BIRD User's Guide: Protocols

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## 6. Protocols

## 6.1 BGP

The Border Gateway Protocol is the routing protocol used for backbone level routing in the today's Internet. Contrary to the other protocols, its convergence doesn't rely on all routers following the same rules for route selection, making it possible to implement any routing policy at any router in the network, the only restriction being that if a router advertises a route, it must accept and forward packets according to it.

BGP works in terms of autonomous systems (often abbreviated as AS). Each AS is a part of the network with common management and common routing policy. It is identified by a unique 16-bit number. Routers within each AS usually communicate with each other using either a interior routing protocol (such as OSPF or RIP) or an interior variant of BGP (called iBGP). Boundary routers at the border of the AS communicate with their peers in the neighboring AS'es via exterior BGP (eBGP).

Each BGP router sends to its neighbors updates of the parts of its routing table it wishes to export along with complete path information (a list of AS'es the packet will travel through if it uses the particular route) in order to avoid routing loops.

BIRD supports all requirements of the BGP4 standard as defined in RFC 4271 ftp://ftp.rfc-editor.org/in-notes/rfc4271.txt It also supports the community attributes (RFC 1997 ftp://ftp.rfc-editor.org/in-notes/rfc1997.txt), capability negotiation (RFC 3392 ftp://ftp.rfc-editor.org/in-notes/rfc3392.txt), MD5 password authentication (RFC 2385 ftp://ftp.rfc-editor.org/in-notes/rfc2385.txt), route reflectors (RFC 4456 ftp://ftp.rfc-editor.org/in-notes/rfc4456.txt), multiprotocol extensions (RFC 4760 ftp://ftp.rfc-editor.org/in-notes/rfc4760.txt), and 4B AS numbers (RFC 4893 ftp://ftp.rfc-editor.org/in-notes/rfc4893.txt).

For IPv6, it uses the standard multiprotocol extensions defined in RFC 2283 ftp://ftp.rfc-editor.org/in-notes/rfc2283.txt including changes described in the latest draft ftp://ftp.rfc-editor.org/internet-drafts/draft-ietf-idr-bgp4-multiprotocol-v2-05.txt and applied to IPv6 according to RFC 2545 ftp://ftp.rfc-editor.org/in-notes/rfc2545.txt.

### Route selection rules

BGP doesn't have any simple metric, so the rules for selection of an optimal route among multiple BGP routes with the same preference are a bit more complex and they are implemented according to the following algorithm. It starts the first rule, if there are more "best" routes, then it uses the second rule to choose among them and so on.

- Prefer route with the highest Local Preference attribute.
- Prefer route with the shortest AS path.
- Prefer IGP origin over EGP and EGP over incomplete.
- Prefer the lowest value of the Multiple Exit Discriminator.
- Prefer internal routes over external ones.
- Prefer the route with the lowest value of router ID of the advertising router.

### Configuration

Each instance of the BGP corresponds to one neighboring router. This allows to set routing policy and all the other parameters differently for each neighbor using the following configuration parameters:

`local as number`

  Define which AS we are part of. (Note that contrary to other IP routers, BIRD is able to act as a router located in multiple AS'es simultaneously, but in such cases you need to tweak the BGP paths manually in the filters to get consistent behavior.) This parameter is mandatory.

`neighbor ip as number`

  Define neighboring router this instance will be talking to and what AS it's located in. Unless you use the `multihop` clause, it must be directly connected to one of your router's interfaces. In case the neighbor is in the same AS as we are, we automatically switch to iBGP. This parameter is mandatory.

`multihop number via ip`

  Configure multihop BGP to a neighbor which is connected at most number hops far and to which we should route via our direct neighbor with address ip. Default: switched off.

`next hop self`

  Avoid calculation of the Next Hop attribute and always advertise our own source address (see below) as a next hop. This needs to be used only occasionally to circumvent misconfigurations of other routers. Default: disabled.

`missing lladdr self|drop|ignore`

  Next Hop attribute in BGP-IPv6 sometimes contains just the global IPv6 address, but sometimes it has to contain both global and link-local IPv6 addresses. This option specifies what to do if BIRD have to send both addresses but does not know link-local address. This situation might happen when routes from other protocols are exported to BGP, or when improper updates are received from BGP peers. `self` means that BIRD advertises its own local address instead. `drop` means that BIRD skips that prefixes and logs error. `ignore` means that BIRD ignores the problem and sends just the global address (and therefore forms improper BGP update). Default: `self`, unless BIRD is configured as a route server (option `rs client`), in that case default is `drop`, because route servers usually does not forward packets ifselves.

`source address ip`

  Define local address we should use for next hop calculation. Default: the address of the local end of the interface our neighbor is connected to.

`password string`

  Use this password for MD5 authentication of BGP sessions. Default: no authentication. Password has to be set by external utility (e.g. setkey(8)) on BSD systems.

`passive switch`

  Standard BGP behavior is both initiating outgoing connections and accepting incoming connections. In passive mode, outgoing connections are not initiated. Default: off.

`rr client`

  Be a route reflector and treat the neighbor as a route reflection client. Default: disabled.

`rr cluster id IPv4 address`

  Route reflectors use cluster id to avoid route reflection loops. When there is one route reflector in a cluster it usually uses its router id as a cluster id, but when there are more route reflectors in a cluster, these need to be configured (using this option) to use a common cluster id. Clients in a cluster need not know their cluster id and this option is not allowed for them. Default: the same as router id.

`rs client`

  Be a route server and treat the neighbor as a route server client. A route server is used as a replacement for full mesh EBGP routing in Internet exchange points in a similar way to route reflectors used in IBGP routing. BIRD does not implement obsoleted RFC 1863, but uses ad-hoc implementation, which behaves like plain EBGP but reduces modifications to advertised route attributes to be transparent (for example does not prepend its AS number to AS PATH attribute and keep MED attribute). Default: disabled.

`enable route refresh switch`

  When BGP speaker changes its import filter, it has to re-examine all routes received from its neighbor against the new filter. As these routes might not be available, there is a BGP protocol extension Route Refresh (specified in RFC 2918) that allows BGP speaker to request re-advertisement of all routes from its neighbor. This option specifies whether BIRD advertises this capability and accepts such requests. Even when disabled, BIRD can send route refresh requests. Default: on.

`interpret communities switch`

  RFC 1997 demands that BGP speaker should process well-known communities like no-export (65535, 65281) or no-advertise (65535, 65282). For example, received route carrying a no-adverise community should not be advertised to any of its neighbors. If this option is enabled (which is by default), BIRD has such behavior automatically (it is evaluated when a route is exported to the BGP protocol just before the export filter). Otherwise, this integrated processing of well-known communities is disabled. In that case, similar behavior can be implemented in the export filter. Default: on.

`enable as4 switch`

  BGP protocol was designed to use 2B AS numbers and was extended later to allow 4B AS number. BIRD supports 4B AS extension, but by disabling this option it can be persuaded not to advertise it and to maintain old-style sessions with its neighbors. This might be useful for circumventing bugs in neighbor's implementation of 4B AS extension. Even when disabled (off), BIRD behaves internally as AS4-aware BGP router. Default: on.

`capabilities switch`

  Use capability advertisement to advertise optional capabilities. This is standard behavior for newer BGP implementations, but there might be some older BGP implementations that reject such connection attempts. When disabled (off), features that request it (4B AS support) are also disabled. Default: on, with automatic fallback to off when received capability-related error.

`advertise ipv4 switch`

  Advertise IPv4 multiprotocol capability. This is not a correct behavior according to the strict interpretation of RFC 4760, but it is widespread and required by some BGP implementations (Cisco and Quagga). This option is relevant to IPv4 mode with enabled capability advertisement only. Default: on.

`route limit number`

  The maximal number of routes that may be imported from the protocol. If the route limit is exceeded, the connection is closed with error. Default: no limit.

`disable after error switch`

  When an error is encountered (either locally or by the other side), disable the instance automatically and wait for an administrator to fix the problem manually. Default: off.

`hold time number`

  Time in seconds to wait for a Keepalive message from the other side before considering the connection stale. Default: depends on agreement with the neighboring router, we prefer 240 seconds if the other side is willing to accept it.

`startup hold time number`

  Value of the hold timer used before the routers have a chance to exchange open messages and agree on the real value. Default: 240 seconds.

`keepalive time number`

  Delay in seconds between sending of two consecutive Keepalive messages. Default: One third of the hold time.

`connect retry time number`

  Time in seconds to wait before retrying a failed attempt to connect. Default: 120 seconds.

`start delay time number`

  Delay in seconds between protocol startup and the first attempt to connect. Default: 5 seconds.

`error wait time number,number`

  Minimum and maximum delay in seconds between a protocol failure (either local or reported by the peer) and automatic restart. Doesn't apply when `disable after error` is configured. If consecutive errors happen, the delay is increased exponentially until it reaches the maximum. Default: 60, 300.

`error forget time number`

  Maximum time in seconds between two protocol failures to treat them as a error sequence which makes the `error wait time` increase exponentially. Default: 300 seconds.

`path metric switch`

  Enable comparison of path lengths when deciding which BGP route is the best one. Default: on.

`prefer older switch`

  Standard route selection algorithm breaks ties by comparing router IDs. This changes the behavior to prefer older routes (when both are external and from different peer). For details, see RFC 5004. Default: off.

`default bgp_med number`

  Value of the Multiple Exit Discriminator to be used during route selection when the MED attribute is missing. Default: 0.

`default bgp_local_pref number`

  A default value for the Local Preference attribute. It is used when a new Local Preference attribute is attached to a route by the BGP protocol itself (for example, if a route is received through eBGP and therefore does not have such attribute). Default: 100 (0 in pre-1.2.0 versions of BIRD).

### Attributes

BGP defines several route attributes. Some of them (those marked with ``I`' in the table below) are available on internal BGP connections only, some of them (marked with ``O`') are optional.

`bgppath `bgp_path``

  Sequence of AS numbers describing the AS path the packet will travel through when forwarded according to the particular route. In case of internal BGP it doesn't contain the number of the local AS.

`int `bgp_local_pref` [I]`

  Local preference value used for selection among multiple BGP routes (see the selection rules above). It's used as an additional metric which is propagated through the whole local AS.

`int `bgp_med` [O]`

  The Multiple Exit Discriminator of the route is an optional attribute which is used on on external (inter-AS) links to convey to an adjacent AS the optimal entry point into the local AS. The received attribute may be also propagated over internal BGP links (and this is default behavior). The attribute value is zeroed when a route is exported from a routing table to a BGP instance to ensure that the attribute received from a neighboring AS is not propagated to other neighboring ASes. A new value might be set in the export filter of a BGP instance. See RFC 4451 ftp://ftp.rfc-editor.org/in-notes/rfc4451.txt for further discussion of BGP MED attribute.

`enum `bgp_origin``

  Origin of the route: either `ORIGIN_IGP` if the route has originated in an interior routing protocol or `ORIGIN_EGP` if it's been imported from the `EGP` protocol (nowadays it seems to be obsolete) or `ORIGIN_INCOMPLETE` if the origin is unknown.

`ip `bgp_next_hop``

  Next hop to be used for forwarding of packets to this destination. On internal BGP connections, it's an address of the originating router if it's inside the local AS or a boundary router the packet will leave the AS through if it's an exterior route, so each BGP speaker within the AS has a chance to use the shortest interior path possible to this point.

`void `bgp_atomic_aggr` [O]`

  This is an optional attribute which carries no value, but the sole presence of which indicates that the route has been aggregated from multiple routes by some router on the path from the originator.

`clist `bgp_community` [O]`

  List of community values associated with the route. Each such value is a pair (represented as a `pair` data type inside the filters) of 16-bit integers, the first of them containing the number of the AS which defines the community and the second one being a per-AS identifier. There are lots of uses of the community mechanism, but generally they are used to carry policy information like "don't export to USA peers". As each AS can define its own routing policy, it also has a complete freedom about which community attributes it defines and what will their semantics be.

`quad `bgp_originator_id` [O]`

  This attribute is created by the route reflector when reflecting the route and contains the router ID of the originator of the route in the local AS.

`clist `bgp_cluster_list` [O]`

  This attribute contains a list of cluster IDs of route reflectors. Each route reflector prepends its cluster ID when reflecting the route.

### Example

    protocol bgp {
            local as 65000;                      # Use a private AS number
            neighbor 62.168.0.130 as 5588;       # Our neighbor ...
            multihop 20 via 62.168.0.13;         # ... which is connected indirectly
            export filter {                      # We use non-trivial export rules
                    if source = RTS_STATIC then { # Export only static routes
                            # Assign our community
                            bgp_community.add((65000,5678));
                            # Artificially increase path length
                            # by advertising local AS number twice
                            if bgp_path ~ [= 65000 =] then
                                    bgp_path.prepend(65000);
                            accept;
                    }
                    reject;
            };
            import all;
            source address 62.168.0.1;      # Use a non-standard source address
    }

## 6.2 Device

The Device protocol is not a real routing protocol. It doesn't generate any routes and it only serves as a module for getting information about network interfaces from the kernel.

Except for very unusual circumstances, you probably should include this protocol in the configuration since almost all other protocols require network interfaces to be defined for them to work with.

### Configuration

`scan time number`

  Time in seconds between two scans of the network interface list. On systems where we are notified about interface status changes asynchronously (such as newer versions of Linux), we need to scan the list only in order to avoid confusion by lost notification messages, so the default time is set to a large value.

`primary [ "mask" ] prefix`

  If a network interface has more than one network address, BIRD has to choose one of them as a primary one, because some routing protocols (for example OSPFv2) suppose there is only one network address per interface. By default, BIRD chooses the lexicographically smallest address as the primary one.

  This option allows to specify which network address should be chosen as a primary one. Network addresses that match prefix are preferred to non-matching addresses. If more `primary` options are used, the first one has the highest preference. If "mask" is specified, then such `primary` option is relevant only to matching network interfaces.

  In all cases, an address marked by operating system as secondary cannot be chosen as the primary one.

As the Device protocol doesn't generate any routes, it cannot have any attributes. Example configuration looks like this:

    protocol device {
            scan time 10;           # Scan the interfaces often
            primary "eth0" 192.168.1.1;
            primary 192.168.0.0/16;
    }

## 6.3 Direct

The Direct protocol is a simple generator of device routes for all the directly connected networks according to the list of interfaces provided by the kernel via the Device protocol.

The question is whether it is a good idea to have such device routes in BIRD routing table. OS kernel usually handles device routes for directly connected networks by itself so we don't need (and don't want) to export these routes to the kernel protocol. OSPF protocol creates device routes for its interfaces itself and BGP protocol is usually used for exporting aggregate routes. Although there are some use cases that use the direct protocol (like abusing eBGP as an IGP routing protocol), in most cases it is not needed to have these device routes in BIRD routing table and to use the direct protocol.

The only configurable thing about direct is what interfaces it watches:

`interface pattern [, ...]`

  By default, the Direct protocol will generate device routes for all the interfaces available. If you want to restrict it to some subset of interfaces (for example if you're using multiple routing tables for policy routing and some of the policy domains don't contain all interfaces), just use this clause.

Direct device routes don't contain any specific attributes.

Example config might look like this:

    protocol direct {
            interface "-arc*", "*";         # Exclude the ARCnets
    }

## 6.4 Kernel

The Kernel protocol is not a real routing protocol. Instead of communicating with other routers in the network, it performs synchronization of BIRD's routing tables with the OS kernel. Basically, it sends all routing table updates to the kernel and from time to time it scans the kernel tables to see whether some routes have disappeared (for example due to unnoticed up/down transition of an interface) or whether an `alien' route has been added by someone else (depending on the `learn` switch, such routes are either ignored or accepted to our table).

Unfortunately, there is one thing that makes the routing table synchronization a bit more complicated. In the kernel routing table there are also device routes for directly connected networks. These routes are usually managed by OS itself (as a part of IP address configuration) and we don't want to touch that. They are completely ignored during the scan of the kernel tables and also the export of device routes from BIRD tables to kernel routing tables is restricted to prevent accidental interference. This restriction can be disabled using `device routes` switch.

If your OS supports only a single routing table, you can configure only one instance of the Kernel protocol. If it supports multiple tables (in order to allow policy routing; such an OS is for example Linux 2.2), you can run as many instances as you want, but each of them must be connected to a different BIRD routing table and to a different kernel table.

### Configuration

`persist switch`

  Tell BIRD to leave all its routes in the routing tables when it exits (instead of cleaning them up).

`scan time number`

  Time in seconds between two consecutive scans of the kernel routing table.

`learn switch`

  Enable learning of routes added to the kernel routing tables by other routing daemons or by the system administrator. This is possible only on systems which support identification of route authorship.

`device routes switch`

  Enable export of device routes to the kernel routing table. By default, such routes are rejected (with the exception of explicitly configured device routes from the static protocol) regardless of the export filter to protect device routes in kernel routing table (managed by OS itself) from accidental overwriting or erasing.

`kernel table number`

  Select which kernel table should this particular instance of the Kernel protocol work with. Available only on systems supporting multiple routing tables.

The Kernel protocol doesn't define any route attributes.

A simple configuration can look this way:

    protocol kernel {
            import all;
            export all;
    }

Or for a system with two routing tables:

    protocol kernel {               # Primary routing table
            learn;                  # Learn alien routes from the kernel
            persist;                # Don't remove routes on bird shutdown
            scan time 10;           # Scan kernel routing table every 10 seconds
            import all;
            export all;
    }

    protocol kernel {               # Secondary routing table
            table auxtable;
            kernel table 100;
            export all;
    }

## 6.5 OSPF

### Introduction

Open Shortest Path First (OSPF) is a quite complex interior gateway protocol. The current IPv4 version (OSPFv2) is defined in RFC 2328 ftp://ftp.rfc-editor.org/in-notes/rfc2328.txt and the current IPv6 version (OSPFv3) is defined in RFC 5340 ftp://ftp.rfc-editor.org/in-notes/rfc5340.txt It's a link state (a.k.a. shortest path first) protocol -- each router maintains a database describing the autonomous system's topology. Each participating router has an identical copy of the database and all routers run the same algorithm calculating a shortest path tree with themselves as a root. OSPF chooses the least cost path as the best path.

In OSPF, the autonomous system can be split to several areas in order to reduce the amount of resources consumed for exchanging the routing information and to protect the other areas from incorrect routing data. Topology of the area is hidden to the rest of the autonomous system.

Another very important feature of OSPF is that it can keep routing information from other protocols (like Static or BGP) in its link state database as external routes. Each external route can be tagged by the advertising router, making it possible to pass additional information between routers on the boundary of the autonomous system.

OSPF quickly detects topological changes in the autonomous system (such as router interface failures) and calculates new loop-free routes after a short period of convergence. Only a minimal amount of routing traffic is involved.

Each router participating in OSPF routing periodically sends Hello messages to all its interfaces. This allows neighbors to be discovered dynamically. Then the neighbors exchange theirs parts of the link state database and keep it identical by flooding updates. The flooding process is reliable and ensures that each router detects all changes.

### Configuration

In the main part of configuration, there can be multiple definitions of OSPF area witch different id included. These definitions includes many other switches and multiple definitions of interfaces. Definition of interface may contain many switches and constant definitions and list of neighbors on nonbroadcast networks.

    protocol ospf <name> {
            rfc1583compat <switch>;
            tick <num>;
            area <id> {
                    stub cost <num>;
                    networks {
                            <prefix>;
                            <prefix> hidden;
                    }
                    stubnet <prefix>;
                    stubnet <prefix> {
                            hidden <switch>;
                            summary <switch>;
                            cost <num>;
                    }
                    interface <interface pattern> {
                            cost <num>;
                            stub <switch>;
                            hello <num>;
                            poll <num>;
                            retransmit <num>;
                            priority <num>;
                            wait <num>;
                            dead count <num>;
                            dead <num>;
                            rx buffer [normal|large|<num>];
                            type [broadcast|nonbroadcast|pointopoint];
                            strict nonbroadcast <switch>;
                            authentication [none|simple|cryptographic];
                            password "<text>";
                            password "<text>" {
                                    id <num>;
                                    generate from "<date>";
                                    generate to "<date>";
                                    accept from "<date>";
                                    accept to "<date>";
                            };
                            neighbors {
                                    <ip>;
                                    <ip> eligible;
                            };
                    };
                    virtual link <id> {
                            hello <num>;
                            retransmit <num>;
                            wait <num>;
                            dead count <num>;
                            dead <num>;
                            authentication [none|simple|cryptographic];
                            password "<text>";
                    };
            };
    }

`rfc1583compat switch`

  This option controls compatibility of routing table calculation with RFC 1583 ftp://ftp.rfc-editor.org/in-notes/rfc1583.txt. Default value is no.

`area id`

  This defines an OSPF area with given area ID (an integer or an IPv4 address, similarly to a router ID). The most important area is the backbone (ID 0) to which every other area must be connected.

`stub cost num`

  No external (except default) routes are flooded into stub areas. Setting this value marks area stub with defined cost of default route. Default value is no. (Area is not stub.)

`tick num`

  The routing table calculation and clean-up of areas' databases is not performed when a single link state change arrives. To lower the CPU utilization, it's processed later at periodical intervals of num seconds. The default value is 1.

`networks { set }`

  Definition of area IP ranges. This is used in summary LSA origination. Hidden networks are not propagated into other areas.

`stubnet prefix { options }`

  Stub networks are networks that are not transit networks between OSPF routers. They are also propagated through an OSPF area as a part of a link state database. By default, BIRD generates a stub network record for each primary network address on each OSPF interface that does not have any OSPF neighbors, and also for each non-primary network address on each OSPF interface. This option allows to alter a set of stub networks propagated by this router.

  Each instance of this option adds a stub network with given network prefix to the set of propagated stub network, unless option `hidden` is used. It also suppresses default stub networks for given network prefix. When option `summary` is used, also default stub networks that are subnetworks of given stub network are suppressed. This might be used, for example, to aggregate generated stub networks.

`interface pattern`

  Defines that the specified interfaces belong to the area being defined. See interface common option for detailed description.

`virtual link id`

  Virtual link to router with the router id. Virtual link acts as a point-to-point interface belonging to backbone. The actual area is used as transport area. This item cannot be in the backbone.

`cost num`

  Specifies output cost (metric) of an interface. Default value is 10.

`stub switch`

  If set to interface it does not listen to any packet and does not send any hello. Default value is no.

`hello num`

  Specifies interval in seconds between sending of Hello messages. Beware, all routers on the same network need to have the same hello interval. Default value is 10.

`poll num`

  Specifies interval in seconds between sending of Hello messages for some neighbors on NBMA network. Default value is 20.

`retransmit num`

  Specifies interval in seconds between retransmissions of unacknowledged updates. Default value is 5.

`priority num`

  On every multiple access network (e.g., the Ethernet) Designed Router and Backup Designed router are elected. These routers have some special functions in the flooding process. Higher priority increases preferences in this election. Routers with priority 0 are not eligible. Default value is 1.

`wait num`

  After start, router waits for the specified number of seconds between starting election and building adjacency. Default value is 40.

`dead count num`

  When the router does not receive any messages from a neighbor in dead count*hello seconds, it will consider the neighbor down.

`dead num`

  When the router does not receive any messages from a neighbor in dead seconds, it will consider the neighbor down. If both directives dead count and dead are used, dead has precendence.

`rx buffer num`

  This sets the size of buffer used for receiving packets. The buffer should be bigger than maximal size of any packets. Value NORMAL (default) means 2*MTU, value LARGE means maximal allowed packet - 65536.

`type broadcast`

  BIRD detects a type of a connected network automatically, but sometimes it's convenient to force use of a different type manually. On broadcast networks, flooding and Hello messages are sent using multicasts (a single packet for all the neighbors).

`type pointopoint`

  Point-to-point networks connect just 2 routers together. No election is performed there which reduces the number of messages sent.

`type nonbroadcast`

  On nonbroadcast networks, the packets are sent to each neighbor separately because of lack of multicast capabilities.

`strict nonbroadcast switch`

  If set, don't send hello to any undefined neighbor. This switch is ignored on any non-NBMA network. Default is No.

`authentication none`

  No passwords are sent in OSPF packets. This is the default value.

`authentication simple`

  Every packet carries 8 bytes of password. Received packets lacking this password are ignored. This authentication mechanism is very weak.

`authentication cryptographic`

  16-byte long MD5 digest is appended to every packet. For the digest generation 16-byte long passwords are used. Those passwords are not sent via network, so this mechanism is quite secure. Packets can still be read by an attacker.

`password "text"`

  An 8-byte or 16-byte password used for authentication. See password common option for detailed description.

`neighbors { set }`

  A set of neighbors to which Hello messages on nonbroadcast networks are to be sent. Some of them could be marked as eligible.

### Attributes

OSPF defines four route attributes. Each internal route has a `metric`. Metric is ranging from 1 to infinity (65535). External routes use `metric type 1` or `metric type 2`. A `metric of type 1` is comparable with internal `metric`, a `metric of type 2` is always longer than any `metric of type 1` or any `internal metric`. `Internal metric` or `metric of type 1` is stored in attribute `ospf_metric1`, `metric type 2` is stored in attribute `ospf_metric2`. If you specify both metrics only metric1 is used.

Each external route can also carry attribute `ospf_tag` which is a 32-bit integer which is used when exporting routes to other protocols; otherwise, it doesn't affect routing inside the OSPF domain at all. The fourth attribute `ospf_router_id` is a router ID of the router advertising that route/network. This attribute is read-only. Default is `ospf_metric2 = 10000` and `ospf_tag = 0`.

### Example

    protocol ospf MyOSPF {
            rfc1583compat yes;
            tick 2;
            export filter {
                    if source = RTS_BGP then {
                            ospf_metric1 = 100;
                            accept;
                    }
                    reject;
            };
            area 0.0.0.0 {
                    interface "eth*" {
                            cost 11;
                            hello 15;
                            priority 100;
                            retransmit 7;
                            authentication simple;
                            password "aaa";
                    };
                    interface "ppp*" {
                            cost 100;
                            authentication cryptographic;
                            password "abc" {
                                    id 1;
                                    generate to "22-04-2003 11:00:06";
                                    accept from "17-01-2001 12:01:05";
                            };
                            password "def" {
                                    id 2;
                                    generate to "22-07-2005 17:03:21";
                                    accept from "22-02-2001 11:34:06";
                            };
                    };
                    interface "arc0" {
                            cost 10;
                            stub yes;
                    };
                    interface "arc1";
            };
            area 120 {
                    stub yes;
                    networks {
                            172.16.1.0/24;
                            172.16.2.0/24 hidden;
                    }
                    interface "-arc0" , "arc*" {
                            type nonbroadcast;
                            authentication none;
                            strict nonbroadcast yes;
                            wait 120;
                            poll 40;
                            dead count 8;
                            neighbors {
                                    192.168.120.1 eligible;
                                    192.168.120.2;
                                    192.168.120.10;
                            };
                    };
            };
    }

## 6.6 Pipe

### Introduction

The Pipe protocol serves as a link between two routing tables, allowing routes to be passed from a table declared as primary (i.e., the one the pipe is connected to using the `table` configuration keyword) to the secondary one (declared using `peer table`) and vice versa, depending on what's allowed by the filters. Export filters control export of routes from the primary table to the secondary one, import filters control the opposite direction.

The Pipe protocol may work in the opaque mode or in the transparent mode. In the opaque mode, the Pipe protocol retransmits optimal route from one table to the other table in a similar way like other protocols send and receive routes. Retransmitted route will have the source set to the Pipe protocol, which may limit access to protocol specific route attributes. The opaque mode is a default mode.

In transparent mode, the Pipe protocol retransmits all routes from one table to the other table, retaining their original source and attributes. If import and export filters are set to accept, then both tables would have the same content. The mode can be set by `mode` option.

The primary use of multiple routing tables and the Pipe protocol is for policy routing, where handling of a single packet doesn't depend only on its destination address, but also on its source address, source interface, protocol type and other similar parameters. In many systems (Linux being a good example), the kernel allows to enforce routing policies by defining routing rules which choose one of several routing tables to be used for a packet according to its parameters. Setting of these rules is outside the scope of BIRD's work (on Linux, you can use the `ip` command), but you can create several routing tables in BIRD, connect them to the kernel ones, use filters to control which routes appear in which tables and also you can employ the Pipe protocol for exporting a selected subset of one table to another one.

### Configuration

`peer table table`

  Defines secondary routing table to connect to. The primary one is selected by the `table` keyword.

`mode opaque|transparent`

  Specifies the mode for the pipe to work in. Default is opaque.

### Attributes

The Pipe protocol doesn't define any route attributes.

### Example

Let's consider a router which serves as a boundary router of two different autonomous systems, each of them connected to a subset of interfaces of the router, having its own exterior connectivity and wishing to use the other AS as a backup connectivity in case of outage of its own exterior line.

Probably the simplest solution to this situation is to use two routing tables (we'll call them `as1` and `as2`) and set up kernel routing rules, so that packets having arrived from interfaces belonging to the first AS will be routed according to `as1` and similarly for the second AS. Thus we have split our router to two logical routers, each one acting on its own routing table, having its own routing protocols on its own interfaces. In order to use the other AS's routes for backup purposes, we can pass the routes between the tables through a Pipe protocol while decreasing their preferences and correcting their BGP paths to reflect the AS boundary crossing.

    table as1;                              # Define the tables
    table as2;

    protocol kernel kern1 {                 # Synchronize them with the kernel
            table as1;
            kernel table 1;
    }

    protocol kernel kern2 {
            table as2;
            kernel table 2;
    }

    protocol bgp bgp1 {                     # The outside connections
            table as1;
            local as 1;
            neighbor 192.168.0.1 as 1001;
            export all;
            import all;
    }

    protocol bgp bgp2 {
            table as2;
            local as 2;
            neighbor 10.0.0.1 as 1002;
            export all;
            import all;
    }

    protocol pipe {                         # The Pipe
            table as1;
            peer table as2;
            export filter {
                    if net ~ [ 1.0.0.0/8+] then {   # Only AS1 networks
                            if preference>10 then preference = preference-10;
                            if source=RTS_BGP then bgp_path.prepend(1);
                            accept;
                    }
                    reject;
            };
            import filter {
                    if net ~ [ 2.0.0.0/8+] then {   # Only AS2 networks
                            if preference>10 then preference = preference-10;
                            if source=RTS_BGP then bgp_path.prepend(2);
                            accept;
                    }
                    reject;
            };
    }

## 6.7 RIP

### Introduction

The RIP protocol (also sometimes called Rest In Pieces) is a simple protocol, where each router broadcasts (to all its neighbors) distances to all networks it can reach. When a router hears distance to another network, it increments it and broadcasts it back. Broadcasts are done in regular intervals. Therefore, if some network goes unreachable, routers keep telling each other that its distance is the original distance plus 1 (actually, plus interface metric, which is usually one). After some time, the distance reaches infinity (that's 15 in RIP) and all routers know that network is unreachable. RIP tries to minimize situations where counting to infinity is necessary, because it is slow. Due to infinity being 16, you can't use RIP on networks where maximal distance is higher than 15 hosts. You can read more about RIP at http://www.ietf.org/html.charters/rip-charter.html. Both IPv4 (RFC 1723 ftp://ftp.rfc-editor.org/in-notes/rfc1723.txt) and IPv6 (RFC 2080 ftp://ftp.rfc-editor.org/in-notes/rfc2080.txt) versions of RIP are supported by BIRD, historical RIPv1 (RFC 1058 ftp://ftp.rfc-editor.org/in-notes/rfc1058.txt)is not currently supported. RIPv4 MD5 authentication (RFC 2082 ftp://ftp.rfc-editor.org/in-notes/rfc2082.txt) is supported.

RIP is a very simple protocol, and it has a lot of shortcomings. Slow convergence, big network load and inability to handle larger networks makes it pretty much obsolete. (It is still usable on very small networks.)

### Configuration

In addition to options common for all to other protocols, RIP supports the following ones:

`authentication none|plaintext|md5`

  selects authentication method to be used. `none` means that packets are not authenticated at all, `plaintext` means that a plaintext password is embedded into each packet, and `md5` means that packets are authenticated using a MD5 cryptographic hash. If you set authentication to not-none, it is a good idea to add `password` section. Default: none.

`honor always|neighbor|never`

  specifies when should requests for dumping routing table be honored. (Always, when sent from a host on a directly connected network or never.) Routing table updates are honored only from neighbors, that is not configurable. Default: never.

There are two options that can be specified per-interface. First is `metric`, with default one. Second is `mode multicast|broadcast|quiet|nolisten|version1`, it selects mode for rip to work in. If nothing is specified, rip runs in multicast mode. `version1` is currently equivalent to `broadcast`, and it makes RIP talk to a broadcast address even through multicast mode is possible. `quiet` option means that RIP will not transmit any periodic messages to this interface and `nolisten` means that RIP will send to this interface but not listen to it.

The following options generally override behavior specified in RFC. If you use any of these options, BIRD will no longer be RFC-compliant, which means it will not be able to talk to anything other than equally configured BIRD. I have warned you.

`port number`

  selects IP port to operate on, default 520. (This is useful when testing BIRD, if you set this to an address >1024, you will not need to run bird with UID==0).

`infinity number`

  selects the value of infinity, default is 16. Bigger values will make protocol convergence even slower.

`period number`

  specifies the number of seconds between periodic updates. Default is 30 seconds. A lower number will mean faster convergence but bigger network load. Do not use values lower than 10.

`timeout time number`

  specifies how old route has to be to be considered unreachable. Default is 4*`period`.

`garbage time number`

  specifies how old route has to be to be discarded. Default is 10*`period`.

### Attributes

RIP defines two route attributes:

`int `rip_metric``

  RIP metric of the route (ranging from 0 to `infinity`). When routes from different RIP instances are available and all of them have the same preference, BIRD prefers the route with lowest `rip_metric`. When importing a non-RIP route, the metric defaults to 5.

`int `rip_tag``

  RIP route tag: a 16-bit number which can be used to carry additional information with the route (for example, an originating AS number in case of external routes). When importing a non-RIP route, the tag defaults to 0.

### Example

    protocol rip MyRIP_test {
            debug all;
            port 1520;
            period 10;
            garbage time 60;
            interface "eth0" { metric 3; mode multicast; };
            interface "eth*" { metric 2; mode broadcast; };
            honor neighbor;
            authentication none;
            import filter { print "importing"; accept; };
            export filter { print "exporting"; accept; };
    }

## 6.8 Static

The Static protocol doesn't communicate with other routers in the network, but instead it allows you to define routes manually. This is often used for specifying how to forward packets to parts of the network which don't use dynamic routing at all and also for defining sink routes (i.e., those telling to return packets as undeliverable if they are in your IP block, you don't have any specific destination for them and you don't want to send them out through the default route to prevent routing loops).

There are three types of static routes: `classical' routes telling to forward packets to a neighboring router, device routes specifying forwarding to hosts on a directly connected network and special routes (sink, blackhole etc.) which specify a special action to be done instead of forwarding the packet.

When the particular destination is not available (the interface is down or the next hop of the route is not a neighbor at the moment), Static just uninstalls the route from the table it is connected to and adds it again as soon as the destination becomes adjacent again.

The Static protocol has no configuration options. Instead, the definition of the protocol contains a list of static routes:

`route prefix via ip`

  Static route through a neighboring router.

`route prefix via "interface"`

  Static device route through an interface to hosts on a directly connected network.

`route prefix drop|reject|prohibit`

  Special routes specifying to drop the packet, return it as unreachable or return it as administratively prohibited.

Static routes have no specific attributes.

Example static config might look like this:

    protocol static {
            table testable;                  # Connect to a non-default routing table
            route 0.0.0.0/0 via 62.168.0.13; # Default route
            route 62.168.0.0/25 reject;      # Sink route
            route 10.2.0.0/24 via "arc0";    # Secondary network
    }

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