BGP Questions

BGP Quick Summary:

Protocol type: Path Vector
Type: EGP (External Gateway Protocol)
Packet Types: Open, Update, KeepAlive, Notification
Administrative Distance: eBGP: 20; iBGP: 200
Transport: TCP port 179
Neighbor States: Idle -> Active -> Connect -> Open Sent -> Open Confirm -> Established
1 – Idle: the initial state of a BGP connection. In this state, the BGP speaker is waiting for a BGP start event, generally either the establishment of a TCP connection or the re-establishment of a previous connection. Once the connection is established, BGP moves to the next state.
2 – Connect: In this state, BGP is waiting for the TCP connection to be formed. If the TCP connection completes, BGP will move to the OpenSent stage; if the connection cannot complete, BGP goes to Active
3 – Active: In the Active state, the BGP speaker is attempting to initiate a TCP session with the BGP speaker it wants to peer with. If this can be done, the BGP state goes to OpenSent state.
4 – OpenSent: the BGP speaker is waiting to receive an OPEN message from the remote BGP speaker
5 – OpenConfirm: Once the BGP speaker receives the OPEN message and no error is detected, the BGP speaker sends a KEEPALIVE message to the remote BGP speaker
6 – Established: All of the neighbor negotiations are complete. You will see a number, which tells us the number of prefixes the router has received from a neighbor or peer group.
Path Selection Attributes: (highest) Weight > (highest) Local Preference > Originate > (shortest) AS Path > Origin > (lowest) MED > External > IGP Cost > eBGP Peering > (highest) Router ID
(Originate: prefer routes that it installed into BGP by itself over a route that another router installed in BGP)
Authentication: MD5
BGP Origin codes: i – IGP (injected by “network” statement), e – EGP, ? – Incomplete
AS number range: Private AS range: 64512 – 65535, Globally (unique) AS: 1 – 64511

Local Preference (LocalPrf) Attribute:
+ Sent to all iBGP neighbor (not be exchanged between eBGP neighbors)
+ Used to choose the path to external BGP neighbors
+ Higher value is preferred
+ Default value is 100

Note: Although Local Preference attribute is only sent to all iBGP neighbor and it is not exchanged between eBGP neighbors but we can apply it to an eBGP neighbor (inbound direction) to affect our local AS choice.

For example in the topology above, we can use Local Preference on R2 (inbound direction) for the R2-R4 eBGP connection (command like “neighbor 192.168.24.4 route-map LOCAL-PREF-150 in”) to affect routing decision on R1 toward R4.

Unlike Weight attribute, Local Preference attribute is advertised to all iBGP neighbors.

MED Attribute:
+ Optional nontransitive attribute (nontransitive means that we can only advertise MED to routers that are one AS away)
+ Sent through ASes to external BGP neighbors
+ Lower value is preferred (it can be considered the external metric of a route)
+ Default value is 0

Route Reflector (RR)

IBGP routers do not re-advertise routes learned via IBGP to other IBGP peers. One of the two solutions for this is configuring a Route Reflector.

A BGP route reflector is an IBGP speaker that reflects or repeats routes learned from IBGP peers to some of its other IBGP peers.

 The route reflector can have three type of peerings:
+ EBGP neighbor
+ IBGP client neighbor
+ IBGP non-client neighbor

When a route reflector forwards a route, there are a couple of rules:
+ A route learned from a non-RR client is advertised to RR clients but not to non-RR clients.
+ A route learned from a RR client is advertised to both RR clients and non-RR clients. Even the RR client that advertised the route will receive a copy and discards it because it sees itself as the originator.
+ A route learned from an EBGP neighbor is advertised to both RR clients and non-RR clients.

All configuration of the route reflector is done on the RR itself. The configuration that identifies which IBGP peers are RR clients (or non-clients) is done from the RR only.

To avoid a single point of failure, redundant route reflectors are typically used.