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In general, BGP compares MEDs of routes received from the same AS only. NOTE: The current implementation supports using the compare-different-as-med command to force BGP to compare MED values of routes received from different ASs. • LOCAL_PREF The LOCAL_PREF attribute is exchanged between IBGP peers only; therefore, it is not advertised to any other AS. It indicates the priority of a BGP router. LOCAL_PREF is used to determine the best route for traffic leaving the local AS. When a BGP router obtains from several IBGP peers multiple routes to the same destination, but with different next hops, it considers the route with the highest LOCAL_PREF value as the best route. As shown in Figure 51, traffic from AS20 to AS10 travels through Router C that is selected according to LOCAL_PREF. Figure 51 LOCAL_PREF attribute Local_pref = 100 Router B 8.0.0.0 EBGP Router A 2.1.1.1 3.1.1.1 EBGP AS 10 IBGP Next_hop = 2.1.1.1 Local_pref = 100 IBGP Router D D = 8.0.0.0 Next_hop = 3.1.1.1 IBGP Local_pref = 200 Router C Local_pref = 200 AS 20 • COMMUNITY The COMMUNITY attribute is a group of specific data. A route can carry one or more COMMUNITY attribute values (each of which is represented by a four-byte integer). The receiving router processes the route (for example, determining whether to advertise the route and the scope for advertising the route) based on the COMMUNITY attribute values. This simplifies routing policy usage and facilitates management and maintenance. Well-known community attributes are as follows: { INTERNET-By default, all routes belong to the Internet community. Routes with this attribute can be advertised to all BGP peers. { NO_EXPORT-After received, routes with this attribute cannot be advertised out the local AS or out the local confederation, but can be advertised to other sub-ASs in the confederation. For confederation information, see "Settlements for problems in large scale BGP networks." { NO_ADVERTISE-After received, routes with this attribute cannot be advertised to other BGP peers. { NO_EXPORT_SUBCONFED-After received, routes with this attribute cannot be advertised out the local AS or other ASs in the local confederation. 123

Section	Page
Title Page	1
Layer 3 - IP Routing Configuration Guide	3
IP routing basics	11
Overview	11
Routing table	11
Dynamic routing protocols	12
Routing preference	12
Load sharing	13
Route backup	13
Route recursion	13
Route redistribution	14
Displaying and maintaining a routing table	14
Configuring static routing	16
Introduction	16
Static route	16
Default route	16
Static route configuration items	16
Configuring a static route	17
Configuring BFD for static routes	18
BFD control mode	18
BFD echo mode	19
Displaying and maintaining static routes	19
Static route configuration examples	20
Basic static route configuration example	20
Network requirements	20
Configuration procedure	20
BFD for static routes configuration example (direct next hop)	22
Network requirements	22
Configuration procedure	22
BFD for static routes configuration example (indirect next hop)	24
Network requirements	24
Configuration procedure	24
Configuring RIP	27
Overview	27
RIP route entries	27
RIP timers	27
Routing loop prevention	27
RIP operation	28
RIP versions	28
RIP message format	28
RIPv1 message format	29
RIPv2 message format	29
RIPv2 authentication message format	29
Supported RIP features	30
Protocols and standards	30
RIP configuration task list	30
Configuring RIP basic functions	31
Configuration prerequisites	31
Enabling RIP and a RIP interface	31
Configuring the interface behavior	32
Configuring a RIP version	32
Setting the DSCP value for RIP packets	33
Configuring RIP route control	34
Configuring an additional routing metric	34
Configuring RIPv2 route summarization	34
Enabling RIPv2 route automatic summarization	34
Advertising a summary route	35
Disabling host route reception	35
Advertising a default route	35
Configuring inbound or outbound route filtering	36
Configuring a priority for RIP	37
Configuring RIP route redistribution	37
Tuning and optimizing RIP networks	37
Configuring RIP timers	38
Configuring split horizon and poison reverse	38
Enabling split horizon	38
Enabling poison reverse	38
Configuring the maximum number of ECMP routes	39
Enabling zero field check on incoming RIPv1 messages	39
Enabling source IP address check on incoming RIP updates	39
Configuring RIPv2 message authentication	40
Specifying a RIP neighbor	40
Configuring RIP-to-MIB binding	41
Configuring the RIP packet sending rate	41
Configuring BFD for RIP	41
Single-hop echo detection mode	41
Bidirectional control detection mode	42
Displaying and maintaining RIP	42
RIP configuration examples	43
Configuring RIP version	43
Network requirements	43
Configuration procedure	43
Configuring RIP route redistribution	44
Network requirements	44
Configuration procedure	45
Configuring an additional metric for a RIP interface	46
Network requirements	46
Configuration procedure	47
Configuring RIP to advertise a summary route	48
Network requirements	48
Configuration procedure	49
Configuring BFD for RIP (single-hop echo detection mode)	50
Network requirements	50
Configuration procedure	51
Configuring BFD for RIP (bidirectional control detection mode)	53
Network requirements	53
Configuration procedure	54
Troubleshooting RIP	57
No RIP updates received	57
Symptom	57
Analysis	57
Solution	57
Route oscillation occurred	57
Symptom	57
Analysis	57
Solution	57
Configuring OSPF	58
Overview	58
Basic concepts	58
Autonomous System	58
OSPF route computation	58
Router ID	59
OSPF packets	59
LSA types	59
Neighbor and Adjacency	59
Area based OSPF network partition	60
Network partition	60
Backbone area and virtual links	60
Stub area	61
NSSA area	62
Comparison between the areas	62
Router types	63
Router classification	63
Route types	63
OSPF network classification	64
OSPF network types	64
NBMA network configuration guidelines	64
DR and BDR	65
Introduction	65
DR and BDR election	65
OSPF packet formats	66
OSPF packet header	66
Hello packet	66
DD packet	67
LSR packet	68
LSU packet	69
LSAck packet	69
LSA header format	70
LSAs formats	70
Supported features	74
Multi-process	74
Authentication	74
OSPF Graceful Restart	74
BFD	75
Protocols and standards	75
OSPF configuration task list	75
Enabling OSPF	76
Configuration prerequisites	76
Configuration procedure	77
Configuring OSPF areas	78
Configuration prerequisites	78
Configuring a stub area	78
Configuring an NSSA area	79
Configuring a virtual link	79
Configuring OSPF network types	80
Configuration prerequisites	80
Configuring the broadcast network type for an interface	80
Configuring the NBMA network type for an interface	81
Configuring the P2MP network type for an interface	81
Configuring the P2P network type for an interface	82
Configuring OSPF route control	82
Configuration prerequisites	82
Configuring OSPF route summarization	83
Configuring route summarization on an ABR	83
Configuring route summarization when redistributing routes into OSPF on an ASBR	83
Configuring OSPF inbound route filtering	84
Configuring ABR Type-3 LSA filtering	84
Configuring an OSPF cost for an interface	84
Configuring the maximum number of OSPF routes	85
Configuring the maximum number of ECMP routes	85
Configuring OSPF preference	86
Configuring OSPF route redistribution	86
Configuring route redistribution into OSPF	86
Configuring OSPF to redistribute a default route	87
Configuring the default parameters for redistributed routes	87
Advertising a host route	88
Tuning and optimizing OSPF networks	88
Configuration prerequisites	88
Configuring OSPF packet timers	88
Specifying LSA transmission delay	89
Specifying SPF calculation interval	90
Specifying the LSA arrival interval	90
Specifying the LSA generation interval	90
Disabling interfaces from receiving and sending OSPF packets	91
Configuring stub routers	91
Configuring OSPF authentication	92
Adding the interface MTU into DD packets	92
Configuring the maximum number of external LSAs in LSDB	93
Enabling compatibility with RFC 1583	93
Logging neighbor state changes	93
Configuring OSPF network management	94
Enabling message logging	94
Enabling the advertisement and reception of opaque LSAs	94
Configuring OSPF to give priority to receiving and processing hello packets	95
Configuring the LSU transmit rate	95
Setting the DSCP value for OSPF packets	96
Enabling OSPF ISPF	96
Configuring OSPF Graceful Restart	96
Configuring the OSPF GR Restarter	96
Configuring the IETF standard OSPF GR Restarter	96
Configuring the non-IETF standard OSPF GR Restarter	97
Configuring the OSPF GR Helper	97
Configuring the IETF standard OSPF GR Helper	97
Configuring the non IETF standard OSPF GR Helper	98
Triggering OSPF Graceful Restart	98
Configuring BFD for OSPF	98
Configuring control packet bidirectional detection	98
Configuring echo packet single-hop detection	99
Displaying and maintaining OSPF	99
OSPF configuration examples	100
Configuring OSPF basic functions	100
Network requirements	100
Configuration procedure	101
Configuring OSPF route redistribution	104
Network requirements	104
Configuration procedure	104
Configuring OSPF to advertise a summary route	105
Network requirements	105
Configuration procedure	106
Configuring an OSPF stub area	108
Network requirements	108
Configuration procedure	108
Configuring an OSPF NSSA area	110
Network requirements	110
Configuration procedure	111
Configuring OSPF DR election	112
Network requirements	112
Configuration procedure	113
Configuring OSPF virtual links	116
Network requirements	116
Configuration procedure	117
Configuring OSPF Graceful Restart	118
Network requirements	118
Configuration procedure	119
Configuring route filtering	120
Network requirements	120
Configuration procedure	121
Configuring BFD for OSPF	123
Network requirements	123
Configuration procedure	123
Troubleshooting OSPF configuration	126
No OSPF neighbor relationship established	126
Symptom	126
Analysis	126
Solution	126
Incorrect routing information	126
Symptom	126
Analysis	126
Solution	126
Configuring BGP	127
BGP overview	127
BGP messages formats	127
Header	127
Open	128
Update	128
Notification	129
Keepalive	129
Route-refresh	129
BGP path attributes	130
Path attributes classification	130
Usage of BGP path attributes	130
BGP route selection	134
Route selection rules	134
Route selection with BGP load balancing	134
BGP route advertisement rules	135
BGP and IGP synchronization	135
Settlements for problems in large scale BGP networks	136
Route summarization	136
Route dampening	136
Peer group	137
Community	137
Route reflector	137
Confederation	138
BGP GR	139
MP-BGP	140
Overview	140
MP-BGP extended attributes	140
Address family	140
Protocols and standards	140
BGP configuration task list	141
Configuring BGP basic functions	142
Configuration prerequisites	142
Creating a BGP connection	142
Specifying the source interface for TCP connections	143
Allowing establishment of EBGP connection to an indirectly connected peer or peer group	144
Controlling route generation	144
Configuration prerequisites	144
Injecting a local network	144
Configuring BGP route redistribution	145
Enabling default route redistribution into BGP	145
Controlling route distribution and reception	146
Configuration prerequisites	146
Configuring BGP route summarization	146
Configuring automatic route summarization	146
Configuring manual route summarization	146
Advertising a default route to a peer or peer group	146
Configuring BGP route distribution/reception filtering policies	147
Configuration prerequisites	147
Configuring BGP route distribution filtering policies	147
Configuring BGP route reception filtering policies	148
Enabling BGP and IGP route synchronization	149
Limiting prefixes received from a peer or peer group	149
Configuring BGP route dampening	150
Configuring a shortcut route	150
Configuring BGP route attributes	151
Configuration prerequisites	151
Specifying a preferred value for routes received	151
Configuring preferences for BGP routes	151
Configuring the default local preference	152
Configuring the MED attribute	152
Configuring the default MED value	152
Enabling the comparison of MED of routes from different ASs	152
Enabling the comparison of MED of routes from each AS	152
Enabling the comparison of MED of routes from confederation peers	154
Configuring the NEXT_HOP attribute	154
Configuring the AS_PATH attribute	155
Permitting local AS number to appear in routes from a peer or peer group	155
Disabling BGP from considering AS_PATH during best route selection	155
Specifying a fake AS number for a peer or peer group	156
Configuring AS number substitution	156
Removing private AS numbers from updates to a peer or peer group	157
Ignoring the first AS number of EBGP route updates	157
Tuning and optimizing BGP networks	157
Configuration prerequisites	157
Configuring the BGP keepalive interval and holdtime	157
Configuring the interval for sending the same update	158
Configuring BGP soft-reset	158
Configuring automatic soft-reset	159
Configuring manual soft-reset	159
Enabling the BGP ORF capability	159
Enabling 4-byte AS number suppression	160
Setting the DSCP value for BGP packets	161
Enabling quick EBGP session reestablishment	161
Enabling MD5 authentication for TCP connections	162
Configuring BGP load balancing	162
Forbiding session establishment with a peer or peer group	162
Configuring a large scale BGP network	163
Configuration prerequisites	163
Configuring BGP peer groups	163
Configuring an IBGP peer group	163
Configuring an EBGP peer group	163
Configuring BGP community	165
Configuring a BGP route reflector	165
Configuring a BGP confederation	166
Configuring a BGP confederation	166
Configuring confederation compatibility	166
Configuring BGP GR	167
Enabling trap	167
Enabling logging of peer state changes	168
Configuring BFD for BGP	168
Displaying and maintaining BGP	168
Displaying BGP	168
Resetting BGP connections	170
Clearing BGP information	170
BGP configuration examples	170
BGP basic configuration example	170
Network requirements	170
Configuration procedure	171
BGP and IGP synchronization configuration example	174
Network requirements	174
Configuration procedure	175
BGP load balancing configuration example	177
Network requirements	177
Configuration procedure	178
BGP community configuration example	180
Network requirements	180
Configuration procedure	180
BGP route reflector configuration example	182
Network requirements	182
Configuration procedure	182
BGP confederation configuration example	184
Network requirements	184
Configuration procedure	184
BGP path selection configuration example	187
Network requirements	187
Configuration procedure	187
BGP GR configuration example	190
Network requirements	190
Configuration procedure	190
BFD for BGP configuration example	191
Network requirements	191
Configuration procedure	192
Troubleshooting BGP	196
BGP peer relationship not established	196
Symptom	196
Analysis	196
Solution	196
Configuring IPv6 static routing	198
Overview	198
IPv6 static routes features	198
Default IPv6 route	198
Configuring an IPv6 static route	198
Displaying and maintaining IPv6 static routes	199
IPv6 static routing configuration example	199
Network requirements	199
Configuration procedure	200
Configuring RIPng	202
Introduction to RIPng	202
RIPng working mechanism	202
RIPng packet format	203
Basic format	203
RTE format	203
RIPng packet processing procedure	204
Request packet	204
Response packet	204
Protocols and standards	204
RIPng configuration task list	204
Configuring RIPng basic functions	205
Configuring RIPng route control	205
Configuring an additional routing metric	205
Configuring RIPng route summarization	206
Advertising a default route	206
Configuring a RIPng route filtering policy	206
Configuring a priority for RIPng	207
Configuring RIPng route redistribution	207
Tuning and optimizing the RIPng network	207
Configuring RIPng timers	207
Configuring split horizon and poison reverse	208
Configuring split horizon	208
Configuring the poison reverse function	208
Configuring zero field check on RIPng packets	209
Configuring the maximum number of ECMP routes	209
Displaying and maintaining RIPng	209
RIPng configuration examples	210
Configuring RIPng basic functions	210
Network requirements	210
Configuration procedure	210
Configuring RIPng route redistribution	212
Network requirements	212
Configuration procedure	212
Configuring OSPFv3	216
Introduction to OSPFv3	216
OSPFv3 overview	216
OSPFv3 packets	216
OSPFv3 LSA types	217
OSPFv3 timers	217
OSPFv3 packet timer	217
LSA delay time	218
SPF timer	218
GR timer	218
OSPFv3 features supported	218
Protocols and standards	218
OSPFv3 configuration task list	218
Enabling OSPFv3	219
Configuration prerequisites	219
Enabling OSPFv3	219
Configuring OSPFv3 area parameters	219
Configuration prerequisites	220
Configuring an OSPFv3 stub area	220
Configuring an OSPFv3 virtual link	220
Configuring OSPFv3 network types	221
Configuration prerequisites	221
Configuring the OSPFv3 network type for an interface	221
Configuring an NBMA or P2MP neighbor	221
Configuring OSPFv3 routing information control	222
Configuration prerequisites	222
Configuring OSPFv3 route summarization	222
Configuring OSPFv3 inbound route filtering	222
Configuring an OSPFv3 cost for an interface	223
Configuring the maximum number of OSPFv3 ECMP routes	223
Configuring a priority for OSPFv3	224
Configuring OSPFv3 route redistribution	224
Tuning and optimizing OSPFv3 networks	225
Configuration prerequisites	225
Configuring OSPFv3 timers	225
Configuring a DR priority for an interface	226
Ignoring MTU check for DD packets	227
Disabling interfaces from receiving and sending OSPFv3 packets	227
Enabling the logging of neighbor state changes	227
Configuring OSPFv3 GR	227
Configuring GR Restarter	228
Configuring GR Helper	228
Displaying and maintaining OSPFv3	229
OSPFv3 configuration examples	230
Configuring OSPFv3 areas	230
Network requirements	230
Configuration procedure	230
Configuring OSPFv3 DR election	233
Network requirements	233
Configuration procedure	234
Configuring OSPFv3 GR	236
Network requirements	236
Configuration procedure	237
Troubleshooting OSPFv3 configuration	238
No OSPFv3 neighbor relationship established	238
Symptom	238
Analysis	238
Process steps	238
Incorrect routing information	238
Symptom	238
Analysis	238
Solution	238
Configuring IPv6 BGP	239
IPv6 BGP overview	239
IPv6 BGP configuration task list	239
Configuring IPv6 BGP basic functions	240
Configuration prerequisites	240
Configuration guidelines	240
Specifying an IPv6 BGP peer	240
Injecting a local IPv6 route	241

Match case Limit results 1 per page

123

In general, BGP compares MEDs of routes received from the same AS only.

NOTE:

The current implementation supports using the

compare-different-as-med

command to force BGP to

compare MED values of routes received from different ASs.

•

LOCAL_PREF

The LOCAL_PREF attribute is exchanged between IBGP peers only; therefore, it is not advertised to

any other AS. It indicates the priority of a BGP router.

LOCAL_PREF is used to determine the best route for traffic leaving the local AS. When a BGP router

obtains from several IBGP peers multiple routes to the same destination, but with different next

hops, it considers the route with the highest LOCAL_PREF value as the best route. As shown

Figure 51

, traffic from AS20 to AS10 travels through Router C that is selected according to

LOCAL_PREF.

Figure 51

LOCAL_PREF attribute

•

COMMUNITY

The COMMUNITY attribute is a group of specific data. A route can carry one or more

COMMUNITY attribute values (each of which is represented by a four-byte integer). The receiving

router processes the route (for example, determining whether to advertise the route and the scope

for advertising the route) based on the COMMUNITY attribute values. This simplifies routing policy

usage and facilitates management and maintenance. Well-known community attributes are as

follows:

{

INTERNET

—By default, all routes belong to the Internet community. Routes with this attribute

can be advertised to all BGP peers.

{

NO_EXPORT

—After received, routes with this attribute cannot be advertised out the local AS or

out the local confederation, but can be advertised to other sub-ASs in the confederation. For

confederation information, see "

Settlements for problems in large scale BGP networks

{

NO_ADVERTISE

—After received, routes with this attribute cannot be advertised to other BGP

peers.

{

NO_EXPORT_SUBCONFED

—After received, routes with this attribute cannot be advertised out

the local AS or other ASs in the local confederation.

EBGP

Router B

Router A

Router C

Router D

D = 8.0.0.0

Next_hop = 3.1.1.1

Local_pref = 200

IBGP

EBGP

2.1.1.1

8.0.0.0

Local_pref = 100

Next_hop = 2.1.1.1

Local_pref = 100

Local_pref = 200

3.1.1.1

AS 20

AS 10

HP 6125G HP 6125G &amp; 6125G/XG Blade Switches Layer 3 - IP Routing Confi - Page 133

Local_pref.

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HP 6125G HP 6125G & 6125G/XG Blade Switches Layer 3 - IP Routing Confi - Page 133