With BGP multipath a router selects a single best-path to a destination prefix and additionally considers one or more equal cost paths to install in the RIB/FIB. If there are multiple equal cost iBGP or eBGP paths to a destination, a router enabled with multipath can install those paths from the BGP table into the IP routing table. There are several details to consider when enabling BGP multipath.
The example BGP router configuration command maximum-paths 2 enables multipath for two eBGP prefixes. If multipath for iBGP is needed the example command maximum-paths ibgp 2 can be used. The feature can also be configured for both eBGP and iBGP but needs to be used with caution, as reminded in the following output. Also, the outputs show the different multipath commands may be mutually exclusive.
R1>
R1>enable
R1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#
R1(config)#router bgp 65100
R1(config-router)#address-family ipv4
R1(config-router-af)#maximum-paths eibgp 2%BGP: This may cause traffic loop if not used properly (command accepted)
------------
R1(config-router-af)#maximum-paths eibgp 2% De-configure ibgp-multipath first
------------
R1(config-router-af)#maximum-paths eibgp 2% De-configure ebgp-multipath first
Two eBGP paths - same AS
In the following scenario, R1 installs two equal cost routes through the AS 65100 and destined to the prefix 172.16.2.0/24 in Site 2. This is accomplished with eBGP multipath configured on R1. Notice that 4-byte BGP AS Numbers are used, and on R1 the neighbor configuration is applied with a peer-session template. The BGP router command maximum-paths 2 ensures that two paths are installed into the RIB.
Configuration:
R1
R1#show run | sec ^router
router bgp 64086.59904
template peer-session EXAMPLE-SESSION
remote-as 65100
exit-peer-session
!
bgp router-id 1.1.1.1
bgp asnotation dot
bgp log-neighbor-changes
neighbor 10.1.0.2 inherit peer-session EXAMPLE-SESSION
neighbor 10.2.0.2 inherit peer-session EXAMPLE-SESSION
!
address-family ipv4
network 172.16.1.0 mask 255.255.255.0
neighbor 10.1.0.2 activate
neighbor 10.2.0.2 activate
maximum-paths 2
exit-address-family
R1#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
description ** to Host **
ip address 172.16.1.2 255.255.255.0
duplex auto
speed auto
media-type rj45
R1#show run int Gi0/1 | sec int
interface GigabitEthernet0/1
description ** to R2 **
ip address 10.1.0.1 255.255.255.252
duplex auto
speed auto
media-type rj45
R1#show run int Gi0/2 | sec int
interface GigabitEthernet0/2
description ** to R3 **
ip address 10.2.0.1 255.255.255.252
duplex auto
speed auto
media-type rj45
R2
R2#show run | sec ^router
router bgp 65100
bgp router-id 2.2.2.2
bgp asnotation dot
bgp log-neighbor-changes
neighbor 10.1.0.1 remote-as 64086.59904
neighbor 10.3.0.1 remote-as 64086.59905
R2#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
description ** to R1 **
ip address 10.1.0.2 255.255.255.252
duplex auto
speed auto
media-type rj45
R2#show run int Gi0/1 | sec int
interface GigabitEthernet0/1
description ** to R4 **
ip address 10.3.0.2 255.255.255.252
duplex auto
speed auto
media-type rj45
R3
R3#show run | sec ^router
router bgp 65100
bgp router-id 3.3.3.3
bgp asnotation dot
bgp log-neighbor-changes
neighbor 10.2.0.1 remote-as 64086.59904
neighbor 10.4.0.1 remote-as 64086.59905
R3#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
description ** to R1 **
ip address 10.2.0.2 255.255.255.252
duplex auto
speed auto
media-type rj45
R3#show run int Gi0/1 | sec int
interface GigabitEthernet0/1
description ** to R4 **
ip address 10.4.0.2 255.255.255.252
duplex auto
speed auto
media-type rj45
R4
R4#show run | sec ^router
router bgp 64086.59905
bgp router-id 4.4.4.4
bgp asnotation dot
bgp log-neighbor-changes
neighbor 10.3.0.2 remote-as 65100
neighbor 10.4.0.2 remote-as 65100
!
address-family ipv4
network 172.16.2.0 mask 255.255.255.0
neighbor 10.3.0.2 activate
neighbor 10.4.0.2 activate
exit-address-family
R4#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
description ** to Server **
ip address 172.16.2.2 255.255.255.0
duplex auto
speed auto
media-type rj45
R4#show run int Gi0/1 | sec int
interface GigabitEthernet0/1
description ** to R2 **
ip address 10.3.0.1 255.255.255.252
duplex auto
speed auto
media-type rj45
R4#show run int Gi0/2 | sec int
interface GigabitEthernet0/2
description ** to R3 **
ip address 10.4.0.1 255.255.255.252
duplex auto
speed auto
media-type rj45
R1#show ip bgp | beg Ne
Network Next Hop Metric LocPrf Weight Path
*> 172.16.1.0/24 0.0.0.0 0 32768 i
*m 172.16.2.0/24 10.2.0.2 0 65100 64086.59905 i « Multipath route in BGP Table (BGP RIB)
*> 10.1.0.2 0 65100 64086.59905 i
R1#show ip route bgp | beg Ga
Gateway of last resort is not set
172.16.0.0/16 is variably subnetted, 3 subnets, 2 masks
B 172.16.2.0/24 [20/0] via 10.2.0.2, 00:57:16 « Two BGP routes to single destination
[20/0] via 10.1.0.2, 00:57:16
R1#show ip bgp 172.16.2.0
BGP routing table entry for 172.16.2.0/24, version 3
Paths: (2 available, best #2, table default)
Multipath: eBGP
Advertised to update-groups:
1
Refresh Epoch 1
65100 64086.59905
10.2.0.2 from 10.2.0.2 (3.3.3.3)
Origin IGP, localpref 100, valid, external, multipath(oldest)« Multipath routes to single destination
rx pathid: 0, tx pathid: 0
Refresh Epoch 1
65100 64086.59905
10.1.0.2 from 10.1.0.2 (2.2.2.2)
Origin IGP, localpref 100, valid, external, multipath, best
rx pathid: 0, tx pathid: 0x0
R1#show ip bgp 172.16.2.0 bestpath« BGP still selects a best path even if multipath is enabled
BGP routing table entry for 172.16.2.0/24, version 3
Paths: (2 available, best #2, table default)
Multipath: eBGP
Advertised to update-groups:
1
Refresh Epoch 1
65100 64086.59905
10.1.0.2 from 10.1.0.2 (2.2.2.2)
Origin IGP, localpref 100, valid, external, multipath, best
rx pathid: 0, tx pathid: 0x0
R1#show ip cef exact-route 172.16.1.1 172.16.2.1
172.16.1.1 -> 172.16.2.1 =>IP adj out of GigabitEthernet0/1, addr 10.1.0.2« Equal cost load sharing, from one host transmit through R2R1#show ip cef exact-route 172.16.1.3 172.16.2.1
172.16.1.3 -> 172.16.2.1 =>IP adj out of GigabitEthernet0/2, addr 10.2.0.2« From another host transmit through R3R1#show ip cef 172.16.2.0 detail
172.16.2.0/24, epoch 0, flags [rib only nolabel, rib defined all labels], per-destination sharing
recursive via 10.1.0.2
attached to GigabitEthernet0/1
recursive via 10.2.0.2
attached to GigabitEthernet0/2
Three eBGP paths - same AS (IPv6)
In the following example, eBGP multipath installs 3 routes received from AS 65100 and destined to the IPv6 network 2001:DB8:B::/64. This means, that the BGP router command maximum-paths 3 is configured on R1 under the IPv6 address-family section. Notice that a BGP Dynamic Neighbor configuration is also applied on R1 for IPv6 peerings.
Configuration:
R1
R1#show run | sec ^router
router bgp 65001
bgp router-id 1.1.1.1
bgp log-neighbor-changes
bgp listen range 2001:DB8::/32 peer-group EXAMPLE-PG
neighbor EXAMPLE-PG peer-group
neighbor EXAMPLE-PG remote-as 65100
!
address-family ipv4
no neighbor EXAMPLE-PG activate
exit-address-family
!
address-family ipv6
maximum-paths 3
network 2001:DB8:A::/64
neighbor EXAMPLE-PG activate
exit-address-family
R1#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
description ** to Host **
no ip address
duplex auto
speed auto
media-type rj45
ipv6 address FE80::2 link-local
ipv6 address 2001:DB8:A::2/64
R1#show run int Gi0/1 | sec int
interface GigabitEthernet0/1
description ** to R2 **
no ip address
duplex auto
speed auto
media-type rj45
ipv6 address FE80::1 link-local
ipv6 address 2001:DB8:1::1/64
R1#show run int Gi0/2 | sec int
interface GigabitEthernet0/2
description ** to R3 **
no ip address
duplex auto
speed auto
media-type rj45
ipv6 address FE80::1 link-local
ipv6 address 2001:DB8:2::1/64
R1#show run int Gi0/3 | sec int
interface GigabitEthernet0/3
description ** to R4 **
no ip address
duplex auto
speed auto
media-type rj45
ipv6 address FE80::1 link-local
ipv6 address 2001:DB8:3::1/64
R1#show run | sec ^ipv6 unicast
ipv6 unicast-routing
R2
R2#show run | sec ^router
router bgp 65100
bgp router-id 2.2.2.2
bgp log-neighbor-changes
neighbor 2001:DB8:1::1 remote-as 65001
neighbor 2001:DB8:4::1 remote-as 65002
!
address-family ipv4
no neighbor 2001:DB8:1::1 activate
no neighbor 2001:DB8:4::1 activate
exit-address-family
!
address-family ipv6
neighbor 2001:DB8:1::1 activate
neighbor 2001:DB8:4::1 activate
exit-address-family
R2#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
description ** to R1 **
no ip address
duplex auto
speed auto
media-type rj45
ipv6 address FE80::2 link-local
ipv6 address 2001:DB8:1::2/64
R2#show run int Gi0/1 | sec int
interface GigabitEthernet0/1
description ** to R5 **
no ip address
duplex auto
speed auto
media-type rj45
ipv6 address FE80::2 link-local
ipv6 address 2001:DB8:4::2/64
R2#show run | sec ^ipv6 unicast
ipv6 unicast-routing
R3
R3#show run | sec ^router
router bgp 65100
bgp router-id 3.3.3.3
bgp log-neighbor-changes
neighbor 2001:DB8:2::1 remote-as 65001
neighbor 2001:DB8:5::1 remote-as 65002
!
address-family ipv4
no neighbor 2001:DB8:2::1 activate
no neighbor 2001:DB8:5::1 activate
exit-address-family
!
address-family ipv6
neighbor 2001:DB8:2::1 activate
neighbor 2001:DB8:5::1 activate
exit-address-family
R3#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
description ** to R1 **
no ip address
duplex auto
speed auto
media-type rj45
ipv6 address FE80::2 link-local
ipv6 address 2001:DB8:2::2/64
R3#show run int Gi0/1 | sec int
interface GigabitEthernet0/1
description ** to R5 **
no ip address
duplex auto
speed auto
media-type rj45
ipv6 address FE80::2 link-local
ipv6 address 2001:DB8:5::2/64
R3#show run | sec ^ipv6 unicast
ipv6 unicast-routing
R5
R5#show run | sec ^router
router bgp 65002
bgp router-id 5.5.5.5
bgp log-neighbor-changes
neighbor 2001:DB8:4::2 remote-as 65100
neighbor 2001:DB8:5::2 remote-as 65100
neighbor 2001:DB8:6::2 remote-as 65100
!
address-family ipv4
no neighbor 2001:DB8:4::2 activate
no neighbor 2001:DB8:5::2 activate
no neighbor 2001:DB8:6::2 activate
exit-address-family
!
address-family ipv6
network 2001:DB8:B::/64
neighbor 2001:DB8:4::2 activate
neighbor 2001:DB8:5::2 activate
neighbor 2001:DB8:6::2 activate
exit-address-family
R5#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
description ** to Server **
no ip address
duplex auto
speed auto
media-type rj45
ipv6 address FE80::2 link-local
ipv6 address 2001:DB8:B::2/64
R5#show run int Gi0/1 | sec int
interface GigabitEthernet0/1
description ** to R2 **
no ip address
duplex auto
speed auto
media-type rj45
ipv6 address FE80::1 link-local
ipv6 address 2001:DB8:4::1/64
R5#show run int Gi0/2 | sec int
interface GigabitEthernet0/2
description ** to R3 **
no ip address
duplex auto
speed auto
media-type rj45
ipv6 address FE80::1 link-local
ipv6 address 2001:DB8:5::1/64
R5#show run int Gi0/3 | sec int
interface GigabitEthernet0/3
description ** to R4 **
no ip address
duplex auto
speed auto
media-type rj45
ipv6 address FE80::1 link-local
ipv6 address 2001:DB8:6::1/64
R5#show run | sec ^ipv6 unicast
ipv6 unicast-routing
R1#show ip bgp ipv6 unicast | beg Ne
Network Next Hop Metric LocPrf Weight Path
*> 2001:DB8:A::/64 :: 0 32768 i
*m 2001:DB8:B::/64 2001:DB8:1::2 0 65100 65002 i « Multipath IPv6 NLRI in BGP table*m 2001:DB8:3::2 0 65100 65002 i
*> 2001:DB8:2::2 0 65100 65002 i
R1#show ip bgp ipv6 unicast summary | beg Ne
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
*2001:DB8:1::2 4 65100 116 117 5 0 0 01:38:42 1
*2001:DB8:2::2 4 65100 116 112 5 0 0 01:38:49 1
*2001:DB8:3::2 4 65100 116 115 5 0 0 01:38:46 1
* Dynamically created based on a listen range command
Dynamically created neighbors: 3, Subnet ranges: 1 « IPv6 Dynamic Neighbors established
BGP peergroup EXAMPLE-PG listen range group members:
2001:DB8::/32
Total dynamically created neighbors: 3/(100 max), Subnet ranges: 1
R1#show ip bgp ipv6 unicast 2001:DB8:B::/64
BGP routing table entry for 2001:DB8:B::/64, version 5
Paths: (3 available, best #3, table default)
Multipath: eBGP
Flag: 0x8000000100
Advertised to update-groups:
1
Refresh Epoch 2
65100 65002
2001:DB8:1::2 (FE80::2) from *2001:DB8:1::2 (2.2.2.2) « IPv6 BGP routes have Global and Link-local next-hop
Origin IGP, localpref 100, valid, external, multipath
rx pathid: 0, tx pathid: 0
Refresh Epoch 2
65100 65002
2001:DB8:3::2 (FE80::2) from *2001:DB8:3::2 (4.4.4.4)
Origin IGP, localpref 100, valid, external, multipath(oldest)
rx pathid: 0, tx pathid: 0
Refresh Epoch 2
65100 65002
2001:DB8:2::2 (FE80::2) from *2001:DB8:2::2 (3.3.3.3)
Origin IGP, localpref 100, valid, external, multipath, best
rx pathid: 0, tx pathid: 0x0
R1#show ipv6 route bgp
IPv6 Routing Table - default - 10 entries
Codes: C - Connected, L - Local, S - Static, U - Per-user Static route
B - BGP, HA - Home Agent, MR - Mobile Router, R - RIP
H - NHRP, I1 - ISIS L1, I2 - ISIS L2, IA - ISIS interarea
IS - ISIS summary, D - EIGRP, EX - EIGRP external, NM - NEMO
ND - ND Default, NDp - ND Prefix, DCE - Destination, NDr - Redirect
RL - RPL, O - OSPF Intra, OI - OSPF Inter, OE1 - OSPF ext 1
OE2 - OSPF ext 2, ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2
la - LISP alt, lr - LISP site-registrations, ld - LISP dyn-eid
lA - LISP away, a - Application
B 2001:DB8:B::/64 [20/0] « Single prefix has three next-hop addresses in IPv6 RIB
via FE80::2, GigabitEthernet0/2
via FE80::2, GigabitEthernet0/3
via FE80::2, GigabitEthernet0/1
Two eBGP paths - different AS
In the following scenario, eBGP on R1 receives multiple equal cost paths but from different ASN neighbors. Namely, R1 peers with R2 (AS 65100) and R3 (AS 65200). On R1, the BGP router command bgp bestpath as-path multipath-relax allows the use of two routes with different AS-Path next-hops to be considered for multipath.
Configuration:
R1
R1#show run | sec ^router
router bgp 65001
bgp router-id 1.1.1.1
bgp log-neighbor-changes
bgp bestpath as-path multipath-relax
neighbor 10.1.0.2 remote-as 65100
neighbor 10.2.0.2 remote-as 65200
!
address-family ipv4
network 172.16.1.0 mask 255.255.255.0
neighbor 10.1.0.2 activate
neighbor 10.2.0.2 activate
maximum-paths 2
exit-address-family
R1#show run int Gi1 | sec int
interface GigabitEthernet1
description ** to Host **
ip address 172.16.1.2 255.255.255.0
negotiation auto
no mop enabled
no mop sysid
R1#show run int Gi2 | sec int
interface GigabitEthernet2
description ** to R2 **
ip address 10.1.0.1 255.255.255.252
negotiation auto
no mop enabled
no mop sysid
R1#show run int Gi3 | sec int
interface GigabitEthernet3
description ** to R3 **
ip address 10.2.0.1 255.255.255.252
negotiation auto
no mop enabled
no mop sysid
R2
R2#show run | sec ^router
router bgp 65100
bgp router-id 2.2.2.2
bgp log-neighbor-changes
neighbor 10.1.0.1 remote-as 65001
neighbor 10.3.0.1 remote-as 65002
!
address-family ipv4
neighbor 10.1.0.1 activate
neighbor 10.3.0.1 activate
exit-address-family
R2#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
description ** to R1 **
ip address 10.1.0.2 255.255.255.252
duplex auto
speed auto
media-type rj45
R2#show run int Gi0/1 | sec int
interface GigabitEthernet0/1
description ** to R4 **
ip address 10.3.0.2 255.255.255.252
duplex auto
speed auto
media-type rj45
R3
R3#show run | sec ^router
router bgp 65200
bgp router-id 3.3.3.3
bgp log-neighbor-changes
neighbor 10.2.0.1 remote-as 65001
neighbor 10.4.0.1 remote-as 65002
!
address-family ipv4
neighbor 10.2.0.1 activate
neighbor 10.4.0.1 activate
exit-address-family
R3#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
description ** to R1 **
ip address 10.2.0.2 255.255.255.252
duplex auto
speed auto
media-type rj45
R3#show run int Gi0/1 | sec int
interface GigabitEthernet0/1
description ** to R4 **
ip address 10.4.0.2 255.255.255.252
duplex auto
speed auto
media-type rj45
R4
R4#show run | sec ^router
router bgp 65002
bgp router-id 4.4.4.4
bgp log-neighbor-changes
neighbor 10.3.0.2 remote-as 65100
neighbor 10.4.0.2 remote-as 65200
!
address-family ipv4
network 172.16.2.0 mask 255.255.255.0
neighbor 10.3.0.2 activate
neighbor 10.4.0.2 activate
exit-address-family
R4#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
description ** to Server **
ip address 172.16.2.2 255.255.255.0
duplex auto
speed auto
media-type rj45
R4#show run int Gi0/1 | sec int
interface GigabitEthernet0/1
description ** to R2 **
ip address 10.3.0.1 255.255.255.252
duplex auto
speed auto
media-type rj45
R4#show run int Gi0/2 | sec int
interface GigabitEthernet0/2
description ** to R3 **
ip address 10.4.0.1 255.255.255.252
duplex auto
speed auto
media-type rj45
R1#show ip bgp | beg Ne
Network Next Hop Metric LocPrf Weight Path
*> 172.16.1.0/24 0.0.0.0 0 32768 i
*m 172.16.2.0/24 10.2.0.2 0 65200 65002 i « Next-hop selected for multipath
*> 10.1.0.2 0 65100 65002 i
R1#show ip bgp 172.16.2.0
BGP routing table entry for 172.16.2.0/24, version 11
Paths: (2 available, best #2, table default)
Multipath: eBGP
Advertised to update-groups:
1
Refresh Epoch 2
65200 65002 « Multipath routes with different neighbor AS
10.2.0.2 from 10.2.0.2 (3.3.3.3)
Origin IGP, localpref 100, valid, external, multipath(oldest)
rx pathid: 0, tx pathid: 0
Updated on Nov 28 2024 18:24:32 UTC
Refresh Epoch 2
65100 65002
10.1.0.2 from 10.1.0.2 (2.2.2.2)
Origin IGP, localpref 100, valid, external, multipath, best
rx pathid: 0, tx pathid: 0x0
Updated on Nov 28 2024 18:24:25 UTC
R1#show ip route bgp | beg Ga
Gateway of last resort is not set
172.16.0.0/16 is variably subnetted, 3 subnets, 2 masks
B 172.16.2.0/24 [20/0] via 10.2.0.2, 00:14:32 « Multipath installed in IP RIB
[20/0] via 10.1.0.2, 00:14:32
Two iBGP paths
In the following example, iBGP multipath is configured with the command maximum-paths ibgp 2 on R1. This means, R1 installs two next-hop addresses received through internal BGP from R2 and R3. The next-hop addresses are for the single destination prefix 172.16.2.0/24 in Site 2.
Configuration:
R1
R1#show run | sec ^router
router bgp 65001
bgp router-id 1.1.1.1
bgp log-neighbor-changes
neighbor 10.1.0.2 remote-as 65001
neighbor 10.2.0.2 remote-as 65001
!
address-family ipv4
network 172.16.1.0 mask 255.255.255.0
neighbor 10.1.0.2 activate
neighbor 10.2.0.2 activate
maximum-paths ibgp 2
exit-address-family
R1#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
description ** to Host **
ip address 172.16.1.2 255.255.255.0
duplex auto
speed auto
media-type rj45
R1#show run int Gi0/1 | sec int
interface GigabitEthernet0/1
description ** to R2 **
ip address 10.1.0.1 255.255.255.252
duplex auto
speed auto
media-type rj45
R1#show run int Gi0/2 | sec int
interface GigabitEthernet0/2
description ** to R3 **
ip address 10.2.0.1 255.255.255.252
duplex auto
speed auto
media-type rj45
R2
R2#show run | sec ^router
router bgp 65001
bgp router-id 2.2.2.2
bgp log-neighbor-changes
neighbor 10.1.0.1 remote-as 65001
neighbor 10.3.0.1 remote-as 65002
!
address-family ipv4
neighbor 10.1.0.1 activate
neighbor 10.1.0.1 next-hop-self
neighbor 10.3.0.1 activate
exit-address-family
R2#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
description ** to R1 **
ip address 10.1.0.2 255.255.255.252
duplex auto
speed auto
media-type rj45
R2#show run int Gi0/1 | sec int
interface GigabitEthernet0/1
description ** to R4 **
ip address 10.3.0.2 255.255.255.252
duplex auto
speed auto
media-type rj45
R3
R3#show run | sec ^router
router bgp 65001
bgp router-id 3.3.3.3
bgp log-neighbor-changes
neighbor 10.2.0.1 remote-as 65001
neighbor 10.4.0.1 remote-as 65002
!
address-family ipv4
neighbor 10.2.0.1 activate
neighbor 10.2.0.1 next-hop-self
neighbor 10.4.0.1 activate
exit-address-family
R3#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
description ** to R1 **
ip address 10.2.0.2 255.255.255.252
duplex auto
speed auto
media-type rj45
R3#show run int Gi0/1 | sec int
interface GigabitEthernet0/1
description ** to R4 **
ip address 10.4.0.2 255.255.255.252
duplex auto
speed auto
media-type rj45
R4
R4#show run | sec ^router
router bgp 65002
bgp router-id 4.4.4.4
bgp log-neighbor-changes
neighbor 10.3.0.2 remote-as 65001
neighbor 10.4.0.2 remote-as 65001
!
address-family ipv4
network 172.16.2.0 mask 255.255.255.0
neighbor 10.3.0.2 activate
neighbor 10.4.0.2 activate
exit-address-family
R4#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
description ** to Server **
ip address 172.16.2.2 255.255.255.0
duplex auto
speed auto
media-type rj45
R4#show run int Gi0/1 | sec int
interface GigabitEthernet0/1
description ** to R2 **
ip address 10.3.0.1 255.255.255.252
duplex auto
speed auto
media-type rj45
R4#show run int Gi0/2 | sec int
interface GigabitEthernet0/2
description ** to R3 **
ip address 10.4.0.1 255.255.255.252
duplex auto
speed auto
media-type rj45
Two iBGP paths - with Route Reflector Add-Path (MPLS)
In the following scenario, Site 1 uses a BGP-free core network with MPLS data forwarding. Due to the BGP Additional Paths feature the Route Reflector R2 advertises two next-hop addresses for the destination prefix 172.16.2.0 /24. This is how R1 receives the BGP next-hop of R4 as well as R5 in order to reach the Server in Site 2.
However, the BGP Add-Path feature on R1 does not install both routes into the RIB/FIB. In other words, although R1 receives two equal cost next-hop addresses in the BGP table for the same destination prefix (Site 2), R1 does not automatically install both routes in the IP routing table. For this reason, the BGP multipath feature is needed on R1 using the command maximum-paths ibgp 2.
Meanwhile, MPLS is enabled on R1, R3, R4 and R5. R2 is a BGP Route Reflector. R1, R4 and R5 share routes through an iBGP neighborship with R2. Such a design is called a BGP-free core because R3 does not have BGP enabled. Importantly, R3 has no knowledge of the Site 1 Host network (172.16.1.0 /24) or the Site 2 network (172.16.2.0 /24). R3 just forwards MPLS encapsulated traffic between R1, R4 and R5.
Configuration:
R1
R1#show run | sec router ospf
router ospf 10
router-id 1.1.1.1
network 1.1.1.1 0.0.0.0 area 0
network 10.1.0.0 0.0.0.3 area 0
R1#show run | sec ^router bgp
router bgp 65001
bgp router-id 1.1.1.1
bgp log-neighbor-changes
neighbor 2.2.2.2 remote-as 65001
neighbor 2.2.2.2 update-source Loopback10
!
address-family ipv4
bgp additional-paths receive
network 172.16.1.0 mask 255.255.255.0
neighbor 2.2.2.2 activate
maximum-paths ibgp 2
exit-address-family
R1#show run int Lo10 | sec int
interface Loopback10
ip address 1.1.1.1 255.255.255.255
R1#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
description ** to Host **
ip address 172.16.1.2 255.255.255.0
duplex auto
speed auto
media-type rj45
R1#show run int Gi0/1 | sec int
interface GigabitEthernet0/1
description ** to R3 **
ip address 10.1.0.1 255.255.255.252
duplex auto
speed auto
media-type rj45
mpls ip
R1#show run | sec ^mpls
mpls label range 100 199
mpls ldp router-id Loopback10 force
R1#show run all | sec ^mpls ip$
mpls ip
R2 (RR)
R2#show run | sec ^router ospf
router ospf 10
router-id 2.2.2.2
network 2.2.2.2 0.0.0.0 area 0
network 10.2.0.0 0.0.0.3 area 0
R2#show run | sec ^router bgp
router bgp 65001
bgp router-id 2.2.2.2
bgp log-neighbor-changes
neighbor 1.1.1.1 remote-as 65001
neighbor 1.1.1.1 update-source Loopback10
neighbor 4.4.4.4 remote-as 65001
neighbor 4.4.4.4 update-source Loopback10
neighbor 5.5.5.5 remote-as 65001
neighbor 5.5.5.5 update-source Loopback10
!
address-family ipv4
bgp additional-paths select all
bgp additional-paths send
neighbor 1.1.1.1 activate
neighbor 1.1.1.1 route-reflector-client
neighbor 1.1.1.1 advertise additional-paths all
neighbor 4.4.4.4 activate
neighbor 4.4.4.4 route-reflector-client
neighbor 5.5.5.5 activate
neighbor 5.5.5.5 route-reflector-client
exit-address-family
R2#show run int Lo10 | sec int
interface Loopback10
ip address 2.2.2.2 255.255.255.255
R2#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
description ** to R3 **
ip address 10.2.0.1 255.255.255.252
duplex auto
speed auto
media-type rj45
R3
R3#show run | sec router ospf
router ospf 10
router-id 3.3.3.3
network 3.3.3.3 0.0.0.0 area 0
network 10.1.0.0 0.0.0.3 area 0
network 10.2.0.0 0.0.0.3 area 0
network 10.3.0.0 0.0.0.3 area 0
network 10.4.0.0 0.0.0.3 area 0
R3#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
description ** to R1 **
ip address 10.1.0.2 255.255.255.252
duplex auto
speed auto
media-type rj45
mpls ip
R3#show run int Gi0/1 | sec int
interface GigabitEthernet0/1
description ** to RR R2 **
ip address 10.2.0.2 255.255.255.252
duplex auto
speed auto
media-type rj45
R3#show run int Gi0/2 | sec int
interface GigabitEthernet0/2
description ** to PE R4 **
ip address 10.3.0.1 255.255.255.252
duplex auto
speed auto
media-type rj45
mpls ip
R3#show run int Gi0/3 | sec int
interface GigabitEthernet0/3
description ** to PE R5 **
ip address 10.4.0.1 255.255.255.252
duplex auto
speed auto
media-type rj45
mpls ip
R3#show run | sec ^mpls
mpls label range 300 399
mpls ldp router-id Loopback10 force
R3#show run all | sec ^mpls ip$
mpls ip
R4 (PE)
R4#show run | sec ^router ospf
router ospf 10
router-id 4.4.4.4
network 4.4.4.4 0.0.0.0 area 0
network 10.3.0.0 0.0.0.3 area 0
R4#show run | sec ^router bgp
router bgp 65001
bgp router-id 4.4.4.4
bgp log-neighbor-changes
neighbor 2.2.2.2 remote-as 65001
neighbor 2.2.2.2 update-source Loopback10
neighbor 2.2.2.2 next-hop-self
neighbor 10.5.0.2 remote-as 65002
R4#show run int Lo10 | sec int
interface Loopback10
ip address 4.4.4.4 255.255.255.255
R4#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
description ** to R3 **
ip address 10.3.0.2 255.255.255.252
duplex auto
speed auto
media-type rj45
mpls ip
R4#show run int Gi0/1 | sec int
interface GigabitEthernet0/1
description ** to R6 **
ip address 10.5.0.1 255.255.255.252
duplex auto
speed auto
media-type rj45
R4#show run | sec ^mpls
mpls label range 400 499
mpls ldp router-id Loopback10 force
R4#show run all | sec ^mpls ip$
mpls ip
R5 (PE)
R5#show run | sec ^router ospf
router ospf 10
router-id 5.5.5.5
network 5.5.5.5 0.0.0.0 area 0
network 10.4.0.0 0.0.0.3 area 0
R5#show run | sec ^router bgp
router bgp 65001
bgp router-id 5.5.5.5
bgp log-neighbor-changes
neighbor 2.2.2.2 remote-as 65001
neighbor 2.2.2.2 update-source Loopback10
neighbor 2.2.2.2 next-hop-self
neighbor 10.6.0.2 remote-as 65002
R5#show run int Lo10 | sec int
interface Loopback10
ip address 5.5.5.5 255.255.255.255
R5#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
description ** to R3 **
ip address 10.4.0.2 255.255.255.252
duplex auto
speed auto
media-type rj45
mpls ip
R5#show run int Gi0/1 | sec int
interface GigabitEthernet0/1
description ** to R6 **
ip address 10.6.0.1 255.255.255.252
duplex auto
speed auto
media-type rj45
R5#show run | sec ^mpls
mpls label range 500 599
mpls ldp router-id Loopback10 force
R5#show run all | sec ^mpls ip$
mpls ip
R6
R6#show run | sec ^router
router bgp 65002
bgp router-id 6.6.6.6
bgp log-neighbor-changes
network 172.16.2.0 mask 255.255.255.0
neighbor 10.5.0.1 remote-as 65001
neighbor 10.6.0.1 remote-as 65001
R6#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
description ** to R4 **
ip address 10.5.0.2 255.255.255.252
duplex auto
speed auto
media-type rj45
R6#show run int Gi0/1 | sec int
interface GigabitEthernet0/1
description ** to R5 **
ip address 10.6.0.2 255.255.255.252
duplex auto
speed auto
media-type rj45
R6#show run int Gi0/2 | sec int
interface GigabitEthernet0/2
description ** to Server **
ip address 172.16.2.2 255.255.255.0
duplex auto
speed auto
media-type rj45
R2#show ip bgp neighbors 1.1.1.1 | sec Addi
Additional Paths send capability: advertised« BGP Additional Paths are advertised from RR R2 to R1
Additional Paths receive capability: received
R1#show ip bgp 172.16.2.0
BGP routing table entry for 172.16.2.0/24, version 9
Paths: (2 available, best #1, table default)
Multipath: iBGP
Not advertised to any peer
Refresh Epoch 1
65002
4.4.4.4 (metric 3) from 2.2.2.2 (2.2.2.2) « R1 receives next-hop addresses from RR R2
Origin IGP, metric 0, localpref 100, valid, internal, multipath, best « R1 uses BGP multipath to install both next-hops in local RIB/FIB
Originator: 4.4.4.4, Cluster list: 2.2.2.2
rx pathid: 0x0, tx pathid: 0x0
Refresh Epoch 1
65002
5.5.5.5 (metric 3) from 2.2.2.2 (2.2.2.2)
Origin IGP, metric 0, localpref 100, valid, internal, multipath(oldest)
Originator: 5.5.5.5, Cluster list: 2.2.2.2
rx pathid: 0x1, tx pathid: 0
R1#show ip route 172.16.2.0
Routing entry for 172.16.2.0/24
Known via "bgp 65001", distance 200, metric 0
Tag 65002, type internal
Last update from 5.5.5.5 00:14:18 ago
Routing Descriptor Blocks:
5.5.5.5, from 2.2.2.2, 00:14:18 ago « R1 receives next-hop addresses from RR R2
Route metric is 0, traffic share count is 1
AS Hops 1
Route tag 65002
MPLS label: none
* 4.4.4.4, from 2.2.2.2, 00:14:18 ago
Route metric is 0, traffic share count is 1
AS Hops 1
Route tag 65002
MPLS label: none
R1#show ip cef 172.16.2.0 detail
172.16.2.0/24, epoch 0, flags [rib only nolabel, rib defined all labels], per-destination sharing
recursive via 4.4.4.4
nexthop 10.1.0.2 GigabitEthernet0/1 label 302-(local:104) « MPLS label switching is used between R1 and PE routers
recursive via 5.5.5.5
nexthop 10.1.0.2 GigabitEthernet0/1 label 303-(local:105)
Host#ping 172.16.2.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.2.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 3/3/4 ms « Host can reach the ServerHost#trace 172.16.2.1 probe 1
Type escape sequence to abort.
Tracing the route to 172.16.2.1
VRF info: (vrf in name/id, vrf out name/id)
1 172.16.1.2 2 msec
2 *
3 10.4.0.2 2 msec
4 10.6.0.2 4 msec
5 172.16.2.1 4 msec
R1#show mpls forwarding-table
Local Outgoing Prefix Bytes Label Outgoing Next Hop
Label Label or Tunnel Id Switched interface
100 Pop Label 10.4.0.0/30 0 Gi0/1 10.1.0.2
101 Pop Label 10.3.0.0/30 0 Gi0/1 10.1.0.2
102 Pop Label 10.2.0.0/30 0 Gi0/1 10.1.0.2
103 301 2.2.2.2/32 0 Gi0/1 10.1.0.2
104 302 4.4.4.4/32 0 Gi0/1 10.1.0.2 « MPLS forwarding table on R1 with labels to reach PE routers
105 303 5.5.5.5/32 0 Gi0/1 10.1.0.2
106 Pop Label 3.3.3.3/32 0 Gi0/1 10.1.0.2
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