aci multisite BUM traffic handlingf across sites steps and GIPo

14.01 2020 | by massimiliano

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aci multisite BUM traffic handling across sites steps   the deployment of VXLAN allows the use of a logical abstraction […]

aci multisite BUM traffic handling across sites steps

 

the deployment of VXLAN allows the use of a logical abstraction so that endpoints separated by multiple Layer 3 hops can communicate as if they were part of the same logical Layer 2 domain

 

those endpoints must be capable of sourcing Layer 2 multidestination frames received by all the other endpoints connected to the same Layer 2 segment, regardless of their actual physical location

 

this capability can be achieved either by using the native multicast replication functions offered by the Layer 3 infrastructure interconnecting the endpoints (this is the approach adopted in the Cisco ACI Multi-Pod architecture) or by enabling head-end replication functions (often referred to as HER) on the source VXLAN TEP (VTEP) devices, which create multiple unicast copies of each BUM frame to be sent to all the remote VTEPs on which those endpoints are connected.

 

ACI Multi-Site-capable spine switches performing the HER function, because the interconnected fabrics may be deployed around the world and it would be difficult to ensure proper multicast support across the entire interconnecting network infrastructure.

 

NOTE that transmission of Layer 2 BUM frames across sites is required only for the specific bridge domains that are stretched with flooding enabled

 

 

STEPS:

 

1)

EP1, belonging to a specific bridge domain, generates a Layer 2 BUM frame

 

2)

Depending on the type of frame and the corresponding bridge domain settings, the leaf may need to flood the traffic in that specific Layer 2 domain; as a consequence, the frame is VXLAN encapsulated and sent to the specific multicast group (called GIPo) associated with the bridge domain within the fabric along one of the specific multidestination trees associated to that GIPo, so it can reach all the other leaf and spine nodes 

 

3)

One of the spine nodes connected to the external intersite network is elected as the designated forwarder for that specific bridge domain (this election is held between the spine nodes using IS-IS protocol exchanges); the designated forwarder is responsible for replicating each BUM frame for that bridge domain to all the remote sites with endpoints connected to the same stretched bridge domain. The designated forwarder is responsible for replicating each BUM frame for that bridge domain to all the remote sites with endpoints connected to the same stretched bridge domain

 

4)

The designated forwarder makes a copy of the BUM frame and sends it to the remote sites; the destination IP address used when VXLAN encapsulating the packet is a special IP address (HER-ETEP) identifying each remote site and is used specifically for the transmission of BUM traffic across sites.

The HER-ETEP is another anycast IP address defined on all the remote spine nodes that are connected to the intersite network (each site uses a unique HER-ETEP address).

The source IP address for the VXLAN encapsulated packet is instead the anycast DP-ETEP address deployed on all the local spine nodes connected to the intersite network

 

NOTE that the HER-ETEP (referred to as multicast data TEP in the Cisco ACI Multi-Site GUI) is yet another IP address that must be sent to the Layer 3 network connecting the fabrics

 

5)

One of the remote spine nodes receives the packet, translates the VNID value contained in the header to the locally significant VNID value associated with the same bridge domain, and sends the traffic to the site along one of the local multidestination trees defined for the bridge domain

 

6)

The traffic is forwarded within the site and reaches all the spine and leaf nodes with endpoints actively connected to the specific bridge domain

 

7)

The receiving leaf nodes use the information contained in the VXLAN header to learn the site location for endpoint EP1 that sourced the BUM frame; They also send the BUM frame to all the local interfaces associated with the bridge domain, so that endpoint EP2 can receive it