Path attributes defined. - Well-known mandatory (recognized and always sent – 010) - Well-known discretionary (recognized not necessarily sent – 010 if sent at all) - Optional transitive (may or may not be recognized, set partial bit if not recognized and pass onwards – 11?) - Optional non-transitive (if not recognized, quietly discard – 100)
The local preference attribute is exchanged in IBGP peering sessions only and is not passed to outside AS
. The MED represents the external metric of a route - 32 bit, non-negative -- not propagated beyond neighboring AS Because the MED is a nontransitive BGP attribute, the AS that receives a route with an associated MED does not forward the MED value to other ASs . The MED is used in the route selection process only within the AS . . If the route needs to be readvertised to another AS, the MED value must be reset to zero , unless the associated export policy sets an outgoing MED value. If a MED is received over an external BGP link, it is propagated over internal links to other BGP systems within the AS.
Whats Origin? - well known mandatory - igp (isis/ospf) - egp (egp) - incomplete (all others – static, rip, etc) - 80% igp in current Internet table.
Inform other BGP speaker that the local system choose a less specific route without selecting a more Specific route that is included in it. For example select route 22.214.171.124/16 which include the 126.96.36.199/24 in it To route packet to 188.8.131.52/24 network. Length = 0
- next_hop, MED is unchanged when sent to a neighboring AS in the confederation Local_pref is passed through such a connection
Marker: This 16-octet field contains a value that the receiver of the message can predict. If the Type of the message is OPEN, or if the OPEN message carries no Authentication Information (as an Optional Parameter), then the Marker must be all ones. Otherwise, the value of the marker can be predicted by some a computation specified as part of the authentication mechanism (which is specified as part of the Authentication Information) used. The Marker can be used to detect loss of synchronization between a pair of BGP peers, and to authenticate incoming BGP messages. Length: This 2-octet unsigned integer indicates the total length of the message, including the header, in octets. Thus, e.g., it allows one to locate in the transport-level stream the (Marker field of the) next message. The value of the Length field must always be at least 19 and no greater than 4096, and may be further constrained, depending on the message type. No &quot;padding&quot; of extra data after the message is allowed, so the Length field must have the smallest value required given the rest of the message. Type: This 1-octet unsigned integer indicates the type code of the message. The following type codes are defined: 1 - OPEN 2 - UPDATE 3 - NOTIFICATION 4 - KEEPALIVE
Version --A 1-byte unsigned integer that indicates the version of the BGP protocol, such as BGP3 or BGP4. During the neighbor negotiation, BGP peers agree on a BGP version number. BGP peers will try to negotiate the highest common version that they both support. Cisco Systems provides the option of predefining the version negotiated to cut down on the negotiation process. Setting the version statically is usually used when the versions of the BGP peers are already known. My Autonomous System --A 2-byte field that indicates the AS number of the BGP router. Hold Time --The maximum amount of time in seconds that may elapse between the receipt of successive KEEPALIVE or UPDATE messages. The hold timer is a counter that increments from zero to the hold time value. Receipt of a KEEPALIVE or UPDATE message causes the hold timer to reset to zero. If the hold time for a particular neighbor is exceeded, the neighbor would be considered dead. The hold time is a 2-byte unsigned integer. The BGP router negotiates with its neighbor to set the hold time at whichever value is lower--its own hold time or its neighbor's. The hold time could be 0, in which case the hold timer and the KEEPALIVE timers are never reset--that is, these timers never expire, and the connection is considered to be always up. If not set to zero, the minimum recommended hold time is three seconds. BGP Identifier --A 4-byte unsigned integer that indicates the sender's ID. In Cisco's implementation, this is usually the router ID (RID), which is calculated as the highest IP address on the router or the highest loopback address at BGP session startup. ( Loopback address is Cisco's representation of the IP address of a virtual software interface that is considered to be up at all times, irrespective of the state of any physical interface.) Optional Parameters --This is a variable length field that indicates a list of optional parameters used in BGP neighbor session negotiation. This field is represented by the triplet <Parameter Type, Parameter Length, Parameter Value> with lengths of 1-byte, 1-byte, and variable length, respectively. An example of optional parameters is the authentication information parameter (type1), which is used to authenticate the session with a BGP peer. Optional Parameter Length --This is a 1-byte unsigned integer that indicates the total length in bytes of the Optional Parameters field. A length value of 0 indicates that no Optional Parameters are present.
Unfeasible Routes Length: This 2-octets unsigned integer indicates the total length of the Withdrawn Routes field in octets. Its value must allow the length of the Network Layer Reachability Information field to be determined as specified below. A value of 0 indicates that no routes are being withdrawn from service, and that the WITHDRAWN ROUTES field is not present in this UPDATE message. Withdrawn Routes: This is a variable length field that contains a list of IP address prefixes for the routes that are being withdrawn from service. Each IP address prefix is encoded as a 2-tuple of the form <length, prefix>, whose fields are described below: +---------------------------+ | Length (1 octet) | +---------------------------+ | Prefix (variable) | +---------------------------+ The use and the meaning of these fields are as follows: a) Length: The Length field indicates the length in bits of the IP address prefix. A length of zero indicates a prefix that matches all IP addresses (with prefix, itself, of zero octets). b) Prefix: The Prefix field contains IP address prefixes followed by enough trailing bits to make the end of the field fall on an octet boundary. Note that the value of trailing bits is irrelevant. Total Path Attribute Length: This 2-octet unsigned integer indicates the total length of the Path Attributes field in octets. Its value must allow the length of the Network Layer Reachability field to be determined as specified below. A value of 0 indicates that no Network Layer Reachability Information field is present in this UPDATE message. Path Attributes: A variable length sequence of path attributes is present in every UPDATE. Each path attribute is a triple <attribute type, attribute length, attribute value> of variable length. Attribute Type is a two-octet field that consists of the Attribute Flags octet followed by the Attribute Type Code octet. 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Attr. Flags |Attr. Type Code| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
FSM states. - idle (will refuse incoming connection attempts) - connect (listening) - active (trying to establish connection) - opensent (waiting for open from peer) - openconfirm (waiting notification or keepalive – handshake) - established (steady state)
. The MED represents the external metric of a route
Each routing table is identified by a name, which consists of the protocol family followed by a period and small, nonnegative integer. The protocol family can be inet (Internet), iso (ISO), or mpls (MPLS).The following names are reserved for the default routing tables maintained by the JUNOS software: inet.0 --Default unicast routing table instance-name. inet.0 --Unicast routing table for a particular routing instance inet.1 --Multicast forwarding cache inet.3 --MPLS routing table for path information mpls.0 --MPLS routing table for label-switched path (LSP) next hops If Multiprotocol Border Gateway Protocol (MBGP) is enabled, inet.2 is used for subaddress family indicator (SAFI) 2 routes
Introduction toBorder Gateway Protocol (BGP 4) Version 1.2