VXLAN Fabric underlay routing - Why IGP session between leaf switches?

[Carlos Gonzalez Garcia]

Hi

I'm analyzing an existing VXLAN EVPN fabric with the following caracteristics:

- VPC between leaf swtiches

- OSPF as the underlay IGP.

Each leaf has an OSPF session with each spine, but also with the peer leaf over the VPC Peer-link. Why this last session is necessary? Currently there are no prefixes announced on it.

Thanks in advance for your help,

Carlos

[M02@rt37]

hello @Carlos Gonzalez Garcia 

If you do # sh ip ospf interface brief you see Po1 ?

Note that, IGP session between Leaf it's mandatory. Through VPC peer-LINK ? not necessary! It this model, the best way it is to use a vPC peer-link virtual !

[Carlos Gonzalez Garcia]

Hi

Thanks for your help. I guessed that IGP session between Leaf was mandatory, but didn't found any documentacion about it. Do you know any?
Didn't know about the feature you mentioned -- vPC with Fabric Peering for VXLAN EVPN. I will investigate it to improve this design.

Regarding the OSPF session:

leaf1# sh ip ospf neighbors
OSPF Process ID UNDERLAY VRF default
Total number of neighbors: 3
Neighbor ID Pri State Up Time Address Interface
100.64.1.102 1 FULL/ - 1y29w 100.64.10.5 Vlan3900
100.64.1.1 1 FULL/ - 1y29w 100.64.11.100 Eth1/51
100.64.1.2 1 FULL/ - 1y29w 100.64.12.100 Eth1/52


leaf1# sh ip ospf interface brief
Interface ID Area Cost State Neighbors Status
Vlan3900 3 0.0.0.0 400 P2P 1 up


Vlan3900 is a regular one (not asociated with VNI)

leaf1# sh ip arp 100.64.10.5
IP ARP Table
Total number of entries: 1
Address Age MAC Address Interface Flags
100.64.10.5 00:10:30 b4de.3100.aae7 Vlan3900


leaf1# sh mac address-table address b4de.3100.aae7
VLAN MAC Address Type age Secure NTFY Ports
---------+-----------------+--------+---------+------+----+-----------------
G 3900 b4de.3100.aae7 static - F F vPC Peer-Link(R)

Thanks again,

Carlos

[AshSe]

Dear Carlos,

As you know that vPC peer-link is a L2 link and thus no P2P OSPF neighborship can be formed. Also, there is no Broadcast Multiaccess Network (BMA) there in the setup. This means that OSPF is used to exchange the routes amongst all the Leaf and the Spine nodes only. I wonder if you can see OSPF between vPC peer switches. 

May I request you to label your diagram with the interface IP addresses including loopback addresses for improvised understanding.