BGP

=Introduction=

BGP is needed for redundancy of servers. BGP is not used for providing redundancy to users for internet access.

BGP neighbors are not discovered. They must be configured manually on both sides of the connection. TCP port 179 is used. Only one session remains if both connection attempts succeed.
 * Session establishment facts


 * BGP is an advanced path vector protocol and has following advantages:

Reliable updates Triggered updates only Rich metric (Path attributes) Scalable to massive networks

BGP States
Idle          Neighbor is not responding Active        Attempting to connect Connect       TCP session established OpenSent      Open message sent OpenConfirm   Response received Established   Adjacency established

BGP Messages
Open Update Keepalive      Sent every 60 seconds Notification   Always indicate something is wrong

Protocol Specifications
Customer connected to multiple Internet service providers (ISPs). Service provider networks (Transit autonomous system). Network cores of very large enterprise networks (distribution or core layer)as a backup or redundant routing protocol due to its stability.
 * Usage applications

=BGP Route selection criteria = Mnemonics: N-WLLA-OMNI

Full Internet BGP routing table is more than 300K routes and a BGP router can receive multiple copies of that routing table from multiple providers, router has to compare those multiple entries and select only the best route for the routing table. It uses the BGP Best Path Selection Algorithm to do this. Routes installed by different BGP instances are compared by the general algorithm, i.e. route distances are compared and the route with lower distance is preferred.

=Filter with Route Maps=

Route maps are very powerful filtering tools, they can be used to accomplish the following tasks:

Filter on IP prefixes coming from a specific autonomous system Filter on other BGP attributes Modify BGP attributes

Match clauses in the BGP route map can be based on the following:

IP network numbers and subnet masks (prefix list or access list) Route originator Next hop Origin code Tag value attached to an Interior Gateway Protocol (IGP) route Autonomous system path Community IGP route type

With a route map, the following can be set:

Origin Next hop Weight Community Local preference MED

You can apply a route map on incoming or outgoing routing information for a neighbor. The routing information must be permitted by the route map to be accepted. If the route map has no statement explicitly permitting a route, the route is implicitly denied and dropped.

The syntax required is as follows:

Router(config-router)# neighbor ip-address route-map name in|out

= What is a Route Reflector? =

= Synchronization =

= Auto-Summarization =

= MED vs Local Preference =

= iBGP vs eBGP =

= Troubleshooting and Monitoring =


 * BGP route not installing, route reasons:

Synchronization is enabled & route knowing by IGP Not Sync Next Hop inaccessible AS path includes the local AS Rejection by inbound policy

= R&S Quick Notes = When using Communities, don’t forget “neighbor send-community” Know your attributes and the direction which applied, when to used what. “aggregate address” needs a more specific prefix in the BGP table for aggregate to be advertised. Synchronization issue has 3 solutions, 1- Load BGP on all transit routers, 2- GRE tunnel, 3- Redistribution BGP>IGP. “no bgp nexthop trigger” – Disables next-hop tracking between scanner intervals. “no bgp fast-ext-fallover” – Force the router to wait for the dead-timer to expire, before generating notification messages, when a connected peer goes down. “neighbor fall-over” – Will check neighbor connenctivity between scanner intervals, aka BGP Fast Peering. Only the Holdtime is sent in update-msg. Two neighbors will use the lowest holdtime and then calculate the keepalive from that. Know your Regular Expressions Know the difference between Peer-Groups and Peer-Templates

=LAB=


 * Complete BGP Lab:

=References=