BGP

BGP (Border Gateway Protocol) -

BGP is considered a "Path Vector" Routing protocol.

BGP was not built to route 'within' an Autonomous System (AS), but rather to route between AS's.

BGP ASN Numbers -

1 - 65535

Private range: 64512 - 65535

BGP (Border Gateway Protocol) TCP port -

TCP 179

BGP Router, Neighbor relationship -

BGP router = speaker

Neighbor relationship = peer

iBGP Peers -

BGP neighbors within the same AS

eBGP Peers -

BGP neighbors connecting separate AS

BGP True Benefit -

Controlling how traffic 'enters' the local AS, rather than how traffic 'exits' it.

Default BGP max-hop -

Max of 1 hop away. Can be overwritten by using 'eBGP-multihop' option with the 'neighbor' command.

AD for eBGP route -

20

AD for iBGP and locally originated routes -

200

BGP peer message: OPEN -

Sent to initiate connection. Contains several parameters:

• BGP Version - must match between BGP peers

• Local AS Number

• BGP Router ID

BGP peer message: KEEPALIVE -

Sent every 60 sec to ensure that the remote peer is still available.

If a router does not receive a KEEPALIVE from a peer for a Hold-time period (default 180 sec) router declares that peer dead.

BGP peer message: UPDATE -

Used to exchange routers between peers.

BGP peer message: NOTIFICATION -

Sent when there is a fatal error condition.

If NOTIFICATION is received, BGP peer session is torn down and reset

BGP Finite-State Machine (FSM) process to establish peer session. -

IDLE

CONNECT

ACTIVE

OpenSent

OpenConfirm

ESTABLISHED

BGP FSM state: Idle -

Initial BGP state

BGP FSM state: Connect -

BGP waits for a TCP connection with the remote peer.

If successful, an OPEN message is sent.

If unsuccessful, session is placed in Active state.

BGP FSM state: Active -

BGP attempts to initiate a TCP connection with remote peer.

If successful, an OPEN message is sent.

If unsuccessful, BGP will wait for a ConnectRetry timer to expire, and place session back in Connect.

BGP FSM state: OpenSent -

BGP Both established the TCP connection AND send an OPEN message, and is waiting for a reply OPEN message. Once it receives a reply OPEN message, the BGP peer will send a KEEPALIVE message.

BGP FSM state: OpenConfirm -

BGP listens for a reply KEEPALIVE message

BGP FSM state: Established -

BGP session is fully established.

UPDATE messages containing routing information will now be sent.

BGP Peer is stuck in ACTIVE state: potential problems. -

• No IP connectivity (no route to host)

• Incorrect 'neighbor' statement

• Access-list filtering TCP port 179

Fundamental BGP rule before route is advertised to eBPG peer. -

Route must be in Routing Table before BGP will advertise the network to an eBGP peer.

BGP Confederation -

Sub-AS's or AS's within an AS

BGP Peer-group -

Simplify configuration of groups of neighbors, assuming those neighbors share identical settings.

Conserve processor/memory resources by sending updates to all peer-group members simultaneously, as opposed to sending individual updates to each neighbor.

BGP Attribute Types -

Well-known Mandatory

Well-known Discretionary

Optional Transitive

Optional Non-Transitive

BGP Attribute Type: Well-known Mandatory -

Standard attributes supported by all BGP implementations and always included in every BGP update

BGP Attribute Type: Well-known Discretionary -

Standard attributes supported by all BGP implementations and are optionally included BGP updates

BGP Attribute Type: Optional Transitive -

Optional attribute that may not be supported by all implementations of BGP.

Transitive indicates that a non-compliant BGP router will forward the unsupported attribute unchanged, when sending updates to peers.

BGP Attribute Type: Optional Non-Transitive -

Optional attribute that may not be supported by all implementations of BGP.

Non-Transitive indicates that a non-compliant BGP router will strip out the unsupported attribute, when sending updates to peers.

BGP Attribute: AS-Path -

Well-known Mandatory.

Identifies the list or path of traversed AS's to reach a particular destination

BGP Attribute: Next-Hop -

Well-known Mandatory.

Identifies the next hop IP address to reach a particular destination.

BGP Attribute: Origin -

Well-known Mandatory.

Identifies the originator of the route.

BGP Attribute: Local Preference -

Well-known Discretionary.

Provides a preference to determine the best path for outbound traffic.

BGP Attribute: Atomic Aggregate -

Well-known Discretionary.

Identifies routes that have been summarized, or aggregated.

BGP Attribute: Aggregator -

Optional Transitive.

Identifies the BGP router that performed an address aggregation.

BGP Attribute: Community -

Optional Transitive.

Tags routes that share common characteristics into communities.

BGP Attribute: Multi-Exit-Discriminator (MED) -

Optional Non-Transitive.

Provides a preference to eBGP peers to a specific inbound router

BGP Attribute: Weight (Cisco proprietary) -

Similar to Local Preference, provides a local weight to determine the best path for outbound traffic.

BGP Best Path Determination -

Weight - Which router has the highest weight?

Local Preference - Which route has the highest local preference?

Locally Originated - Did the local router originate this route? In other words, is the next hop to the destination 0.0.0.0?

AS-Path - Which router has the shortest AS-Path?

Origin Code - Where did the route originate? IGP, EGP, or unknown?

MED - Which path has the lowest MED?

BGP Route Type - Is this an eBGP or iBGP route? eBGP routes are preferred.

Age - Which route is the oldest? Oldest is preferred.

Router ID - Which route originated from the router with the lowest BGP router ID?

Peer IP Address - Which route originated from the router with the lowest IP?

When applying attributes: where are Weight and Local Preference applied? -

Applied to inbound routes, dictating the best outbound path.

When applying attributes: where are AS-Path and MED applied? -

Applied to outbound routes, dictating best inbound path.

Local Preference Attribute -

Applied to inbound external routes, dictating best outbound path.

Passed on to iBGP peers when sending updates. Informs iBGP routers how to exit the AS, if multiple paths exist.

Local Preference (32 bit) number range -

0 - 4294967295

Highest is preferred.

Default = 100

AS-Path Attribute -

Applied to outbound routes, indicating the best inbound path.

2 things can be accomplished with this attribute: pre-pend and filter.

AS-Path Prepend -

Add to existing AS-Path resulting in a longer AS-Path.

Makes route less desirable for inbound traffic.

MED (MultiExit Discriminator) attribute -

Applied to outbound routes, dictating the best inbound path into the AS.

Lower metric preferred.

Default = 0.

Redistributed Routes -

Automatically summarized when added to BGP