Hi Kevin,
Border Gateway Protocol v4 is an Exterior Gateway Protocol type (versus OSPF for example which is an Interior Gateway Protocol type).
1 BGP is made to connect IGPs between them and RFCs advise against OSPF to OSPF redistributions
2 BGP provide lot of attributes to manage the routing policy
3 BGP is a loop free protocol (versus OSPF to OSPF redistributions)
4 BGP provide routing interoperability between different equipments (OSPF to OSPF is Cisco proprietary)
So BGP has been choosed to provide interoperability, a robust, loop free and standard routing protocol over the Alcanet international backbone.
BGP basics
BGP use TCP connections to establish a routing path between 2 routers. This is so a point-to-point protocol versus OSPF, RIP or EIGRP for examples which are using broadcast or multicast to establish a routing neighboring.
There are 2 types of BGP peers :
1. External BGP peers, which provide a routing path between 2 routers belonging to 2 different Autonomous Systems
2. Internal BGP peers, which are mandatory to provide a BGP routing path between 2 routers belonging to the same AS each of one possessing some EBGP sessions
BGP is a path-vector (sometime called distance-vector) protocol with enhancements as reliable updates, triggered updates only and rich metrics (called path attributes).
Cisco BGP complete decision process :
1. Only consider paths with reachable NEXT_HOPs
2. Do not consider iBGP path if not synchronized
3. Highest WEIGHT
4. Highest LOCAL_PREF
5. Prefer locally originated route
6. Shortest AS_PATH
7. Lowest ORIGIN code: IGP < EGP < incomplete
8. Lowest Multi-Exit Discriminator (MED)
8a. If BGP deterministic-med, order the paths before comparing
8b. If BGP always-compare-med, then compare it for all paths
8c. Considered only if paths are from the same neighbor AS
9. Prefer an External path over an Internal one
10. Lowest IGP metric to the NEXT_HOP
11. If multipath is enabled, the router may install up to N parallel paths in the routing table
12. For E-BGP paths, select the "oldest" to minimize route-flap
13. Lowest Router-ID Originator-ID is considered for reflected routes
14. Shortest Cluster-List Client must be aware of RR attributes !
15. Lowest neighbor IP address
Administrative distances
Route Source Default Distance
Connected interface 0
Static route 1
Enhanced IGRP summary route 5
External BGP 20
Internal Enhanced IGRP 90
IGRP 100
OSPF 110
IS-IS 115
RIP 120
EGP 140
Internal BGP 200
Unknown 255
Rate this if Helps you in Understanding !!
Best Regards,
Guru Prasad R
