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MED Multi-Exit Discriminator, NEXT_HOP attribute

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{ Filter routes-By using an AS path list, you can filter routes based on AS numbers contained in the AS_PATH attribute. For more information about AS path list, see "Configuring routing policies." • NEXT_HOP The NEXT_HOP attribute may not be the IP address of a directly-connected router. Its value is determined as follows: { When a BGP speaker advertises a self-originated route to a BGP peer, it sets the address of the sending interface as the NEXT_HOP. { When a BGP speaker sends a received route to an EBGP peer, it sets the address of the sending interface as the NEXT_HOP. { When a BGP speaker sends a route received from an EBGP peer to an IBGP peer, it does not modify the NEXT_HOP attribute. If load balancing is configured, BGP modifies the NEXT_HOP attribute for the equal-cost routes. For load balancing information, see "BGP load balancing." Figure 45 NEXT_HOP attribute • MED (Multi-Exit Discriminator) BGP advertises the MED attribute between two neighboring ASs, each of which does not advertise the attribute to any other AS. Similar to metrics used by IGPs, MED is used to determine the best route for traffic going into an AS. When a BGP router obtains multiple routes to the same destination, but with different next hops from different EBGP peers, it considers the route with the smallest MED value the best route given that other conditions are the same. As shown in Figure 46, traffic from AS 10 to AS 20 travels through Router B that is selected according to MED. 160

Section	Page
Title Page	1
Contents	3
IP routing basics	12
Routing table	12
Dynamic routing protocols	13
Route preference	13
Load sharing	14
Route backup	14
Route recursion	14
Route redistribution	14
Configuring the maximum number of ECMP routes	15
Displaying and maintaining a routing table	15
Configuring static routing	17
Configuring a static route	17
Configuring BFD for static routes	18
Bidirectional control mode	18
Single-hop echo mode	19
Configuring static route FRR	20
Configuration guidelines	20
Configuration procedure	20
Displaying and maintaining static routes	21
Static route configuration examples	21
Basic static route configuration example	21
Network requirements	21
Configuration procedure	22
Verifying the configuration	22
BFD for static routes configuration example (direct next hop)	23
Network requirements	23
Configuration procedure	24
Verifying the configuration	24
BFD for static routes configuration example (indirect next hop)	25
Network requirements	25
Configuration procedure	26
Verifying the configuration	26
Static route FRR configuration example	28
Network requirements	28
Configuration procedure	28
Verifying the configuration	28
Configuring a default route	30
Configuring RIP	31
Overview	31
RIP route entries	31
Routing loop prevention	31
RIP operation	31
RIP versions	32
Protocols and standards	32
RIP configuration task list	32
Configuring basic RIP	33
Enabling RIP	33
Controlling RIP reception and advertisement on interfaces	34
Configuring a RIP version	34
Configuring RIP route control	35
Configuring an additional routing metric	35
Configuring RIPv2 route summarization	36
Enabling RIPv2 automatic route summarization	36
Advertising a summary route	36
Disabling host route reception	37
Advertising a default route	37
Configuring received/redistributed route filtering	38
Configuring a preference for RIP	38
Configuring RIP route redistribution	38
Tuning and optimizing RIP networks	39
Configuration prerequisites	39
Configuring RIP timers	39
Configuring split horizon and poison reverse	40
Enabling split horizon	40
Enabling poison reverse	40
Configuring the maximum number of ECMP routes	40
Enabling zero field check on incoming RIPv1 messages	41
Enabling source IP address check on incoming RIP updates	41
Configuring RIPv2 message authentication	42
Configuring the RIP packet sending rate	42
Configuring RIP GR	42
Configuring BFD for RIP	43
Configuring RIP FRR	43
Configuration restrictions and guidelines	44
Configuration prerequisites	44
Configuration procedure	44
Displaying and maintaining RIP	44
RIP configuration examples	45
Configuring basic RIP	45
Network requirements	45
Configuration procedure	45
Configuring RIP route redistribution	47
Network requirements	47
Configuration procedure	47
Configuring an additional metric for a RIP interface	49
Network requirements	49
Configuration procedure	49
Configuring RIP to advertise a summary route	51
Network requirements	51
Configuration procedure	51
Configuring BFD for RIP (single-hop echo detection)	53
Network requirements	53
Configuration procedure	54
Verifying the configuration	55
Configuring RIP FRR	56
Network requirements	56
Configuration procedure	56
Verifying the configuration	57
Configuring OSPF	59
Overview	59
OSPF packets	59
LSA types	60
OSPF areas	60
Backbone area and virtual links	61
Stub area and totally stub area	62
NSSA area and totally NSSA area	62
Router types	62
Route types	63
Route calculation	64
OSPF network types	64
DR and BDR	64
DR and BDR election	65
Protocols and standards	65
OSPF configuration task list	66
Enabling OSPF	67
Configuration prerequisites	67
Configuration guidelines	67
Configuration procedure	68
Configuring OSPF areas	68
Configuring a stub area	69
Configuring an NSSA area	69
Configuring a virtual link	70
Configuring OSPF network types	70
Configuration prerequisites	71
Configuring the broadcast network type for an interface	71
Configuring the NBMA network type for an interface	71
Configuring the P2MP network type for an interface	72
Configuring the P2P network type for an interface	73
Configuring OSPF route control	73
Configuration prerequisites	73
Configuring OSPF route summarization	73
Configuring route summarization on an ABR	73
Configuring route summarization when redistributing routes into OSPF on an ASBR	74
Configuring inbound OSPF route filtering	74
Configuring Type-3 LSA filtering	75
Configuring an OSPF cost for an interface	75
Configuring the maximum number of ECMP routes	76
Configuring OSPF preference	76
Configuring OSPF route redistribution	77
Configuring OSPF to redistribute routes from another routing protocol	77
Configuring OSPF to redistribute a default route	77
Configuring default parameters for redistributed routes	78
Advertising a host route	78
Tuning and optimizing OSPF networks	78
Configuration prerequisites	78
Configuring OSPF timers	79
Specifying LSA transmission delay	79
Specifying SPF calculation interval	80
Specifying the LSA arrival interval	80
Specifying the LSA generation interval	81
Disabling interfaces from receiving and sending OSPF packets	81
Configuring stub routers	82
Configuring OSPF authentication	82
Adding the interface MTU into DD packets	83
Configuring the maximum number of external LSAs in LSDB	83
Configuring OSPF exit overflow interval	84
Enabling compatibility with RFC 1583	84
Logging neighbor state changes	84
Configuring OSPF network management	85
Configuring the LSU transmit rate	85
Enabling OSPF ISPF	86
Configuring OSPF GR	86
Configuring the OSPF GR Restarter	86
Configuring the IETF OSPF GR Restarter	86
Configuring the non-IETF OSPF GR Restarter	87
Configuring OSPF GR Helper	87
Configuring the IETF OSPF GR Helper	87
Configuring the non-IETF OSPF GR Helper	88
Triggering OSPF GR	88
Configuring BFD for OSPF	88
Configuring bidirectional control detection	88
Configuring single-hop echo detection	89
Configuring OSPF FRR	89
Configuration prerequisites	90
Configuration guidelines	90
Configuring OSPF FRR to calculate a backup next hop using the LFA algorithm	90
Configuring OSPF FRR to specify a backup next hop using a routing policy	90
Displaying and maintaining OSPF	91
OSPF configuration examples	92
Configuring basic OSPF	92
Network requirements	92
Configuration procedure	92
Verifying the configuration	93
Configuring OSPF route redistribution	95
Network requirements	95
Configuration procedure	95
Verifying the configuration	95
Configuring OSPF to advertise a summary route	96
Network requirements	96
Configuration procedure	97
Configuring an OSPF stub area	99
Network requirements	99
Configuration procedure	100
Configuring an OSPF NSSA area	102
Network requirements	102
Configuration procedure	102
Configuring OSPF DR election	104
Network requirements	104
Configuration procedure	104
Configuring OSPF virtual links	108
Network requirements	108
Configuration procedure	108
Configuring OSPF GR	110
Network requirements	110
Configuration procedure	110
Verifying the configuration	111
Configuring BFD for OSPF	112
Network requirements	112
Configuration procedure	113
Verifying the configuration	114
Configuring OSPF FRR	115
Network requirements	115
Configuration procedure	115
Verifying the configuration	116
Troubleshooting OSPF configuration	117
No OSPF neighbor relationship established	117
Symptom	117
Analysis	117
Solution	117
Incorrect routing information	118
Symptom	118
Analysis	118
Solution	118
Configuring IS-IS	119
Overview	119
Terminology	119
IS-IS address format	119
NSAP	119
Area address	120
System ID	120
SEL	120
Routing method	120
NET	120
IS-IS area	121
Level-1 and Level-2	121
Route leaking	122
IS-IS network types	123
Network types	123
DIS and pseudonodes	123
IS-IS PDUs	124
PDU	124
Hello PDU	124
LSP	124
SNP	124
CLV	125
Protocols and standards	125
IS-IS configuration task list	126
Configuring basic IS-IS	127
Configuration prerequisites	127
Enabling IS-IS	127
Configuring the IS level and circuit level	127
Configuring P2P network type for an interface	128
Configuring IS-IS route control	128
Configuration prerequisites	128
Configuring IS-IS link cost	129
Configuring an IS-IS cost for an interface	129
Configuring a global IS-IS cost	129
Enabling automatic IS-IS cost calculation	130
Specifying a preference for IS-IS	130
Configuring the maximum number of ECMP routes	130
Configuring IS-IS route summarization	131
Advertising a default route	131
Configuring IS-IS route redistribution	132
Configuring IS-IS route filtering	132
Filtering routes calculated from received LSPs	132
Filtering redistributed routes	133
Configuring IS-IS route leaking	133
Tuning and optimizing IS-IS networks	133
Configuration prerequisites	133
Specifying intervals for sending IS-IS hello and CSNP packets	134
Specifying the IS-IS hello multiplier	134
Configuring a DIS priority for an interface	134
Disabling an interface from sending/receiving IS-IS packets	135
Enabling an interface to send small hello packets	135
Configuring LSP parameters	135
Configuring LSP timers	135
Specifying LSP lengths	137
Enabling LSP flash flooding	137
Enabling LSP fragment extension	137
Controlling SPF calculation interval	138
Configuring convergence priorities for specific routes	138
Setting the LSDB overload bit	139
Configuring system ID to host name mappings	139
Configuring a static system ID to host name mapping	139
Configuring dynamic system ID to host name mapping	140
Enabling the logging of neighbor state changes	140
Enabling IS-IS ISPF	141
Enhancing IS-IS network security	141
Configuration prerequisites	141
Configuring neighbor relationship authentication	141
Configuring area authentication	142
Configuring routing domain authentication	142
Configuring IS-IS GR	143
Configuring BFD for IS-IS	144
Configuring IS-IS FRR	144
Configuration prerequisites	145
Configuration guidelines	145
Configuring IS-IS FRR to automatically calculate a backup next hop	145
Configuring IS-IS FRR using a routing policy	145
Displaying and maintaining IS-IS	146
IS-IS configuration examples	146
Basic IS-IS configuration example	146
Network requirements	146
Configuration procedure	147
Verifying the configuration	148
DIS election configuration example	151
Network requirements	151
Configuration procedure	151
IS-IS route redistribution configuration example	155
Network requirements	155
Configuration procedure	155
IS-IS authentication configuration example	158
Network requirements	158
Configuration procedure	159
IS-IS GR configuration example	161
Network requirements	161
Configuration procedure	161
Verifying the configuration	161
BFD for IS-IS configuration example	162
Network requirements	162
Configuration procedure	163
Verifying the configuration	164
IS-IS FRR configuration example	165
Network requirements	165
Configuration procedure	165
Verifying the configuration	166
Configuring BGP	169
Overview	169
BGP speaker and BGP peer	169
BGP message types	169
BGP path attributes	170
BGP route selection	174
BGP route advertisement rules	174
BGP load balancing	174
Settlements for problems in large-scale BGP networks	175
MP-BGP	178
MP-BGP extended attributes	178
Address family	179
BGP configuration views	179
Protocols and standards	180
BGP configuration task list	180
Configuring basic BGP	183
Enabling BGP	183
Configuring a BGP peer	184
Configuring an IPv4 BGP peer	184
Configuring an IPv6 BGP peer	184
Configuring a BGP peer group	185
Configuring an IBGP peer group	185
Configuring an EBGP peer group	186
Specifying the source interface for TCP connections	190
Generating BGP routes	191
Injecting a local network	192
Redistributing IGP routes	192
Controlling route distribution and reception	193
Configuring BGP route summarization	194
Configuring automatic route summarization	194
Configuring manual route summarization	194
Advertising optimal routes in the IP routing table	195
Advertising a default route to a peer or peer group	195
Limiting routes received from a peer or peer group	196
Configuring BGP route filtering policies	197
Configuration prerequisites	197
Configuring BGP route distribution filtering policies	197
Configuring BGP route reception filtering policies	200
Configuring BGP route dampening	202
Controlling BGP path selection	203
Specifying a preferred value for routes received	203
Configuring preferences for BGP routes	204
Configuring the default local preference	205
Configuring the MED attribute	206
Configuring the default MED value	206
Enabling MED comparison for routes from different ASs	207
Enabling MED comparison for routes on a per-AS basis	208
Enabling MED comparison for routes from confederation peers	209
Configuring the NEXT_HOP attribute	210
Configuring the AS_PATH attribute	212
Permitting local AS number to appear in routes from a peer or peer group	212
Disabling BGP from considering AS_PATH during best route selection	213
Advertising a fake AS number to a peer or peer group	213
Configuring AS number substitution	214
Removing private AS numbers from updates sent to an EBGP peer or peer group	215
Ignoring the first AS number of EBGP route updates	216
Tuning and optimizing BGP networks	217
Configuring the keepalive interval and hold time	217
Configuring the interval for sending updates for the same route	218
Enabling BGP to establish an EBGP session over multiple hops	219
Enabling immediate reestablishment of direct EBGP connections upon link failure	220
Enabling 4-byte AS number suppression	220
Configuring MD5 authentication for BGP	221
Configuring BGP load balancing	222
Disabling BGP to establish a session to a peer or peer group	223
Configuring BGP soft-reset	223
Enabling route-refresh	224
Saving updates	225
Configuring manual soft-reset	226
Protecting an EBGP peer when memory usage reaches level 2 threshold	227
Configuring a large-scale BGP network	228
Configuring BGP community	228
Configuring a BGP route reflector	230
Configuring a BGP confederation	230
Configuring a BGP confederation	231
Configuring confederation compatibility	231
Configuring BGP GR	231
Enabling trap	232
Enabling logging of session state changes	233
Configuring BFD for BGP	233
Displaying and maintaining BGP	234
IPv4 BGP configuration examples	237
Basic BGP configuration example	237
Network requirements	237
Configuration considerations	237
Configuration procedure	237
Verifying the configuration	241
BGP and IGP route redistribution configuration example	241
Network requirements	241
Configuration considerations	241
Configuration procedure	241
Verifying the configuration	243
BGP route summarization configuration example	244
Network requirements	244
Configuration procedure	244
Verifying the configuration	246
BGP load balancing configuration example	247
Network requirements	247
Configuration considerations	247
Configuration procedure	247
Verifying the configuration	249
BGP community configuration example	250
Network requirements	250
Configuration procedure	250
Verifying the configuration	252
BGP route reflector configuration example	253
Network requirements	253
Configuration procedure	253
Verifying the configuration	254
BGP confederation configuration example	255
Network requirements	255
Configuration procedure	255
Verifying the configuration	257
BGP path selection configuration example	259
Network requirements	259
Configuration procedure	259
BGP GR configuration example	263
Network requirements	263
Configuration procedure	263
Verifying the configuration	264
BFD for BGP configuration example	264
Network requirements	264
Configuration procedure	265
Verifying the configuration	266
IPv6 BGP configuration examples	268
IPv6 BGP basic configuration example	268
Network requirements	268
Configuration procedure	269
Verifying the configuration	270
IPv6 BGP route reflector configuration example	271
Network requirements	271
Configuration procedure	272
Verifying the configuration	273
BFD for IPv6 BGP configuration example	274
Network requirements	274
Configuration procedure	275
Verifying the configuration	276
Troubleshooting BGP	278
Symptom	278
Analysis	278
Solution	278
Configuring PBR	279
Introduction to PBR	279
Policy	279
if-match clause	279
apply clause	279
Relationship between the match mode and clauses on the node	279
PBR and Track	280
PBR configuration task list	280
Configuring a policy	280
Creating a node	280
Configuring match criteria for a node	280
Configuring actions for a node	281
Configuring PBR	281
Displaying and maintaining PBR	281
Packet type-based interface PBR configuration example	282
Network requirements	282
Configuration procedure	282
Verifying the configuration	283
Configuring IPv6 static routing	285
Configuring an IPv6 static route	285
Configuring BFD for IPv6 static routes	286
Bidirectional control mode	286
Single-hop echo mode	287
Displaying and maintaining IPv6 static routes	288
IPv6 static routing configuration examples	288
Basic IPv6 static route configuration example	288
Network requirements	288
Configuration procedure	289
Verifying the configuration	289
BFD for IPv6 static routes configuration example (direct next hop)	290
Network requirements	290

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{

Filter routes

—By using an AS path list, you can filter routes based on AS numbers contained in

the AS_PATH attribute. For more information about AS path list, see "Configuring routing

policies."

•

NEXT_HOP

The NEXT_HOP attribute may not be the IP address of a directly-connected router. Its value is

determined as follows:

{

When a BGP speaker advertises a self-originated route to a BGP peer, it sets the address of the

sending interface as the NEXT_HOP.

{

When a BGP speaker sends a received route to an EBGP peer, it sets the address of the sending

interface as the NEXT_HOP.

{

When a BGP speaker sends a route received from an EBGP peer to an IBGP peer, it does not

modify the NEXT_HOP attribute. If load balancing is configured, BGP modifies the NEXT_HOP

attribute for the equal-cost routes. For load balancing information, see "

BGP load balancing

Figure 45

NEXT_HOP attribute

•

MED (Multi-Exit Discriminator)

BGP advertises the MED attribute between two neighboring ASs, each of which does not advertise

the attribute to any other AS.

Similar to metrics used by IGPs, MED is used to determine the best route for traffic going into an AS.

When a BGP router obtains multiple routes to the same destination, but with different next hops

from different EBGP peers, it considers the route with the smallest MED value the best route given

that other conditions are the same. As shown in

Figure 46

, traffic from AS 10 to AS 20 travels

through Router B that is selected according to MED.