WO2007060364A1

WO2007060364A1 - Method for selecting in a router a route between at least two routes related to a common destination network address

Info

Publication number: WO2007060364A1
Application number: PCT/FR2006/051205
Authority: WO
Inventors: Marc Capelle; Xavier Vinet; Frédéric SALAUN; Ivan Valiente
Original assignee: France Telecom
Priority date: 2005-11-22
Filing date: 2006-11-21
Publication date: 2007-05-31
Also published as: FR2893799A1

Abstract

The invention concerns a method whereby at least two routes related to a common destination network address have been announced to a first router by two redundant peer routers. The first router then executes a method for selecting one of the two routes including steps of dropping the routes based on parameters associated with the routes. According to the invention, the method includes, when the dropping steps have not enabled the routes to discriminated: a step of assignment (E0) during which, upon reception of a route, the first router assigns a parameter representing the chronological order of receiving the route; a step (E3) of selecting one of said routes based on the assigned parameter.

Description

Method for selecting in a router a route among at least two routes relating to the same destination network address The invention relates to a method for selecting in a router a route among at least two routes relating to the same destination network address and announced by even routers.

The Internet is split into a plurality of "subnets", each of which has a set of routers and operates under its own technical administration, called "autonomous system". Routers are responsible for routing packets across the network and sharing routing information stored in routing tables. A protocol such as Border Gateway Protocol (BGP), defined in IETF RFC 1771 (for the Internet Engeering Task Force), ensures loopless routing between autonomous systems. A BGP routing protocol makes it possible to have sessions between routers of a network so as to ensure the consistency of the routing tables on all the equipment within a given administrative domain and on the network. set of administrative domains of the Internet network. Two versions of BGP exist: if the two routers belong to the same autonomous system, it is IBGP (Internai BGP); if the two routers belong to different autonomous systems, it is EBGP (External BGP). Two routers, to communicate their routing tables, establish a BGP session. A router with which another router has established a BGP session is called a BGP peer router.

For each BGP session with a peer router, a router receives the routing information advertised by that peer and stores it in an Adj_RIB_In table. Based on this information, such as when removing, adding, or modifying a route, or periodically, the router selects a preferred route from among multiple routes with the same destination network address using a selection algorithm called "tie-breaking" in English. Once the preferred route has been selected, it updates a local route management table (Loc_RIB), updates the information of a routing table (FIB) used for routing packets in its transport plan, sets set of routing information to Announcing each of the BGP peer routers stores for each BGP peer router the information to be advertised in a table (Adj_RIB_Out) and transmits it to each BGP peer router.

Each event occurring on the network, such as closing a BGP session or restarting a device, causes cascading updates of the different routers on the network. Indeed, each BGP router receives route updates, selects the preferred route for each destination network address, updates the routing table of its transport plan and transmits the routing information to each peer router. In order to secure the network, redundant routers have been introduced.

A redundant router, called "backup", has with the router, called "nominal", it secures a large number of common characteristics or attributes.

A route selection algorithm is proposed in the IETF working document referenced draft-ietf-idr-bgp4-26. This algorithm comprises different selection steps according to parameters associated with the routes. For routes advertised by redundant routers, only the last step of the algorithm based on the selection of the route advertised by the router whose IP address is the smallest makes it possible to make the final selection.

When the BGP sessions with the nominal router whose IP address is the smallest and therefore through which routes selected by its peer BGP routers, are closed, for example following a network event, all of its BGP routers peers executes the route selection algorithm and the selected routes become those announced by the backup router. The routes selected by each peer BGP router are forwarded to its transport plan and to its own peer BGP routers. When the BGP sessions of the nominal router are reopened, all of its peer BGP routers will again execute the route selection algorithm and the selected routes will become those advertised by the nominal router. Such a change involves a transfer of the selected new routes to the transport plan of each router and a dissemination of this information to all the BGP peers of this router. This results in an increase in the network load due to exchanges of messages between routers, and also an increase in the load of the routers because of the processing of these messages, which results in a convergence time, that is to say the time corresponding to the time taken by the router to put update its routing tables during a topology change, high. Take the example of routers of road reflectors type also called

"RR" (for "Reflector Road" in English). These devices centralize routing information and centrally manage BGP sessions with each of the BGP routers in the network. Because of this centralization, the routing tables they broadcast are very large. In case of restart of the nominal RR, the convergence time of this is about three times higher than that of the backup RR.

There is therefore a need for a technique to limit changes in the routing tables of all BGP peer routers and reduce the convergence time of routers in case of session reset.

The invention responds to this need by proposing a method for selecting in a router a route among at least two routes, these routes being related to the same destination network address and having been announced by two other routers to the router, these other routers being redundant. The method includes steps of discarding routes based on parameters associated with the routes. It further comprises:

an assignment step during which, on reception of a route, the first router assigns a parameter representative of the chronological order of reception of the route;

a step of selecting one of these routes according to the parameter assigned.

The invention therefore consists in assigning to the router, when receiving a route advertised by a peer router, a parameter representative of the chronological order of reception of the routes by the router, called the local discrimination parameter. This parameter then makes it possible to discriminate two routes, relating to the same destination network address, transmitted by two redundant routers and to avoid the change in the selection of the routes when the nominal router returns to service. Thus, undifferentiated use of redundant routers becomes possible. In one embodiment, the selected route corresponds to the one whose reception is the oldest.

Thus, when the choice must be made between two routes, respectively announced by two separate redundant peer routers, the method will allow to privilege that announced earlier. In the example cited above, this avoids unnecessarily switching on the routes announced by the router that had a malfunction when there is more malfunction.

Advantageously, the selection step according to the assigned parameter is executed before a second selection step according to the network address of the advertiser router of the route.

Thus, the selection step based on the discrimination parameter is carried out before the selection step according to the network address of the router having announced the route and avoids the updating of the routing tables in the transport plan and broadcast selected routes to the peer BGP routers. The invention also relates to a router for routing data packets traversing it, comprising: route management means arranged to receive routes announced by at least one peer router; means for selecting a route, arranged to select a route from a set of announced routes, relating to the same destination network address, the selection means being able to set aside routes according to parameters associated with the routes; roads; characterized in that it further comprises, when said selection means have not allowed to discriminate announced routes: - means for assigning to an advertised route a parameter representative of the chronological order of reception of the route, these management means being arranged to affect the parameter to the road; and in that the selection means are able to select a route according to the parameter. The invention will be better understood with the aid of the following description of a particular embodiment of the route selection method and of the router of the invention, with reference to the appended drawings, in which: FIG. routers; FIG. 2 represents the steps of the method according to the particular embodiment described;

- Figure 3 shows the router according to the particular embodiment described.

FIG. 1 illustrates a network 10 comprising a plurality of routers here referenced 11, 12, 13, 20 and 21. The router li has a first BGP session 201 established with the router 20 and a second BGP session 211 established with the router 21.

In order to secure the network, routers 20 and 21 are redundant. The redundant router 21, called "backup", has with the router 20, said "nominal", it secures a large number of characteristics or common attributes. More precisely, when the nominal router can no longer perform its functions, the backup router is adapted to take over. As a result, the parameters associated with the routes advertised by these routers are identical. The same routing and / or connectivity information is transmitted between the router 20 and the router 11 and between the router 21 and the router 11.

Similarly, the router 12 has a BGP session 202 established with the router 20 and a BGP session 212 established with the router 21. The router 13 has a BGP session 203 established with the router 20 and a BGP session 213 established with the router 21 .

The following description relates to routers 11 and 20 but is applicable here to all routers 12, 13 and 21.

For a router, any router with which it has an established BGP session is referred to as a BGP peer router.

Once the BGP session 201 is established between the routers 20 and 11, each router 20 (respectively 11) receives from its router BGP pair 11 (respectively 20) routing information in a BGP-UPDATE message. This contains different information:

- destination network address list disabled;

- known destination network address list of the peer BGP router; - set of attributes common to the known destination network address list of the peer BGP router.

A route is defined by a destination network address and a set of characteristics specific to the route. These characteristics are known as the attributes of the road and are defined in the IETF working document referenced draft- ietf-idr-bgp4-26.

On receipt of the BGP-UPDATE message, the router 20 (respectively 11) updates a table relating to this BGP session, commonly called Adj-RIB-In, as a function of the information contained in the BGP-UPDATE message. In particular: - it adds the new routes announced in the BGP-UPDATE message,

- in the case where the message contains a route already registered in the Adj-RIB-In table, it updates the attributes of that route or

- it removes from the Adj-RIB-In table the decommissioned routes.

Optionally, complementary information such as the IP address of the router that issued the route is stored in the table, associated with the route.

According to the invention, the router 20 (respectively 11) assigns to a new advertised route a parameter having a local significance to the receiving router. This parameter is created locally by the router and only has meaning for this router. This local setting is a route discriminator based on the router that advertised that route. In the particular embodiment, it is a temporal parameter representative of the chronological order of reception of the routes by the router corresponding to the value at the instant of reception of the route of a reference clock of the network broadcast to all routers. Alternatively, it is the value of a clock internal to the router. In a second embodiment, the discrimination parameter is generated by a counter, managed locally by the router, incrementing each parameter request related to receiving a new advertised route.

The router also updates the Adj-RIB-In table information based on events other than the receipt of the BGP-UPDATE message during the BGP session. For example, during a break in the BGP session following a cut at the transport level, for example a TCP cut, the router deletes all the routing information announced by the peer BGP router of this BGP session contained in the table. Adj-RIB-In.

In the particular example of the description, if a new route is announced or if a route has become unavailable, these routes being relative to a destination network address, the router executes a method of selecting a route relating to this network address. of destination.

The purpose of this method, which will be described in more detail later, is to select a route relative to a destination network address. The selected route is stored in a table of the RIB (Routing Information Base) router and then transferred to a routing table, named FIB (for "Forwarding Information Base"), relating to the transport plan. of the router.

It should be noted that in some implementations, the Adj-RIB-In, Adj-RIB-Out and Local-RIB tables may be common.

The router then announces the selected route based on criteria that will not be detailed here to each peer BGP router. It keeps in memory, for each established BGP session, all the routes advertised in an Adj -RIB-Out table. Note that in Figure 1, only routers 11, 12, 13, 20 and 21 have been shown. A larger number of routers can of course be part of the network.

The method of selecting a route relating to a destination network address will now be described with reference to FIG. It should be noted at the outset that steps E1, E2, E4 and E5 are described in more detail in the IETF working document referenced draft-ietf-idr-bgp4-26 in paragraph

9.1.2.2 which provides an example of a selection process. This algorithm includes different route selection steps based on characteristics and parameters associated with the routes.

For routes announced by redundant routers, only the last step, referenced as step E4 in the description, of the algorithm, based on the selection of the route announced by the router whose IP address is the smallest allows to make the final selection. According to the invention, a prior step EO consists, on receiving a new route, in obtaining a local parameter for discriminating routes and assigning it to the route.

At the end of this prior step, steps E1 and E2 of setting aside routes according to parameters associated with the routes are performed. A first step El, broken down into different successive substeps, aims to set aside routes according to the attributes of these routes. These attributes are parameters associated with the route, such as: AS_PATH, Origin, and MULTI_EXIT_DISC. They are defined in paragraph 4.3 of the working document, referenced above. These substeps are not shown in Figure 2 for reasons of clarity. At the end of each of these substeps, if there remains only one route relative to a destination network address, the method proceeds to a step E5 of writing the selected route in the routing table Local_RIB . Otherwise, the method continues the execution of step E1.

If, at the end of step E1, there remains more than one route relating to the destination network address, the method then comprises a step E2, also broken down into different successive sub-steps, aimed at carrying out a routing of routes also based on parameters associated with the route, such as the origin of the route (external or internal to the autonomous system), a route cost criterion or a BGP-Identifier parameter related to the route; BGP session. At the end of each of these sub-steps, if there remains only one route relative to a destination network address, the method proceeds to a step E5 writing the selected route in the Local_RIB routing table. Otherwise, the method continues the execution of step E2.

If, at the end of step E2, there remains more than one route relative to the destination network address, a step E3 is executed. For routes announced by redundant equipment, steps E1 and E2 did not discriminate the routes announced by the two devices. This step E3 aims to perform this discrimination. It consists in selecting the route according to the local discrimination parameter associated with the route during the EO step. It will be recalled that, in the particular example of the description, the local discrimination parameter is representative of the reception chronological order corresponding to the value of the reference clock of the network at the moment of reception of the route. Step E3 then consists in selecting the route which has been announced first, ie the one associated with the lowest hour value corresponding to the oldest date.

In the variant embodiment in which the local discrimination parameter corresponds to the reception time indicated by an internal clock to the router, the discrimination step E3 also consists in selecting the route associated with the lowest hour value.

In the second embodiment, the local discrimination parameter corresponds to the value of a counter. The counter is managed by the receiving router and therefore only makes sense locally at the receiving router. This counter is incremented by one unit at each parameter request transmitted in step E0. The oldest route then corresponds to the one with the lowest local discrimination parameter value.

At the end of this step, if only one route relative to the destination network address remains, the method proceeds to a step E5 of writing the route selected in the routing table Local_RIB. Otherwise, the process proceeds to a final step E4. This last step of the algorithm called E4 selects the route advertised by the router whose value of the IP address is the lowest. Since the IP addresses of the redundant routers are not identical, this last step allows It is mandatory to discriminate between the two routers if this has not been possible in the previous steps.

In step E5, the route relating to the destination network address is written in the routing table Local_RIB. This method executes on all the routes relating to a destination network address extracted from the set of Adj-RIB-In tables of the router.

This method can also be periodically executed by the router. Router 11 will now be described with reference to FIG. 3. The router comprises a central control unit 33 to which all the modules of the router are connected, this unit being arranged to control the operation of these modules. The router 11 comprises: a first storage module 40 of the routing information announced by a peer BGP router (Adj_RIB_In); a second storage module 42 of the routing information advertised to a peer BGP router (Adj_RIB_Out); a third storage module 41 of the selected routes (Local_RIB). As a variant, these storage modules can be grouped into a single storage module. In addition, the router 11 comprises a BGP protocol implementation module 30, a route selection module 31 and a parameter assignment module 32.

The module 30 comprises an application allowing the router 11 to implement the BGP protocol as defined by the document RFC 1771. This module 30 is arranged in particular for:

- receive BGP messages transmitted by a peer BGP router;

- extract messages received information on routes announced by the peer BGP routers; obtaining from the assignment module 32, for each new route advertised, a local discrimination parameter; assign this parameter to the new advertised route and store the route information in the first storage module 40.

It is also arranged to read the selected routes in the third storage module 42 and announce them to a peer BGP router.

The assignment module 32 comprises an application providing on request of the module 30 the local parameter of discrimination.

The selection module 31 is arranged to execute the steps E1 to E5 of the method described, that is to say to discard routes based on criteria based on the attributes AS_PATH,

Origin, MULTI_EXIT_DISC; set routes based on parameters such as the origin of the route (external or internal to the autonomous system), the cost of the route, or the BGP-Identifier parameter related to the BGP session; - discard routes on the basis of a local discrimination parameter; remove routes based on the IP address of the peer BGP router and store the selected route in the third storage module 41.

The modules 30, 31 and 32 comprise program code instructions for executing the steps of the method described above, intended to be executed by the router 11, when the program is executed on it. The software modules can be stored in or transmitted by a data carrier. This may be a hardware storage medium, for example a CD-ROM, a magnetic diskette or a hard disk, or a transmission medium such as an electrical signal, optical or radio, or a telecommunications network.

The described embodiment is based on a routing protocol of the type

BGP. It therefore applies to different versions of the protocol, for example to the BGP4 protocol which allows the announcement of IPv4 routes between two separate devices and its extensions, in particular the MP-BGP extension for Multi Protocol. BGP. It is an extension to the BGP4 routing protocol so that it can advertise other types of information and is used to advertise IPv6 routes, Virtual Private Network (VPN) routes, or other information. traffic engineering type L2VPN for Layer 2 VPN in English.

The embodiment is also applicable to other routing protocols for sharing routing information, in particular IDRP IS 10747 / OSI.

The present invention applies to all networks using such protocols. The present invention can be implemented in unified networks, that is, operating under a single technical administration, as well as in networks comprising several administrative domains. It can be applied in networks connected to an Internet-type network, or in networks used for private purposes. The invention covers a configuration in which all the routers of the network implement a method according to one embodiment of the invention. However, a configuration in which one of the routers or only part of the routers of the network implements an embodiment of the invention is already proving to be very advantageous.

Claims

A method for selecting in a first router a route among at least two routes, said routes being related to the same destination network address and having been announced by two respective second routers, to the first router, said

5 second routers being redundant, said method comprising steps (E1, E2) for discarding routes as a function of parameters associated with the route, characterized in that the said steps of discarding have not allowed to discriminate said roads, it also includes:

an assignment step (EO) during which, on reception of a route, the first router assigns a parameter representative of the chronological order of reception of said route;

a step (E3) for selecting one of the two routes according to the assigned parameter

2. The method of claim 1, wherein the selected route corresponds to the one whose reception is the oldest.

3. Method according to any one of the preceding claims, wherein the assigned parameter corresponds to the value of a clock at the time of reception of the route by the first router.

20

4. Method according to any one of claims 1 to 2, wherein the assigned parameter corresponds to the value of a local counter to the first router.

5. A method as claimed in any one of the preceding claims, wherein the selection step (E3) according to the assigned parameter is performed with a second selection step (E4) according to the network address of the second router. advertiser of the road.

6. Router (11, 12, 13, 20, 21) for routing data packets therethrough, comprising: route management means (30) arranged to receive routes advertised by at least one peer router; - means (31) for selecting a route, arranged to select a route from a set of announced routes, relating to the same destination network address, said selection means being also able to set aside routes in function of parameters associated with the roads; characterized in that it further comprises, when said selection means have not allowed to discriminate announced routes:

means (32) for assigning to an advertised route a parameter representative of the chronological order of reception of said route, said management means (30) being arranged to assign said parameter to said route; and in that said selection means is adapted to select a route according to said parameter.

A program for selecting a route among at least two routes comprising program code instructions for executing the steps of the method according to claims 1 to 5 to be executed by a router, when said program is executed on the router.

8. A recording medium readable by a router on which the program according to claim 7 is recorded.