MPLS Hub & Spoke (metric) help

[fsebera]

I'm trying to understand HOW net 172.16.4/24 became preferred via 10.0.0.2 in VRF REG3.

It appears (to me) the decision was made based on lowest METRIC value.

I see the (metric) was set somewhere in the MPLS cloud (I think) but cannot discover how/why the different metric values are being applied to this net. I would like to be able to influence these values as well.

As seen by the command PE3#>sh ip bgp vpnv4 all 172.16.4.0

When 172.16.4/24 is received from 10.0.0.2 (PE2) metric of 30 is applied

When 172.16.4/24 is received from 10.0.0.1 (PE1) metric of 40 is applied

PE3 receives this net in from CE Spoke-4 (WAN4) via eBGP VRF REG4

Since this is a hub and spoke topology, PE3 also receives the net from PE1 and PE2 and in this path is where the metric seems to be getting set.

sh ip bgp vpnv4 vrf nei received-routes

or

sh ip bgp vpnv4 vrf nei adv

run on the different PEs always shows this net with a metric value of 0

Please see my simple topology map of this setup.

Thanks for ANY assistance!

Frank

PE3#sh ip bgp vpnv4 vrf REG3     

BGP table version is 252, local router ID is 10.0.0.3

   Network          Next Hop            Metric LocPrf Weight Path

Route Distinguisher: 30:1020 (default for vrf REG3)

*>i0.0.0.0         10.0.0.2                0    100      0 20 i

* i                10.0.0.1                0    100      0 10 i

* i172.16.1.0/24   10.0.0.2                0    100      0 20 10 ?

*>i                10.0.0.1                2    100      0 10 ?

*>i172.16.2.0/24   10.0.0.2                2    100      0 20 ?

* i                10.0.0.1                0    100      0 10 20 ?

* i172.16.3.0/24   10.0.0.2                0    100      0 20 65535 30 ?

* i                10.0.0.1                0    100      0 10 65535 30 ?

*>                 172.17.30.1             0    700      0 30 ?

*>i172.16.4.0/24   10.0.0.2                0    100      0 20 65535 40 ?

* i                10.0.0.1                0    100      0 10 65535 40 ?

* i172.16.10.0/24  10.0.0.2                0    100      0 20 10 i

*>i                10.0.0.1                0    100      0 10 i

----snip---

PE3#sh ip bgp vpnv4 all 172.16.4.0

BGP routing table entry for 10:1:172.16.4.0/24, version 94

Paths: (1 available, best #1, no table)

   Not advertised to any peer   10 65535 40

     10.0.0.1 (metric 40) from 10.0.0.1 (10.0.0.1)

       Origin incomplete, metric 0, localpref 100, valid, internal, best

       Extended Community: RT:10:10

       mpls labels in/out nolabel/248

BGP routing table entry for 20:1:172.16.4.0/24, version 246

Paths: (1 available, best #1, no table)

   Not advertised to any peer

   20 65535 40

     10.0.0.2 (metric 30) from 10.0.0.2 (10.0.0.2)

       Origin incomplete, metric 0, localpref 100, valid, internal, best

       Extended Community: RT:20:20

       mpls labels in/out nolabel/11413

BGP routing table entry for 30:1020:172.16.4.0/24, version 250

Paths: (2 available, best #1, table REG3)

   Advertised to update-groups:

     17

   20 65535 40, imported path from 20:1:172.16.4.0/24

     10.0.0.2 (metric 30) from 10.0.0.2 (10.0.0.2)

       Origin incomplete, metric 0, localpref 100, valid, internal, best

       Extended Community: RT:20:20

       mpls labels in/out nolabel/11413

   10 65535 40, imported path from 10:1:172.16.4.0/24

     10.0.0.1 (metric 40) from 10.0.0.1 (10.0.0.1)

       Origin incomplete, metric 0, localpref 100, valid, internal

       Extended Community: RT:10:10

       mpls labels in/out nolabel/248

[rsimoni · Accepted solution]

Hi Frank,

Path 1 via 10.0.0.2 is better than path 2 via 10.0.0.1 because it has a lower IGP metric to the NEXT_HOP.

In other words for PE3 is easier to get to 10.0.0.2 rather than 10.0.0.1

regards,

Riccardo

[fsebera]

Hi Richardo,

THANKS!!

Didn't realize (brain freeze) the local IGP (ISIS) played a role here but clearly does.

Totally forgot ISIS this was in use, even through "I" set it up :).

Ahhhh, the learning continues!!

Regards

Frank

PE3#sh ip ro

Codes: L - local, 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, H - NHRP, l - LISP

       + - replicated route, % - next hop override

Gateway of last resort is not set

       10.0.0.0/8 is variably subnetted, 9 subnets, 2 masks

i L1    10.0.0.1/32 [115/40] via 10.3.0.2, 2d16h, FastEthernet0/1

i L1    10.0.0.2/32 [115/30] via 10.3.0.2, 2d16h, FastEthernet0/1

C        10.0.0.3/32 is directly connected, Loopback0

i L1    10.0.0.4/32 [115/30] via 10.3.0.2, 2d16h, FastEthernet0/1

i L1    10.0.0.5/32 [115/20] via 10.3.0.2, 2d16h, FastEthernet0/1

i L1    10.1.0.0/24 [115/30] via 10.3.0.2, 2d16h, FastEthernet0/1

i L1    10.2.0.0/24 [115/20] via 10.3.0.2, 2d16h, FastEthernet0/1

C        10.3.0.0/24 is directly connected, FastEthernet0/1

L        10.3.0.1/32 is directly connected, FastEthernet0/1

       192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks

C        192.168.1.0/24 is directly connected, FastEthernet0/0.6

L        192.168.1.12/32 is directly connected, FastEthernet0/0.6

[rsimoni]

Hi Frank,

don't worry  brain freezes happen all the times even to experienced engineers.

Riccardo