This post I would like to show the effect of BGP best-path
cost community on an EIGRP MPLS VPN Layer-3 topology.
More technical information can be found in the following
URLs:
Now let’s have the following topology:
R1, R3 and R4 are all PE routers
R2 is P router
R5 and R6 are CE routers located on VRF RED
All SP routers (R1, R2, R3 and R4) are running ISIS, LDP,
BGP and MP-BGP while PE to CE routing protocol is EIGRP.
R5 is advertising network 192.168.51.0/24 while R6 is
advertising network 192.168.61.0/24 and default route. Note that R5 is multi-homed
and connected to both R3 and R4.
Now let’s have a look on R5 routing table:
|
R5#show ip route
Codes: C - connected, S -
static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O -
OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 -
OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF
external type 2
i - IS-IS, su - IS-IS summary, L1 -
IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate
default, U - per-user static route
o - ODR, P - periodic downloaded
static route
Gateway of last resort is 10.1.35.3 to network
0.0.0.0
D 192.168.61.0/24 [90/133120] via 10.1.35.3, 00:12:25,
FastEthernet0/0
10.0.0.0/24 is subnetted, 3 subnets
D 10.1.16.0 [90/5120] via 10.1.35.3,
00:12:25, FastEthernet0/0
C 10.1.45.0 is directly connected,
FastEthernet0/1
C 10.1.35.0 is directly connected,
FastEthernet0/0
C 192.168.51.0/24 is directly connected,
Loopback1
D*EX 0.0.0.0/0 [170/7680] via
10.1.35.3,
00:12:25, FastEthernet0/0
|
Now let’s look on R4 VRF routing table:
|
R4#show ip route vrf RED
Routing Table: RED
Codes: C - connected, S -
static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O -
OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 -
OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF
external type 2
i - IS-IS, su - IS-IS summary, L1 -
IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate
default, U - per-user static route
o - ODR, P - periodic downloaded
static route
Gateway of last resort is
1.1.1.1 to network 0.0.0.0
B 192.168.61.0/24 [200/156160] via 1.1.1.1,
01:32:13
10.0.0.0/24 is subnetted, 3 subnets
B 10.1.16.0 [200/0] via 1.1.1.1,
01:32:13
C 10.1.45.0 is directly connected,
FastEthernet1/1
B 10.1.35.0 [200/0] via 3.3.3.3,
01:32:13
B 192.168.51.0/24
[200/130560] via 3.3.3.3,
01:32:13
B* 0.0.0.0/0 [200/30720] via 1.1.1.1,
01:32:13
|
Note that R4 prefers R3 as the best path to network
192.168.51.0/24 which is directly connected to R5, which is connected to R4 on
Fa1/1!
Viewing R4 EIGRP topology:
|
R4#show ip eigrp vrf RED
topology
IP-EIGRP Topology Table for
AS(100)/ID(10.1.45.4) Routing Table: RED
Codes: P - Passive, A -
Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 0.0.0.0/0, 1 successors, FD
is 5120
via VPNv4 Sourced (5120/0)
P 10.1.16.0/24, 1 successors,
FD is 2560
via VPNv4 Sourced (2560/0)
P 10.1.45.0/24, 1 successors,
FD is 1280000
via Connected, FastEthernet1/1
P 10.1.35.0/24, 1 successors,
FD is 2560
via VPNv4 Sourced (2560/0)
P 192.168.61.0/24, 1
successors, FD is 130560
via VPNv4 Sourced (130560/0)
P 192.168.51.0/24, 1 successors, FD is 130560
via VPNv4 Sourced
(130560/0)
via 10.1.45.5
(1408000/128000), FastEthernet1/1
|
Network 192.168.51.0/24 is learned from two sources, the
first through MP-BGP and the second through directly connected neighbor, so how
come that R4 choose the MP-BGP over EIGRP?
Looking at MP-BGP route to this network will reveal the
answer:
|
R4#show ip bgp vpnv4 vrf RED
192.168.51.0/24
BGP routing table entry for
4:100:192.168.51.0/24, version 18
Paths: (1 available, best #1,
table RED)
Not advertised to any peer
Local, imported path from
3:100:192.168.51.0/24
3.3.3.3 (metric 20) from 2.2.2.2
(2.2.2.2)
Origin incomplete, metric 130560,
localpref 100, valid, internal, best
Extended Community: RT:100:100 Cost:pre-bestpath:128:130560
0x8800:32768:0 0x8801:100:130560
0x8802:65281:25600 0x8803:65281:1500
Originator: 3.3.3.3, Cluster list:
2.2.2.2
mpls labels in/out nolabel/18
|
In order to change this behavior uses the following command:
|
R4(config)#router bgp 65000
R4(config-router)#bgp
bestpath cost-community ignore
|
And after clearing BGP peering here is the result:
|
R4#show ip route vrf RED
Routing Table: RED
Codes: C - connected, S -
static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O -
OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 -
OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF
external type 2
i - IS-IS, su - IS-IS summary, L1 -
IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate
default, U - per-user static route
o - ODR, P - periodic downloaded
static route
Gateway of last resort is
10.1.45.5 to network 0.0.0.0
D 192.168.61.0/24 [90/1413120] via
10.1.45.5, 00:00:10, FastEthernet1/1
10.0.0.0/24 is subnetted, 3 subnets
D 10.1.16.0 [90/1285120] via 10.1.45.5,
00:00:10, FastEthernet1/1
C 10.1.45.0 is directly connected,
FastEthernet1/1
D 10.1.35.0 [90/1282560] via 10.1.45.5,
00:00:10, FastEthernet1/1
D 192.168.51.0/24
[90/1408000] via 10.1.45.5, 00:00:10, FastEthernet1/1
D*EX 0.0.0.0/0 [170/1287680]
via 10.1.45.5, 00:00:10, FastEthernet1/1
|
And
|
R4#show ip eigrp vrf RED
topology
IP-EIGRP Topology Table for
AS(100)/ID(10.1.45.4) Routing Table: RED
Codes: P - Passive, A -
Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 0.0.0.0/0, 1 successors, FD
is 1287680
via 10.1.45.5 (1287680/7680),
FastEthernet1/1
P 10.1.16.0/24, 1 successors,
FD is 1285120
via 10.1.45.5 (1285120/5120),
FastEthernet1/1
P 10.1.45.0/24, 1 successors,
FD is 1280000
via Connected, FastEthernet1/1
P 10.1.35.0/24, 1 successors,
FD is 1282560
via 10.1.45.5 (1282560/2560),
FastEthernet1/1
P 192.168.61.0/24, 1
successors, FD is 1413120
via 10.1.45.5 (1413120/133120),
FastEthernet1/1
P 192.168.51.0/24, 1
successors, FD is 130560
via 10.1.45.5 (1408000/128000),
FastEthernet1/1
|
No comments:
Post a Comment