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336 lines
12 KiB
ReStructuredText
336 lines
12 KiB
ReStructuredText
.. _bgp:
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###
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BGP
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###
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:abbr:`BGP (Border Gateway Protocol) is one of the Exterior Gateway Protocols
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and the de facto standard interdomain routing protocol. The latest BGP version
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is 4. BGP-4 is described in :rfc:`1771` and updated by :rfc:`4271`. :rfc:`2858`
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adds multiprotocol support to BGP.
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VyOS makes use of :abbr:`FRR (Free Range Routing)` and we would like to thank
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them for their effort!
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Basic Concepts
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==============
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.. _bgp-autonomous-systems:
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Autonomous Systems
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------------------
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From :rfc:`1930`:
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An AS is a connected group of one or more IP prefixes run by one or more
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network operators which has a SINGLE and CLEARLY DEFINED routing policy.
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Each AS has an identifying number associated with it called an :abbr:`ASN
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(Autonomous System Number)`. This is a two octet value ranging in value from 1
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to 65535. The AS numbers 64512 through 65535 are defined as private AS numbers.
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Private AS numbers must not be advertised on the global Internet.
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The :abbr:`ASN (Autonomous System Number)` is one of the essential elements of
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BGP. BGP is a distance vector routing protocol, and the AS-Path framework
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provides distance vector metric and loop detection to BGP.
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.. _bgp-address-families:
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Address Families
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----------------
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Multiprotocol extensions enable BGP to carry routing information for multiple
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network layer protocols. BGP supports an Address Family Identifier (AFI) for
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IPv4 and IPv6.
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.. _bgp-route-selection:
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Route Selection
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---------------
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The route selection process used by FRR's BGP implementation uses the following
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decision criterion, starting at the top of the list and going towards the
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bottom until one of the factors can be used.
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1. **Weight check**
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Prefer higher local weight routes to lower routes.
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2. **Local preference check**
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Prefer higher local preference routes to lower.
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3. **Local route check**
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Prefer local routes (statics, aggregates, redistributed) to received routes.
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4. **AS path length check**
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Prefer shortest hop-count AS_PATHs.
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5. **Origin check**
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Prefer the lowest origin type route. That is, prefer IGP origin routes to
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EGP, to Incomplete routes.
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6. **MED check**
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Where routes with a MED were received from the same AS, prefer the route
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with the lowest MED.
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7. **External check**
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Prefer the route received from an external, eBGP peer over routes received
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from other types of peers.
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8. **IGP cost check**
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Prefer the route with the lower IGP cost.
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9. **Multi-path check**
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If multi-pathing is enabled, then check whether the routes not yet
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distinguished in preference may be considered equal. If
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:cfgcmd:`bgp bestpath as-path multipath-relax` is set, all such routes are
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considered equal, otherwise routes received via iBGP with identical AS_PATHs
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or routes received from eBGP neighbours in the same AS are considered equal.
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10. **Already-selected external check**
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Where both routes were received from eBGP peers, then prefer the route
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which is already selected. Note that this check is not applied if
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:cfgcmd:`bgp bestpath compare-routerid` is configured. This check can
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prevent some cases of oscillation.
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11. **Router-ID check**
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Prefer the route with the lowest `router-ID`. If the route has an
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`ORIGINATOR_ID` attribute, through iBGP reflection, then that router ID is
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used, otherwise the `router-ID` of the peer the route was received from is
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used.
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12. **Cluster-List length check**
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The route with the shortest cluster-list length is used. The cluster-list
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reflects the iBGP reflection path the route has taken.
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13. **Peer address**
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Prefer the route received from the peer with the higher transport layer
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address, as a last-resort tie-breaker.
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.. _bgp-capability-negotiation:
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Capability Negotiation
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----------------------
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When adding IPv6 routing information exchange feature to BGP. There were some
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proposals. :abbr:`IETF (Internet Engineering Task Force)`
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:abbr:`IDR (Inter Domain Routing)` adopted a proposal called Multiprotocol
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Extension for BGP. The specification is described in :rfc:`2283`. The protocol
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does not define new protocols. It defines new attributes to existing BGP. When
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it is used exchanging IPv6 routing information it is called BGP-4+. When it is
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used for exchanging multicast routing information it is called MBGP.
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*bgpd* supports Multiprotocol Extension for BGP. So if a remote peer supports
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the protocol, *bgpd* can exchange IPv6 and/or multicast routing information.
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Traditional BGP did not have the feature to detect a remote peer's
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capabilities, e.g. whether it can handle prefix types other than IPv4 unicast
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routes. This was a big problem using Multiprotocol Extension for BGP in an
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operational network. :rfc:`2842` adopted a feature called Capability
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Negotiation. *bgpd* use this Capability Negotiation to detect the remote peer's
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capabilities. If a peer is only configured as an IPv4 unicast neighbor, *bgpd*
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does not send these Capability Negotiation packets (at least not unless other
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optional BGP features require capability negotiation).
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By default, FRR will bring up peering with minimal common capability for the
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both sides. For example, if the local router has unicast and multicast
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capabilities and the remote router only has unicast capability the local router
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will establish the connection with unicast only capability. When there are no
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common capabilities, FRR sends Unsupported Capability error and then resets the
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connection.
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.. _bgp-router-configuration:
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BGP Router Configuration
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========================
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ASN and Router ID
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-----------------
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.. cfgcmd:: set protocols bgp <asn>
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First of all you must configure BGP router with the :abbr:`ASN (Autonomous
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System Number)`. The AS number is an identifier for the autonomous system.
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The BGP protocol uses the AS number for detecting whether the BGP connection
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is internal or external.
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.. cfgcmd:: set protocols bgp <asn> parameters router-id
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This command specifies the router-ID. If router ID is not specified it will
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use the highest interface IP address.
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Route Selection
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---------------
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.. cfgcmd:: set protocols bgp <asn> parameters bestpath as-path confed
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This command specifies that the length of confederation path sets and
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sequences should be taken into account during the BGP best path
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decision process.
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.. cfgcmd:: set protocols bgp <asn> parameters bestpath as-path multipath-relax
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This command specifies that BGP decision process should consider paths
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of equal AS_PATH length candidates for multipath computation. Without
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the knob, the entire AS_PATH must match for multipath computation.
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.. cfgcmd:: set protocols bgp <asn> parameters bestpath as-path ignore
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Ignore AS_PATH length when selecting a route
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IPv4
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^^^^
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A simple eBGP configuration:
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**Node 1:**
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.. code-block:: none
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set protocols bgp 65534 neighbor 192.168.0.2 ebgp-multihop '2'
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set protocols bgp 65534 neighbor 192.168.0.2 remote-as '65535'
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set protocols bgp 65534 neighbor 192.168.0.2 update-source '192.168.0.1'
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set protocols bgp 65534 address-family ipv4-unicast network '172.16.0.0/16'
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set protocols bgp 65534 parameters router-id '192.168.0.1'
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**Node 2:**
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.. code-block:: none
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set protocols bgp 65535 neighbor 192.168.0.1 ebgp-multihop '2'
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set protocols bgp 65535 neighbor 192.168.0.1 remote-as '65534'
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set protocols bgp 65535 neighbor 192.168.0.1 update-source '192.168.0.2'
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set protocols bgp 65535 address-family ipv4-unicast network '172.17.0.0/16'
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set protocols bgp 65535 parameters router-id '192.168.0.2'
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Don't forget, the CIDR declared in the network statement MUST **exist in your
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routing table (dynamic or static), the best way to make sure that is true is
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creating a static route:**
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**Node 1:**
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.. code-block:: none
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set protocols static route 172.16.0.0/16 blackhole distance '254'
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**Node 2:**
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.. code-block:: none
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set protocols static route 172.17.0.0/16 blackhole distance '254'
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IPv6
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^^^^
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A simple BGP configuration via IPv6.
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**Node 1:**
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.. code-block:: none
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set protocols bgp 65534 neighbor 2001:db8::2 ebgp-multihop '2'
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set protocols bgp 65534 neighbor 2001:db8::2 remote-as '65535'
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set protocols bgp 65534 neighbor 2001:db8::2 update-source '2001:db8::1'
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set protocols bgp 65534 neighbor 2001:db8::2 address-family ipv6-unicast
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set protocols bgp 65534 address-family ipv6-unicast network '2001:db8:1::/48'
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set protocols bgp 65534 parameters router-id '10.1.1.1'
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**Node 2:**
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.. code-block:: none
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set protocols bgp 65535 neighbor 2001:db8::1 ebgp-multihop '2'
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set protocols bgp 65535 neighbor 2001:db8::1 remote-as '65534'
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set protocols bgp 65535 neighbor 2001:db8::1 update-source '2001:db8::2'
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set protocols bgp 65535 neighbor 2001:db8::1 address-family ipv6-unicast
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set protocols bgp 65535 address-family ipv6-unicast network '2001:db8:2::/48'
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set protocols bgp 65535 parameters router-id '10.1.1.2'
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Don't forget, the CIDR declared in the network statement **MUST exist in your
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routing table (dynamic or static), the best way to make sure that is true is
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creating a static route:**
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**Node 1:**
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.. code-block:: none
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set protocols static route6 2001:db8:1::/48 blackhole distance '254'
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**Node 2:**
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.. code-block:: none
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set protocols static route6 2001:db8:2::/48 blackhole distance '254'
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Route Filter
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^^^^^^^^^^^^
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Route filter can be applied using a route-map:
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**Node1:**
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.. code-block:: none
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set policy prefix-list AS65535-IN rule 10 action 'permit'
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set policy prefix-list AS65535-IN rule 10 prefix '172.16.0.0/16'
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set policy prefix-list AS65535-OUT rule 10 action 'deny'
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set policy prefix-list AS65535-OUT rule 10 prefix '172.16.0.0/16'
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set policy prefix-list6 AS65535-IN rule 10 action 'permit'
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set policy prefix-list6 AS65535-IN rule 10 prefix '2001:db8:2::/48'
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set policy prefix-list6 AS65535-OUT rule 10 action 'deny'
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set policy prefix-list6 AS65535-OUT rule 10 prefix '2001:db8:2::/48'
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set policy route-map AS65535-IN rule 10 action 'permit'
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set policy route-map AS65535-IN rule 10 match ip address prefix-list 'AS65535-IN'
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set policy route-map AS65535-IN rule 10 match ipv6 address prefix-list 'AS65535-IN'
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set policy route-map AS65535-IN rule 20 action 'deny'
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set policy route-map AS65535-OUT rule 10 action 'deny'
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set policy route-map AS65535-OUT rule 10 match ip address prefix-list 'AS65535-OUT'
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set policy route-map AS65535-OUT rule 10 match ipv6 address prefix-list 'AS65535-OUT'
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set policy route-map AS65535-OUT rule 20 action 'permit'
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set protocols bgp 65534 neighbor 2001:db8::2 address-family ipv4-unicast route-map export 'AS65535-OUT'
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set protocols bgp 65534 neighbor 2001:db8::2 address-family ipv4-unicast route-map import 'AS65535-IN'
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set protocols bgp 65534 neighbor 2001:db8::2 address-family ipv6-unicast route-map export 'AS65535-OUT'
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set protocols bgp 65534 neighbor 2001:db8::2 address-family ipv6-unicast route-map import 'AS65535-IN'
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**Node2:**
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.. code-block:: none
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set policy prefix-list AS65534-IN rule 10 action 'permit'
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set policy prefix-list AS65534-IN rule 10 prefix '172.17.0.0/16'
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set policy prefix-list AS65534-OUT rule 10 action 'deny'
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set policy prefix-list AS65534-OUT rule 10 prefix '172.17.0.0/16'
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set policy prefix-list6 AS65534-IN rule 10 action 'permit'
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set policy prefix-list6 AS65534-IN rule 10 prefix '2001:db8:1::/48'
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set policy prefix-list6 AS65534-OUT rule 10 action 'deny'
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set policy prefix-list6 AS65534-OUT rule 10 prefix '2001:db8:1::/48'
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set policy route-map AS65534-IN rule 10 action 'permit'
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set policy route-map AS65534-IN rule 10 match ip address prefix-list 'AS65534-IN'
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set policy route-map AS65534-IN rule 10 match ipv6 address prefix-list 'AS65534-IN'
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set policy route-map AS65534-IN rule 20 action 'deny'
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set policy route-map AS65534-OUT rule 10 action 'deny'
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set policy route-map AS65534-OUT rule 10 match ip address prefix-list 'AS65534-OUT'
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set policy route-map AS65534-OUT rule 10 match ipv6 address prefix-list 'AS65534-OUT'
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set policy route-map AS65534-OUT rule 20 action 'permit'
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set protocols bgp 65535 neighbor 2001:db8::1 address-family ipv4-unicast route-map export 'AS65534-OUT'
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set protocols bgp 65535 neighbor 2001:db8::1 address-family ipv4-unicast route-map import 'AS65534-IN'
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set protocols bgp 65535 neighbor 2001:db8::1 address-family ipv6-unicast route-map export 'AS65534-OUT'
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set protocols bgp 65535 neighbor 2001:db8::1 address-family ipv6-unicast route-map import 'AS65534-IN'
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We could expand on this and also deny link local and multicast in the rule 20
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action deny.
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