Cisco IOS software bootup, Open Shortest Path First (OSPF) may create a link-state
advertisement (LSA) for an interface that is in the “interface up, line protocol down” state. This
problem does not occur if you administratively shut down the interface at boot time and then later
bring it up.
Workaround: Administratively shut down the interface, save using the copy
system:running-config nvram:startup-config command, and reboot the router.
Alternate workaround: Exclude the interface address from OSPF by removing the network
statement or restricting your inverse mask.
• CSCdr79512
A summary link-state advertisement (LSA) with an abnormally low metric is generated by an area
border router (ABR). This situation may be spuriously generated. There is no workaround.