Users Guide

Table Of Contents

1422 BGP

Route Origin Community Attribute

The Route Origin Community attribute identifies one or more routers that

advertise routes via BGP. The attribute is transitive across Autonomous

System boundaries.

The Route Origin Community attribute is used to prevent routing loops

when BGP speakers are multi-homed to another site and that site uses the

AS-Override feature. This Route Origin Community attribute identifies the

site from where the routes are originated so that they are not re-distributed

back to the originating site.

The value of the high-order octet of the Type field for the Route Origin

Community can be 0x00, 0x01, or 0x02. The value of the low-order octet

(Sub-type) of the Type field for this community is 0x03 (if represented in

Two-octet AS specific format) and 0x103 (if represented in IPv4 address

specific format).

VPNv4/VRF Route Distribution via MP-BGP

Some administrators may choose to use BGP to redistribute VPN routes.

Each VRF has its own independent address space; meaning that the same

address/net mask can be used in any number of VRFs, where in each VRF the

address, in fact, identifies a different system. But a BGP speaker can only

install and distribute one route for a specific address prefix. If multiple

overlapping routes are received by BGP, only the last received route is installed

in any particular per-site VRF route table. Dell EMC Networking allows BGP

to install and distribute multiple overlapping routes to a single IP address

prefix in different VRFs. This is achieved by the use of a new VPNv4 address

family as discussed below. It is recommended that the administrator use a

policy to determine which sites can advertise and install routes.

VPNv4 Address Family

MP-BGP allows BGP to carry routes from different address families. To allow

BGP to install and distribute overlapping address routes, each address/route

must be made unique. To achieve this, a new VPNv4 address family is

introduced. A VPN-IPv4 address is a 12-byte quantity, beginning with an 8-

byte Route Distinguisher (RD) followed by a 4-byte IPv4 address. The RD

attribute follows the same structuring mechanism as described in the

'Extended Community structure' above.