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Bridging Overview

A vEdge router can act as a transparent bridge, switching traffic between LANs that are part of a VLAN at the local router’s site. To implement bridging, the Viptela architecture defines the concept of a bridge domain. Each bridge domain corresponds to a single VLAN. From a switching point of view, each bridge domain is a separate broadcast domain, and each has its own Ethernet switching table (or MAC table) to use for switching traffic within the broadcast domain. Multiple bridge domains, and hence multiple VLANs, can co-exist on a single vEdge router.

To allow hosts in different bridge domains to communicate with each other, vEdge routers support integrated routing and bridging (IRB). IRB is implemented using logical IRB interfaces, which connect a bridge domain to a VPN, or what might better be called a VPN domain. The VPN domain provides the Layer 3 routing services necessary so that traffic can be exchanged between different VLANs. Each bridge domain can have a single IRB interface and can connect to a single VPN domain, and a single VPN domain can connect to multiple bridge domains on a vEdge router. The route table in the VPN domain provides reachability between all bridge domains which participate in that VPN domain, whether the bridge domain is located on the local router or on a remote router.

The following figure illustrates the components of the Viptela bridging solution, and the remainder of this article describes these components.

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Bridge Domains

In standard transparent bridging, virtual LANs, or VLANs, segregate LANs into logical LANs, and each VLAN is an isolated broadcast domain. All VLAN traffic remains in the VLAN, and it is directed to its destination by means of Ethernet switching tables. The Viptela implementation of bridging overlays the concept of a bridge domain on top of the standard VLAN: A bridge domain comprises a single VLAN, and all the ports within a VLAN are part of a single broadcast domain. Within each broadcast domain, the standard bridging operations of learning, forwarding, flooding, filtering, and aging are performed on VLAN traffic to create and maintain the Ethernet switching table (or MAC table) for that VLAN, and hence for that bridge domain.

Each bridge domain is identified by a number. The VLAN within a bridge domain is identified by an 802.1Q identifier, which is called a VLAN tag or VLAN ID. Frames within a bridge domain can remain untagged, or you can configure VLAN ID to tag the frames. In the Viptela design, the VLAN and the VLAN ID are the property of the bridge domain. They are not the property of an interface or a switching port.

Ports that connect to the WAN segments are associated with a bridge domain. In the Viptela overlay network, these ports are the physical Gigabit Ethernet interfaces on vEdge routers. Specifically, they are the base interfaces, for example, ge-0/0. You cannot use subinterfaces for bridge domain ports.

Each broadcast domain in the Viptela overlay network is uniquely identified by the combination of bridge domain number and VLAN ID (if configured). This design means that The same VLAN ID can be used in different bridge domains on a single vEdge router. For example, the VLAN ID 2 can exist in bridge domain 1 and bridge domain 50. In a situation where the VLAN IDs are different two bridge domains can include the same port interfaces. For example, both (bridge 2, VLAN 2) and (bridge 10, VLAN 23) can include interfaces ge0/0 and ge0/1. Here, these two interfaces effectively become trunk ports. However, because of how interface names are tracked internally, two bridge domains that use the same VLAN ID can have no overlap between the interfaces in the two domains. For example, if (bridge 1, VLAN 2) includes interfaces ge0/0 and ge0/1, these interfaces cannot be in (bridge 50, VLAN 2).

As mentioned above, all member interfaces within a VLAN are part of a single broadcast domain. Within each broadcast domain, the standard transparent bridging operations of learning, forwarding, flooding, filtering, and aging are performed on VLAN traffic to create and maintain the Ethernet switching table, also called the MAC table, for that VLAN.

The Viptela bridging domain architecture lacks the concepts of access ports and trunk ports. However, the Viptela architecure emulates these functions. For a vEdge router that has a single bridge domain, the interfaces in the bridge emulate access ports and so the router is similar to a single switch device. For a vEdge router with multiple bridge domains that are tagged with VLAN IDs, the interfaces in the bridges emulate trunk ports, and you can think of each domain as corresponding to a separate switching device.

Native VLAN

Viptela bridge domains support 802.1Q native VLAN. All traffic sent and received on an interface configured for native VLAN do not have a VLAN tag in its Ethernet frame. That is, they are not tagged with a VLAN ID. If a host is connected on an interface enabled for native VLAN, the bridge domain receives no tagged frames. If the bridge domain connects to a switch that support trunk ports or connects to a hub, the bridge domain might receive both untagged and tagged frames.

Native VLAN is used primarily on trunk ports.

Native VLAN provides backwards compatibility for devices that do not support VLAN tagging. For example, native VLAN allows trunk ports to accept all traffic regardless of what devices are connected to the port. Without native VLAN, the trunk ports would accept traffic only from devices that support VLAN tagging.

​Integrated Routing and Bridging (IRB)

Bridge domains and VLANs provide a means to divide a LAN into smaller broadcast domains. Each VLAN is a separate broadcast domain, and switching within that domain directs traffic to destinations within the VLAN. The result is that hosts within a single bridge domain can communicate among themselves, but cannot communicate with hosts in other VLANs. So, for example, if a business places its departments in a separate VLANs, people within the finance department would be able to communicate only with others in that department, but would not be able to communicate with the manufacturing or engineering department.

The only way for traffic to cross Layer 2 VLAN boundaries to allow communicatation between bridge domains is via Layer 3 routing. This process of marrying switching and routing is done by integrated routing and bridging, or IRB. With IRB, a single vEdge router can pass traffic among different bridge domains on the same router and among bridge domains on remote vEdge routers. The only restriction is that all the bridge domains must reside in the same VPN domain in the overlay network.

The Viptela implementation of IRB connects a Layer 2 bridge domain to a Layer 3 VPN domain via an IRB interface. An IRB interface is a logical interface that inherits all the properties of a regular interface, but it is not associated with a port or with a physical interface. Each IRB interface is named with the stem “irb” and a number that matches the number of a bridge domain. For example, the interface irb2 is the logical interface that connects to bridge domain 2. IRB interfaces cannot have subinterfaces.

You create IRB interfaces within a VPN. A VPN domain supports multiple IRB interfaces.

There is a one-to-one association between an IRB logical interface and a bridge domain: an IRB interface can be associated only with one bridge domain, and a bridge domain can be associated with only one IRB interface. As a result, a bridge domain can be part of only one VPN in the overlay network.

The IP address of an IRB interface is the subnet of the VLAN that resides in the bridge domain. From a switching perspective, the IP address of the IRB interface is part of the bridge domain.

Additional Information

Configuring Bridging and IRB

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