Operation Manual – Routing ProtocolH3C S5600 Series Ethernet Switches Chapter 5 BGP Configuration5-5z Data: Used to further determine the cause of errors. Its content is the error datawhich depends on the specific error code and error subcode. Its length isunfixed.V. KeepaliveIn BGP, Keepalive message keeps BGP connection alive and is exchangedperiodically. A BGP Keepalive message only contains the packet header. Noadditional fields is carried.VI. Route-refreshRoute-refresh messages are used to notify the peers that the route refresh function isavailable and request the peers to resend the routing information of the specifiedaddress family. Figure 5-5 shows the format of a route-refresh message.Figure 5-5 Route-refresh message formatThe fields of a route-refresh message are described as follows.z AFI: Address family identifierz Res: Reserved. This field must not be set.z SAFI: Subsequent address family identifier5.1.2 BGP Route AttributesI. Routes attributes classificationBGP route attributes describe route, so that BGP can filter and choose the routes.In fact, all the BGP route attributes can be classified into the following four categories.z Well-known mandatory attributes, which can be identified by any BGP routers.Route attributes of this type are carried in Update messages. Without theseattributes, routing information goes wrong.z Well-known discretionary attributes, which can be identified by any BGP routers.An Update message can travel with or without this type of attributes.z Optional transitive attributes, which can be transmitted among ASs. Althoughattributes of this type may not be supported by any BGP routers, routes withthem can still be received and be forwarded to BGP speakers.z Optional non-transitive attributes, which is dropped on the BGP routers that donot support them. In this case, the attributes are not forwarded to other BGProuters.Table 5-1 lists basic BGP route attributes and the categories they belong to.
