Ex2 Chapter 12 – BGP basics

The Border Gateway Protocol (BGP) is the core routing protocol of the Internet. It maintains a table of IP networks or 'prefixes' which designate network reachability among autonomous systems (AS). It is described as a path vector protocol. BGP does not use traditional Interior Gateway Protocol (IGP) metrics, but makes routing decisions based on path, network policies and/or rulesets.

Routers use “routing protocols” to exchange routing information with each other.

neighbor ebgp-multihop - To accept and attempt BGP connections to external peers residing on networks that are not directly connected.

no synchronization - if an AS provides transit service to another AS, BGP should not advertise a route until all of the routers within the AS have learned about the route via an IGP. A BGP router with synchronization enabled does not install iBGP learned routes into its routing table if it is not able to validate those routes in its IGP.

next-hop-self - allows you to force BGP to use a specific IP address as the next hop.

ebgp-multihop - in EBGP, neighbor relationships are only formed if we have directly connected networks. We would require to use ebgp-multihop  keyword with neighbor statement so that neighbors which are not directly connected can form relationship with each other. We need to specify a number with ebgp-multihop keyword, number can be between 1-255. This number represents how many hop counts is the router away.

update-source - You only have to use the update-source command when someone is peering to your loopback address. This is true for an iBGP peer and an eBGP peer.

Auto-summary (BGP)
 - if enabled: BGP automatically summarizes routes to classful network boundaries
 - route summarization is used to reduce the amount of routing information in routing tables
 - Automatic summarization applies to connected, static, and redistributed routes.
 - By default, automatic summarization is disabled and BGP accepts subnets redistributed from IGP
 - To advertise and carry subnet routes in BGP when auto-summ is enabled, use an explicit network command to advertise the subnet.

 - does not apply to routes injected into BGP via the network command or through iBGP or eBGP.
MPLS VPNs gives the opportunity to use the same MPLS backbone for different customers or services without each one interacting with the other. It’s pretty common to find different clients using the same private range of IP addresses

RD (Route Distinguisher) - is just that, something to make the of customer A different from the of customer B (is locally significant).
RD transforms an IP address of 32 bits length into an address of 96 bits length, unique in the network.
or 65002:777 

RT (Route Target) - is a 64-bits BGP community used for tagging prefixes. It allows you to control the import/export process within the VRF.

IGP is used to refer to the process running on routers inside an ISP’s network
• Within an Autonomous System
• Carries information about internal infrastructure prefixes
• Examples – OSPF, ISIS, EIGRP
EGP is used to refer to the process running between routers bordering directly connected ISP networks
• Used to convey routing information between Autonomous Systems
• De-coupled from the IGP
• Current EGP is BGP
Routing versus Forwarding
• Routing - building maps and giving directions
• Forwarding - moving packets between interfaces according to the “directions”

Routing table or Routing Information Base (RIB) - is an electronic table (file) or database type object that is stored in a router  or a networked computer. The routing table stores the routes (and in some cases, metrics associated with those routes) to particular network destinations. This information contains the topology of the network immediately around it. The construction of routing tables is the primary goal of routing protocols and static routes.

Forwarding table or Forwarding Information Base (FIB) - is most commonly used in network bridging, routing, and similar functions to find the proper interface to which the input interface should send a packet to be transmitted by the router.

• Neighbours
- AS’s which directly exchange routing information;Routers which exchange routing information
• Announce - send routing information to a neighbour
• Accept - receive and use routing information sent by a neighbour
• Originate - insert routing information into external announcements (usually as a result of the IGP)
• Peers - routers in neighbouring AS’s or within one AS which exchange routing and policy information

Routing Policy

• Used to control traffic flow in and out of an ISP network
• ISP makes decisions on what routing information to accept and discard from its neighbours:
Individual routes
Routes originated by specific ASes
Routes traversing specific ASes
Routes belonging to other groupings
Groupings which you define as you see fit

Border Gateway Protocol BasicsRouting Protocol used to exchange routing information between networks
Described in RFC1771
The Autonomous System is BGP’s fundamental operating unit
Autonomous System (AS)• Collection of networks with same routing policy
• Single routing protocol
• Usually under single ownership, trust and administrative control
• Identified by a unique number
• Autonomous System Number (ASN) - a 16 bit number (1-64511) are assigned by the RIRs
Configuring eBGP

BGP General Operation
• Learns multiple paths via internal and external BGP speakers
• Picks the best path and installs in the forwarding table
• Best path is sent to external BGP neighbours
• Policies applied by influencing the best path selection


BGP: Frequently Asked Questions
BGP @ Wikipedia
Cisco Handbook