Jul
19

Can you solve this puzzle?

R2, R3 and R4 create the service provider network, with MPLS on all three routers, and iBGP at the PE routers.  R1 and R5 are the CE routers.

R2, prefers the BGP next hop of 4.4.4.4 for network 5.5.5.5 (R5 loopback). R4, at 4.4.4.4 is an iBGP neighbor.

R2#show ip route vrf v | inc 5.5.5.0
B 5.5.5.0 [200/409600] via 4.4.4.4, 00:06:47

Is R2 preferring an iBGP learned route, which has an AD of 200, over a EIGRP route, which would have an AD of 90?

Can you identify why the routing for 5.5.5.0 on the VRF of R2 is using BGP instead of EIGRP?

EIGRP PATH with MPLS

Below are the relevant portions of the configuration, which also can serve as a great review of how to configure MPLS VPNs.
R1, CE router:

R1#show run
interface Loopback0
ip address 1.1.1.1 255.255.255.0
!
interface FastEthernet0/0
ip address 10.1.12.1 255.255.255.0
duplex auto
speed auto
!
interface Serial0/0
ip address 10.1.215.1 255.255.255.0
!

router eigrp 1
network 0.0.0.0
no auto-summary

R2, PE Router:

R2#show run
!
ip vrf v
rd 1:1
route-target export 1:1
route-target import 1:1
!
!
interface Loopback0
ip address 2.2.2.2 255.255.255.255
ip ospf 1 area 0
!
interface FastEthernet0/0
ip vrf forwarding v
ip address 10.1.12.2 255.255.255.0
!
interface FastEthernet0/1
ip address 10.1.23.2 255.255.255.0
ip ospf 1 area 0
mpls ip
!
router eigrp 1
no auto-summary
!
address-family ipv4 vrf v
redistribute bgp 234 metric 1 10000 1 1 1
network 10.1.12.2 0.0.0.0
auto-summary
autonomous-system 1
exit-address-family
!
router ospf 1
log-adjacency-changes
!
router bgp 234
no bgp default ipv4-unicast
bgp log-neighbor-changes
neighbor 4.4.4.4 remote-as 234
neighbor 4.4.4.4 update-source Loopback0
!
address-family vpnv4
neighbor 4.4.4.4 activate
neighbor 4.4.4.4 send-community extended
exit-address-family
!
address-family ipv4 vrf v
redistribute eigrp 1
no synchronization
exit-address-family
!
ip forward-protocol nd
!

R3, P router:

R3#show run

interface Loopback0
ip address 3.3.3.3 255.255.255.255
!
interface FastEthernet0/0
ip address 10.1.34.3 255.255.255.0
mpls ip
!
interface FastEthernet0/1
ip address 10.1.23.3 255.255.255.0
mpls ip
!
router ospf 1
log-adjacency-changes
network 0.0.0.0 255.255.255.255 area 0
!

R4: PE Router

R4#show run
!
ip vrf v
rd 1:1
route-target export 1:1
route-target import 1:1
!
!
interface Loopback0
ip address 4.4.4.4 255.255.255.255
ip ospf 1 area 0
!
interface FastEthernet0/0
ip address 10.1.34.4 255.255.255.0
ip ospf 1 area 0
mpls ip
!
interface FastEthernet0/1
ip vrf forwarding v
ip address 10.1.45.4 255.255.255.0
!
router eigrp 1
no auto-summary
!
address-family ipv4 vrf v
redistribute bgp 234 metric 1 1 1 1 1
network 10.1.45.4 0.0.0.0
auto-summary
autonomous-system 1
exit-address-family
!
router ospf 1
log-adjacency-changes
!
router bgp 234
no bgp default ipv4-unicast
bgp log-neighbor-changes
neighbor 2.2.2.2 remote-as 234
neighbor 2.2.2.2 update-source Loopback0
!
address-family vpnv4
neighbor 2.2.2.2 activate
neighbor 2.2.2.2 send-community extended
exit-address-family
!
address-family ipv4 vrf v
redistribute eigrp 1
no synchronization
exit-address-family

R5: CE Router

R5#show run
!
interface Loopback0
ip address 5.5.5.5 255.255.255.0
!
interface Serial0/0
ip address 10.1.215.5 255.255.255.0
clock rate 64000
!
interface FastEthernet0/1
ip address 10.1.45.5 255.255.255.0
!
router eigrp 1
network 0.0.0.0
no auto-summary
!

Now for a couple show commands on R1:

R1#show ip route eigrp
5.0.0.0/24 is subnetted, 1 subnets
D 5.5.5.0 [90/435200] via 10.1.12.2, 00:19:08, FastEthernet0/0
10.0.0.0/24 is subnetted, 3 subnets
D 10.1.45.0 [90/307200] via 10.1.12.2, 00:19:08, FastEthernet0/0
R1#

R1#show ip eigrp topology
IP-EIGRP Topology Table for AS(1)/ID(10.1.215.1)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status

P 1.1.1.0/24, 1 successors, FD is 128256
via Connected, Loopback0
P 5.5.5.0/24, 1 successors, FD is 435200
via 10.1.12.2 (435200/409600), FastEthernet0/0
via 10.1.215.5 (2297856/128256), Serial0/0
P 10.1.12.0/24, 1 successors, FD is 281600
via Connected, FastEthernet0/0
P 10.1.45.0/24, 1 successors, FD is 307200
via 10.1.12.2 (307200/281600), FastEthernet0/0
via 10.1.215.5 (2195456/281600), Serial0/0
P 10.1.215.0/24, 1 successors, FD is 2169856
via Connected, Serial0/0
R1#

And some on R2, the PE router:

R2#show ip route vrf v

Routing Table: v
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 not set

1.0.0.0/24 is subnetted, 1 subnets
D 1.1.1.0 [90/409600] via 10.1.12.1, 00:31:48, FastEthernet0/0
5.0.0.0/24 is subnetted, 1 subnets
B 5.5.5.0 [200/409600] via 4.4.4.4, 00:02:34
10.0.0.0/24 is subnetted, 3 subnets
C 10.1.12.0 is directly connected, FastEthernet0/0
B 10.1.45.0 [200/0] via 4.4.4.4, 00:31:48
D 10.1.215.0 [90/2195456] via 10.1.12.1, 00:31:21, FastEthernet0/0

R2#show ip eigrp vrf v topology
IP-EIGRP Topology Table for AS(1)/ID(10.1.12.2) Routing Table: v

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status

P 1.1.1.0/24, 1 successors, FD is 409600
via 10.1.12.1 (409600/128256), FastEthernet0/0
P 5.5.5.0/24, 1 successors, FD is 409600
via VPNv4 Sourced (409600/0)
P 10.1.12.0/24, 1 successors, FD is 281600
via Connected, FastEthernet0/0
P 10.1.45.0/24, 1 successors, FD is 281600
via VPNv4 Sourced (281600/0)
P 10.1.215.0/24, 1 successors, FD is 2195456
via 10.1.12.1 (2195456/2169856), FastEthernet0/0
R2#

Take a minute to post your thoughts, and as always, happy studies.

....

 

It has been a few days, and we have received lots of great ideas.   Thank you.

When R4 receives the routes in VRF v, the EIGRP metrics are copied into extended BGP attributes, and include the information for metric, AS, route-type and more.  The iBGP updates from R4 to R2 contain all those attributes.   When R2 receives the updates, if the route type is internal (from EIGRP attributes) and the source EIGRP AS matches the local EIGRP AS we are importing to, it will then be up to the  metric to determine the best path.

If we decreased the bandwidth statement on R4 Fa0/1, or used an offset list (2,000,000 more should do the trick) on R5 out Fa0/1 (towards R4), the increase in metric would cause R2 to prefer the path through R1 for 5.5.5.0/24 instead of using the MPLS backbone.

BGP updates that contain the cost community attribute will use the EIGRP AD instead of the iBGP AD of 200 to compare routes on metric alone. In that light, another option, would be to tell R2 to ignore cost-community, with the BGP router command:

bgp bestpath cost-community ignore

Let's take a look at the results.

Here is the baseline for before any changes:

R2#show ip route vrf v | inc 5.5.5
B 5.5.5.0 [200/409600] via 4.4.4.4, 00:02:29
R2#show ip bgp vpnv4 all 5.5.5.0
BGP routing table entry for 1:1:5.5.5.0/24, version 8
Paths: (1 available, best #1, table v)
Flag: 0x820
Not advertised to any peer
Local
4.4.4.4 (metric 21) from 4.4.4.4 (4.4.4.4)
Origin incomplete, metric 409600, localpref 100, valid, internal, best
Extended Community: RT:1:1 Cost:pre-bestpath:128:409600 0x8800:32768:0
0x8801:1:153600 0x8802:65281:256000 0x8803:65281:1500
mpls labels in/out nolabel/19
R2#

Now we will remove the default behavior

R2(config)#router bgp 234
R2(config-router)#bgp bestpath cost-community ignore

Cleared BGP sessions and routing tables, and waited a minute before the following show commands:

R2#show ip route vrf v | inc 5.5.5
D 5.5.5.0 [90/2323456] via 10.1.12.1, 00:00:08, FastEthernet0/0
R2#show ip bgp vpnv4 all 5.5.5.0
BGP routing table entry for 1:1:5.5.5.0/24, version 8
Paths: (2 available, best #2, table v)
Flag: 0x820
Advertised to update-groups:
1
Local
4.4.4.4 (metric 21) from 4.4.4.4 (4.4.4.4)
Origin incomplete, metric 409600, localpref 100, valid, internal
Extended Community: RT:1:1 Cost:pre-bestpath:128:409600 0x8800:32768:0
0x8801:1:153600 0x8802:65281:256000 0x8803:65281:1500
mpls labels in/out 20/19
Local
10.1.12.1 from 0.0.0.0 (2.2.2.2)
Origin incomplete, metric 2323456, localpref 100, weight 32768, valid, sourced, best
Extended Community: RT:1:1
Cost:pre-bestpath:128:2323456 (default-2145160191) 0x8800:32768:0
0x8801:1:665600 0x8802:65282:1657856 0x8803:65281:1500
mpls labels in/out 20/nolabel
R2#

After setting it back to defaults, we could then try an offset list on R5 advertising to R4:

R5(config)#router eigrp 1
R5(config-router)#offset-list 0 out 2000000 fastEthernet 0/1

Cleared BGP sessions and routing tables, and waited a minute before the following show commands:

R2#show ip route vrf v | inc 5.5.5
D 5.5.5.0 [90/2323456] via 10.1.12.1, 00:06:28, FastEthernet0/0
R2#show ip bgp vpnv4 all 5.5.5.0
BGP routing table entry for 1:1:5.5.5.0/24, version 12
Paths: (1 available, best #1, table v)
Flag: 0x820
Advertised to update-groups:
1
Local
10.1.12.1 from 0.0.0.0 (2.2.2.2)
Origin incomplete, metric 2323456, localpref 100, weight 32768, valid, sourced, best
Extended Community: RT:1:1
Cost:pre-bestpath:128:2323456 (default-2145160191) 0x8800:32768:0
0x8801:1:665600 0x8802:65282:1657856 0x8803:65281:1500
mpls labels in/out 31/nolabel
R2#

After resetting all that, implementing the following on R4, and then clearing BGP and routing, we issue the show commands again.

R4(config)#int fa 0/1
R4(config-if)#bandwidth 100

R2#show ip route vrf v | inc 5.5.5
D 5.5.5.0 [90/2323456] via 10.1.12.1, 00:00:05, FastEthernet0/0
R2#show ip bgp vpnv4 all 5.5.5.0
BGP routing table entry for 1:1:5.5.5.0/24, version 20
Paths: (1 available, best #1, table v)
Flag: 0x820
Advertised to update-groups:
1
Local
10.1.12.1 from 0.0.0.0 (2.2.2.2)
Origin incomplete, metric 2323456, localpref 100, weight 32768, valid, sourced, best
Extended Community: RT:1:1
Cost:pre-bestpath:128:2323456 (default-2145160191) 0x8800:32768:0
0x8801:1:665600 0x8802:65282:1657856 0x8803:65281:1500
mpls labels in/out 23/nolabel
R2#

Thanks again to all who contributed. I encourage all RS candidates to lab this up, as well as practice MPLS with OSPF at the CEs.

Marcel posted a comment, reminding us of an excellent document written by Petr, on this topic and more. The original post from Petr which includes the link to free .PDF for this document may be found by clicking here. Thanks Marcel!

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