cisco bgp-4 command and configuration handbook

382 Pages·2001·55.58 MB·English

Cisco Systems

CCIE™ Professional Development

Cisco’ BGP-4 Command and
Configuration Handbook

The comprehensive, hands-on guide
to all Cisco 10S* Software BGP-4 commands

iscopress.com William R. Parkhurst, Ph.D., CCIE
Contents at a Glance

Chapter1 Route Aggregation 2
Chapter2 — Auto-summary 38

Chapter3 BGP-Specific Commands 44
Chapter 4 Default Information 94

Chapter5 BGP Administrative Distance 102
Chapter 6 BGP Route Filtering 110
Chapter7 BGP Load Balancing 112
Chapter 8 Neighbor Configuration 118
Chapter9 Role Advertisement 242
Chapter 10 Route Redistrioution 252
Chapter 11 Address Summarization 270
Chapter 12 Synchronization 272

Chapter13 BGP Timers 278

Chapter 14 BGP Show Gommands 284
Chapter 15 BGP Clear Commands 912
Chapter 16 BGP Debug Commands 318
Appendix A RFC 1771-A Border Gateway Protocol 4 330
Appendix 8 Regular Expressions 344
Appendix Route Map Logic 348

Command index 358

Index 260
mmrrexrs "|

Route Aggregation

14 aggregate-address address mask

as in the BGP table, This Loran of the ageregate-
gate and all the more-specie soutes dl une part of

Cisco 108 Software Release: 10.0

Configuration Example 1: Aggregating Local Routes
eTolly somos mes, Ia Fig
2.160 throughs 172.163

For this exemple, we sill
the four networks

Figure 1-1
| Chapter 1: Route Aggregation

Router A
router bgp 1

nesghtor 10.1.1.2 renote.as 2
miter © “a
Anterface Loopback ©

Sp address 172.16.0.7 255.255.255.0

Anterface Loopback 1
4p address 172.16.1.1 255.255.255.0
interface Loopback 2

ip address 172.16.2.1 255.255.285.0
i

interface loopback 9

Ap address 172.18.3.1 285.256.255.0

router bgp 2
network 172.16...
network 172.1621.

network 172.16.2.0 mask 255.255.255,
network 172.16.3.0 mask 255.255.255.0

neighbor 10.1.1.1 renste-as 1

mask 255.255.255.0

Four loopbacks have been created on Ronicr B to simulate the locally sourced routes tha
will be aggregated. A BGP router can advertise an aggregate only if at least one specific

route of the aggregate is in the BGP table, The BGP network commands are necessary on
Router I in order to plice more-specific routes of the aggregate into the BGP table, Before
ng the loopback prefixes, verily thal the specific routes are in the BGP tbles on

FEPAAshow ip BOP
BG? table version 1¢ 16, local router ID ts 172.17.1.1

Seatun coage! 6 aupecencod, dl auauua, ATRIREY? alld > Bag, sat
Grigin codes: i - IGP, @ - EGP, ? - incor € w *
°
network niaxt Hop werete Lecert moiont oder
we inpteee2s athe ° Hees
Detectors senna : Ba
Dinetezees iene ° bea
igtesevs wna 4 te

Mio La
BGP tadle version is 6, iocel router ID is
Status codes: 8 sugsressed, d damped, m history, * valsd, > best, 1

Origin codes: 1 TGP, ¢ - EGP, 7 - incomplete

Netware Next Hep Metric Lactet Weight Path
43172. 18.0.0/28 0.0.0.0 ° 32768 4
ss 17ete1.8/24 0.0.0.0 ® 2700 4
+> 172.10.2.0/24 0.0.0.0 e 92760 4
sa v7etea0/24 0.0.0.0 e S278 1
1-1: eggregate-address address mask

[Now modily the BGP configuration on Rovier B tw enable the advertisement of the
‘aggregate:

| Router 8
| router bap
| “network 172.16.8

naek 255,259.265.0
| network 172.16.1.0 nask 255.255.255,

network 172.16.2,0 nask 288.285 .255,
network 172.16.9,0 nask 255,255,255.0

‘The configuration for Router B contains « network command for every prelix tht is part
of the aggregate, Because we need oaly one more-specitic route in the BGP table in order
to gemerate the prefix, we could have used only one metwork covmmand, The problem with
using oaly one network comand is that ifthe network goes down, the mare-specifie rate
is withdrawn from the BGP table. Ifthe only specific route in the BGP table is withdrassn,
the aggregate is withdrawn as well. By using a network command for every prefix that is
‘contained in the aggregate, the aggregate is advertised as long as one of the more-specitic
routes is up.

Verification
Verify that the aggregate address is in both the Rourer A and B BGP tables:

FETESRNON Sp BP
BGP table version is 18, loca? rovter TD ie 172.17.1.1
Statue codes: * supprested, d daeoea, h nietory, * valid, > beat, ¢ - inteenal
Origin codes: + TGP, 2 FOP, 2 inconpiece

Network Next bop wetric LeoPet Naight Path
23 972.10.0.0/26 10.1.1.2 o 2

H6.7.0/2¢
$6.2.0/2¢
16.3.9/2¢

4s 8, local router 10 ¢ 172.76,9.1
Status ages: © uppressed, daspes, fh history, * valid, > best, 2 - internal
Origin codes: = - TOP, 2 - ‘EOP, 7 - ircorpioze

narwork next hop wetsie LecPrt Neaghe Path
46.9.0/22 0.0.8.8, o 32768 2

172.16,7,0/28
so 472.18.2.9/26
3 170.16.9.0/2¢
3 Chapter 1: Roule Aggregation

Examine the specific information for the aygreyate on Router A:

256 255 25E.0

Perawshow ip bop 172-76:

uP routing table every for 172.16.0.0/22, varaion 16
Paths: (1 avaulaole, best #1, table Default-1>-Aouting -
Not advertised to ary peer
2, (aggregated ty 2 172.16.9.1)
40.1,1.2 fron 1.1.1.2 (172.16.9.1)

his indicates that the AS,

formation for the aggregate has been lost. This does nal pose any problems in this |
example, because the routes that comprise the aggrewate and the aggregate itself originate
oon the sume router, The next example more clearly illustrates the atomic ageregate |
attribute, Also notice that the aggregate for this example contains only the four prefixes
172.16.0,0/24 through 172.16.3,0/24, We could have used an aggregate with x shorter
‘mask. Hor example, we could use # 16-hit mask when forming the aggregate on Router B,
ais shown in the following configuration. This is not recommended, chough, because you are
aggregating routes that might not belong to you.

Tevter 8
router bop 2

network 172.16.0.0 ask 255.256.255,
network 172.76. 265.256.255,
network 172.16.2 255.255.255.0
network 172.16 265.256. 255,

Verify that the aggregate is being advertised: ]

=E-BHshow Sp bop
age tab.e version in 29, local vouter 10 48 172.17.1.1 |

Status codes: € suppressed, d damped, n nistory, * valid, = best, + - intern 4
Origin cones: i - IGP, © - EG, ? - incomplete

Network Next Hop wotric Locket Weight Path q
+> 172.16,0.0/28 e a2k
we T2AG.1 0/28 181-1 0 O2i
serene 2.m28 18.1. ai

ce r7et6.8.0/24 18st.

ivshow Ip route
Coces: € - cannacted, § - static, 1 - IORP, A. MIP, W - mobile, & - BAP
D.- CIGAP, =X'- EZCAP exzernal, O - OSPF, IA- OSFF arter area
Nt. GSEF' usm external sype 1, NO - OSPF NSSA extorrel type 2

examine the IP routing table on Router B |
1-1: aggrogate-address address mask 7

(Continued)
EY - O8PF external lyse 1, £2 » OSPF external type 2, & EOP
4381S, Lt - 18-18 Levar-1, 12 - 15-78 level 2, *. candidate default
U- per-user static raite, o + Ont

Gatenay of last ~esort 4s not set

132 is subnetcee, | sueners
7 comacted, Leoshack®
1g is varigbly subnetien, 5 sumers, 2 s20%0

Co 172.18,0.0/z6 45 careetiy connected, Lonapaex1
G17B-48'* eree As airectiy conmcted, LoaabacK?
© 172.16.2.0/26 8 airestiy eonnacted, Laoabao4,
© rate a.eree a8 airesriy connected, Looobacss
18.0.0,0/99 a8 subnetied, 7 suonets
C___10.1.1.0 is dirsetly comectes, serial

t Pec aan 1-2 illustrates the purpose of the route to Null 0.

Figure 1-2 Rosse io Null 0 Is Needed to Prevent Routing Loops

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TEED UZ? (Rare)

ro... | sorte, oceans
a ae

Assume thar network 172.16.2,0/24 an Rowter fis dosen, Router B withdraws this route

From the BGP table bur still advertises the aggregate. Assume that Router B hay « default

soute pointing to Router A and that there is: no BGP route for 172.

on
Router B.

Router B receives the packet, it inspects the IP routing tuble to determine how (orate Uke
‘packet. Because 172.16.2,0/24 is down, there is no route in the routing table. Router B
sends the pocket to che default route, which is toward Router A. When Router A receives
the packet, itis sent hack to B, and B sends it back to A. This process continues until the
8 Chapter 1: Route Aggregation

“PYL in the TP packet goes to 0. With the Noli 0 route for 172.16.0.0/22 on Router B, the
packet is simply dropped if network 172,16.2.0/24 is down.

Configuration Example 2: Aggregating Redistributed Routes
Instead of using the network command to place the more-specitic routes inthe BGP table,
we could simply redistribuic the connected routes and then aggregate them, as ia Example
1. Modify the BGP configuration on Router B using redisirihution instead of the network
command. The no auto-summary commund is needed to prevent the router from
generating a classful summary of the 172.16.0.0/16 network (see Chupler 2, “Auto
for more information}

| router
router bgp 2

aggregate-adcress 172.15.0.0 258.255.252.0

Verification
Inspect the BGP tables on Reuters A snd B to verify that the summary is being advertised:

Firawahow ip Bap
gp table version is 18, local reuter 1D 4s 172. 16.2.1

Status codes: s suppressed, d canpes, fh history, * velid, > best, 1 - intecnal
Origin codes: 2 - 16P, 0 - =a, 7 - incomplete

Network Next Hop Metric LevPr! weight #ath
+ 1O4A.8190 | 19.1.1.1 Ey o1?
& woae 20 sree 7
+> 172.16.0.0/28 0.0.0.0 20 276 2

2
Ee 32768
we 32768

oe
4 172.16.2.0/28 0.0)
+ 172. 16.3.0/28 9.0)

Configuration Example 3: Aggregating BGP Learned Routes
In Figure |-3, Router A is leaming the prefixes 172.16,0.0/24 through 172. 16.3.0/24 from
Router B. Bezause these routes sre now in the BGP table on Router A, Router A can
aggregate thetn into the 172.16.0.0/22 prefix.
+t: aggregate-addrass address mask

Figure 1-3 Ageregatine BGP Leumed Ronies

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FoURE A
router bop +

neighbor 18.1-1.2

ip address 172.16.
t

interface Leopbuck
Ap adarees 172.16.

14 1eopbtek
ip address 172.16.

interface Leopback
Ap address 172.18.
router bgp 2

network 172.16,
network 172.18.1.8
network 172.18.2.8
netwark 172.18.3.8

router bgp 6559
Linatanor 172.17.1

neighbor 10.1.1.4 renate-aa 4

sine

1
4.4 285.255.255.0

2
2.1 256,256,268.

resk 268.285.288.0
mask 255, 266.255.6
ImeSK 255.265.255.0
Imesk 265,255,256
410 Chapter 1: Route Aggregation

Vorification

‘The BGP tables on Routers A and C shuuld now contain the aggregate address:

RAT a ba
OP table version is 6, PID 26 9172.17.11

Status codes: 5 supers ee, nistery, * veLid, > best, i = internal
‘raga codes: 3 - 10P, @- G9, 2 incomplete

Network Nat top Metric Leer wesynt Path
> 172.16.0.0/28 10.4.3.2 ® ozs

Seales aia 10.7.7?
sr vmalse 2.ajze 0.1.1.2
se a72.10..9)2¢ 19.1.1.2
resc#ehow 4p Bap

BGP table version is $2, local couter 10 i 172.17.1.2
Slutus coves: 2 suppressed, © camped, h history, * wid, > bet
origin cones: 1 15P, 6 EP, 7 - dncomilete

2 = Anternat

cari Nxt Hop et
$2 17216.0.0/9472.47.4.4

Sei72 61.0108 172.104
v9 172.16,2.0/20 172.47.4.1
pireie.s.ni24 a7ea7t1

Notice that in the BGP table on Router C, the path information for the specitie routes is
Gifferent from the path for the ayerogate route. Router A is forming the agerceate, so it
looks as though Router A “owns” this route. If you check the specific route information fae
the aggregate on Router C in the following aulput, you can see thar the route carcies the
atomie sitribute, signifying that there has been s loss of AS path information:

TINGHEHOW Ip Dap 172.16.0.0 255-255-050.0
BGP rosting teble enty for 172,16.8.0/22, version 28
Pathe: (1 available, weet ¥)

T72LIT.A1)

Af you aced to retain the AS path information fbr the aggregate, see sect