IP Prefix Lists
IP prefix lists accommodate mechanisms to bout two apparatus of an IP route:
■ The avenue prefix (the subnet number)
■ The prefix breadth (the subnet mask)
The redistribute command cannot anon advertence a prefix list, but a avenue map can accredit to a
prefix account by application the bout command.
A prefix account itself has agnate characteristics to a avenue map. The account consists of one or more
statements with the aforementioned argument name. Anniversary account has a arrangement cardinal to acquiesce abatement of
individual commands, and admittance of commands into a accurate arrangement position. Each
command has a admittance or abjure action—but because it is acclimated alone for analogous packets, the
permit or abjure keyword aloof implies whether a avenue is akin (permit) or not (deny). The
generic command syntax is as follows:
ip prefix-list list-name [seq seq-value] {deny network/length | admittance network/
length}[ge ge-value] [le le-value]
The sometimes catchy and absorbing allotment of alive with prefix lists is that the acceptation of the
network/length, ge-value, and le-value ambit changes depending on the syntax. The network/
length ambit ascertain the ethics to use to bout the avenue prefix. For example, a network/length
of 10.0.0.0/8 agency “any avenue that begins with a 10 in the aboriginal octet.” The ge and le options are
used for allegory to the prefix length—in added words, to the cardinal of bifold 1s in the subnet
mask. For instance, ge 20 le 22 matches alone routes whose masks are /20, /21, or /22. So, prefix
list argumentation can be abbreviated into a two-step allegory action for anniversary route:
1. The route’s prefix charge be aural the ambit of addresses adumbrated by the prefix-list command’s
network/length parameters.
2. The route’s prefix breadth charge bout the ambit of prefixes adumbrated by the prefix-list
command.
The potentially catchy allotment of the argumentation relates to alive the ambit of prefix lengths arrested by
this logic. The ambit is authentic by the ge-value and le-value parameters, which angle for greaterthan-
or-equal-to and less-than-or-equal-to. Table 10-4 formalizes the logic, including the default
values for ge-value and le-value. In the table, agenda that conf-length refers to the prefix length
configured in the network/prefix (required) parameter, and route-length refers to the prefix length
of a avenue actuality advised by the prefix list.
306 Affiliate 10: IGP Avenue Redistribution, Avenue Summarization, and Absence Routing
Several examples can absolutely advice attach bottomward prefix account logic. The afterward routes will be examined
by a array of prefix lists, with the routes numbered for easier reference:
1. 10.0.0.0/8
2. 10.128.0.0/9
3. 10.1.1.0/24
4. 10.1.2.0/24
5. 10.128.10.4/30
6. 10.128.10.8/30
Next, Table 10-5 shows the after-effects of seven altered one-line prefix lists activated to these six
example routes. The table lists the analogous ambit in the prefix-list commands, abbreviating the
first allotment of the commands. The table explains which of the six routes would bout the listed prefix
list, and why.
Table 10-4 LE and GE Ambit on IP Prefix List, and the Adumbrated Ambit of Prefix Lengths
Prefix Account Ambit Ambit of Prefix Lengths
Neither conf-length = route-length
Only le conf-length <= route-length <= le-value
Only ge ge-value <= route-length <= 32
Both ge and le ge-value <= route-length <= le-value
Table 10-5 Archetype Prefix Lists Activated to the Account of Routes
prefix-list Command
Parameters
Routes
Matched Results
10.0.0.0/8 1 Without ge or le configured, both the prefix (10.0.0.0)
and breadth (8) charge be an exact match.
10.128.0.0/9 None Without ge or le configured, the prefix (10.128.0.0)
and breadth (9) charge be an exact match, alone the second
route in the account is akin by this prefix list.
10.0.0.0/8 ge 9 2–6 The 10.0.0.0/8 agency “all routes whose aboriginal octet is
10,” finer apery an abode range. The
prefix breadth charge be amid 9 and 32, inclusive.
10.0.0.0/8 ge 24 le 24 3, 4 The 10.0.0.0/8 agency “all routes whose aboriginal octet is
10,” and the prefix ambit is 24 to 24—meaning only
routes with prefix breadth 24.
Route Maps, Prefix Lists, and Authoritative Ambit 307
Administrative Distance
A distinct router can apprentice routes application assorted IP acquisition protocols, as able-bodied as via affiliated and
static routes. When a router learns a accurate avenue from assorted sources, the router cannot use
the metrics to actuate the best route, because the metrics are based on altered units. So, the
router uses anniversary route’s authoritative ambit (AD) to actuate which is best, with the lower
number actuality better. Table 10-6 lists the absence AD ethics for the assorted acquisition sources.
prefix-list Command
Parameters
Routes
Matched Results
10.0.0.0/8 le 28 1–4 The prefix breadth needs to be amid 8 and 28,
inclusive.
0.0.0.0/0 None 0.0.0.0/0 agency “match all prefixes, with prefix length
of absolutely 0.” So, it would bout all routes’ prefixes,
but none of their prefix lengths. Alone a absence route
would bout this prefix list.
0.0.0.0/0 le 32 All The ambit adumbrated by 0.0.0.0/0 is all IPv4 addresses.
The le 32 again implies any prefix breadth amid 0 and
32, inclusive. This is the syntax for “match all” prefix
list logic.
Table 10-6 Authoritative Distances
Route Blazon Authoritative Distance
Connected 0
Static 1
EIGRP arbitrary avenue 5
EBGP 20
EIGRP (internal) 90
IGRP 100
OSPF 110
IS-IS 115
RIP 120
EIGRP (external) 170
iBGP 200
Unreachable 255
Table 10-5 Archetype Prefix Lists Activated to the Account of Routes (Continued)
The defaults can be afflicted by application the ambit command. The command differs amidst all
three IGPs covered in this book. The all-encompassing versions of the ambit router subcommand for RIP,
EIGRP, and OSPF, respectively, are as follows:
distance distance
distance eigrp internal-distance external-distance
distance ospf {[intra-area dist1] [inter-area dist2] [external dist3]}
As you can see, EIGRP and OSPF can set a altered AD depending on the blazon of avenue as well,
whereas RIP cannot. You can additionally use the ambit command to set a router’s appearance of the AD per
route, as is covered after in this chapter.