Information Technology Reference
In-Depth Information
This prefix list is also provided in Chapter 6, with a detailed explanation of each prefix.
The second classification is prefix blocks that IANA has not yet allocated to a registry for
assignment. It is very common to see these prefixes advertised into the global BGP table;
however, they are invalid. This list changes periodically as IANA allocates blocks to ARIN,
APNIC, and RIPE. The current allocation status for the IPv4 address space is available at
www.iana.org.
A sample bogon list is not presented here because of the list's dynamic nature. If an ISP
will filter bogon prefixes, which is recommended, the appropriate prefix list should be built
based on the allocation status from the IANA website.
Graded Route Flap Dampening
The BGP route dampening feature was discussed in Chapter 3, “Tuning BGP Performance.”
The default configuration for BGP dampening is a “flat and gentle” approach. All prefixes
are treated equally, regardless of the prefix or its length. The reality, however, is that all pre-
fixes are not equal and should not be treated equally. An /8 represents far more hosts than a
/24. Even though a 60-minute maximum suppression time for a /24 might be reasonable,
this same period might be unacceptable for an /8.
In addition, some prefixes are essential to the Internet's operation. These prefixes are for the
DNS root servers. If access to the DNS root servers is lost, all name resolution fails, effec-
tively cutting off Internet access. RIPE has made specific recommendations for deploying
BGP route dampening that are sensitive to prefix length and the DNS system. The RIPE
recommendation is RIPE-229. Although the specific address assignment of the DNS root
servers is very stable, before the root server addresses are excluded from the dampening
process, they should be verified on the RIPE website (www.ripe.net). The configuration is
shown in Example 9-12.
Example 9-12
Graded BGP Route Dampening Configuration
router bgp 100
...
bgp dampening route-map graded-dampening
...
!
route-map graded-dampening deny 10
match ip address prefix-list ROOTSERVERS
!
route-map graded-dampening deny 20
match ip address prefix-list GTLDSERVERS
!
