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diff --git a/docs/routing/bgp.rst b/docs/routing/bgp.rst index 4fac4889..37cf3c92 100644 --- a/docs/routing/bgp.rst +++ b/docs/routing/bgp.rst @@ -1,13 +1,177 @@ .. _bgp: +### BGP ---- +### :abbr:`BGP (Border Gateway Protocol) is one of the Exterior Gateway Protocols and the de facto standard interdomain routing protocol. The latest BGP version is 4. BGP-4 is described in :rfc:`1771` and updated by :rfc:`4271`. :rfc:`2858` adds multiprotocol support to BGP. +VyOS makes use of :abbr:`FRR (Free Range Routing)` and we would like to thank +them for their effort! + +Basic Concepts +============== + +.. _bgp-autonomous-systems: + +Autonomous Systems +------------------ + +From :rfc:`1930`: + + An AS is a connected group of one or more IP prefixes run by one or more + network operators which has a SINGLE and CLEARLY DEFINED routing policy. + +Each AS has an identifying number associated with it called an :abbr:`ASN +(Autonomous System Number)`. This is a two octet value ranging in value from 1 +to 65535. The AS numbers 64512 through 65535 are defined as private AS numbers. +Private AS numbers must not be advertised on the global Internet. + +The :abbr:`ASN (Autonomous System Number)` is one of the essential elements of +BGP. BGP is a distance vector routing protocol, and the AS-Path framework +provides distance vector metric and loop detection to BGP. + +.. _bgp-address-families: + +Address Families +---------------- + +Multiprotocol extensions enable BGP to carry routing information for multiple +network layer protocols. BGP supports an Address Family Identifier (AFI) for +IPv4 and IPv6. + +.. _bgp-route-selection: + +Route Selection +--------------- + +The route selection process used by FRR's BGP implementation uses the following +decision criterion, starting at the top of the list and going towards the +bottom until one of the factors can be used. + +1. **Weight check** + + Prefer higher local weight routes to lower routes. + +2. **Local preference check** + + Prefer higher local preference routes to lower. + +3. **Local route check** + + Prefer local routes (statics, aggregates, redistributed) to received routes. + +4. **AS path length check** + + Prefer shortest hop-count AS_PATHs. + +5. **Origin check** + + Prefer the lowest origin type route. That is, prefer IGP origin routes to + EGP, to Incomplete routes. + +6. **MED check** + + Where routes with a MED were received from the same AS, prefer the route + with the lowest MED. :ref:`bgp-med`. + +7. **External check** + + Prefer the route received from an external, eBGP peer over routes received + from other types of peers. + +8. **IGP cost check** + + Prefer the route with the lower IGP cost. + +9. **Multi-path check** + + If multi-pathing is enabled, then check whether the routes not yet + distinguished in preference may be considered equal. If + :clicmd:`bgp bestpath as-path multipath-relax` is set, all such routes are + considered equal, otherwise routes received via iBGP with identical AS_PATHs + or routes received from eBGP neighbours in the same AS are considered equal. + +10. **Already-selected external check** + + Where both routes were received from eBGP peers, then prefer the route + which is already selected. Note that this check is not applied if + :clicmd:`bgp bestpath compare-routerid` is configured. This check can + prevent some cases of oscillation. + +11. **Router-ID check** + + Prefer the route with the lowest `router-ID`. If the route has an + `ORIGINATOR_ID` attribute, through iBGP reflection, then that router ID is + used, otherwise the `router-ID` of the peer the route was received from is + used. + +12. **Cluster-List length check** + + The route with the shortest cluster-list length is used. The cluster-list + reflects the iBGP reflection path the route has taken. + +13. **Peer address** + + Prefer the route received from the peer with the higher transport layer + address, as a last-resort tie-breaker. + +.. _bgp-capability-negotiation: + +Capability Negotiation +---------------------- + +When adding IPv6 routing information exchange feature to BGP. There were some +proposals. :abbr:`IETF (Internet Engineering Task Force)` +:abbr:`IDR (Inter Domain Routing)` adopted a proposal called Multiprotocol +Extension for BGP. The specification is described in :rfc:`2283`. The protocol +does not define new protocols. It defines new attributes to existing BGP. When +it is used exchanging IPv6 routing information it is called BGP-4+. When it is +used for exchanging multicast routing information it is called MBGP. + +*bgpd* supports Multiprotocol Extension for BGP. So if a remote peer supports +the protocol, *bgpd* can exchange IPv6 and/or multicast routing information. + +Traditional BGP did not have the feature to detect a remote peer's +capabilities, e.g. whether it can handle prefix types other than IPv4 unicast +routes. This was a big problem using Multiprotocol Extension for BGP in an +operational network. :rfc:`2842` adopted a feature called Capability +Negotiation. *bgpd* use this Capability Negotiation to detect the remote peer's +capabilities. If a peer is only configured as an IPv4 unicast neighbor, *bgpd* +does not send these Capability Negotiation packets (at least not unless other +optional BGP features require capability negotiation). + +By default, FRR will bring up peering with minimal common capability for the +both sides. For example, if the local router has unicast and multicast +capabilities and the remote router only has unicast capability the local router +will establish the connection with unicast only capability. When there are no +common capabilities, FRR sends Unsupported Capability error and then resets the +connection. + +.. _bgp-router-configuration: + +BGP Router Configuration +======================== + +ASN and Router ID +----------------- + +.. cfgcmd:: set protocols bgp '<ASN>' + +First of all you must configure BGP router with the :abbr:`ASN (Autonomous +System Number)`. The AS number is an identifier for the autonomous system. The +BGP protocol uses the AS number for detecting whether the BGP connection is +internal or external. + +.. cfgcmd:: set protocols bgp '<ASN>' parameters router-id + +This command specifies the router-ID. If router ID is not specified it will use +the highest interface IP address. + + IPv4 ^^^^ |