Juniper® JNCIA Exam Cram Notes : Routing Protocols

6. Routing Fundamentals

6.3 Routing Protocols

There are three locally configured routes. They are static route, aggregated route and generated route. These routes are not learned through a dynamic routing protocol but are manually entered by you, the administrator.

The Junos OS routing protocol process assigns a default preference value (also known as an administrative distance) to each route that the routing table receives. The default value depends on the source of the route. The preference value is a value from 0 through 4,294,967,295 (232 -1), with a lower value indicating a more preferred route.

Given below are the route preference values (administrative distances for various protocols):

Directly Connected:0
Static route: 5
OSPF internal: 10
IS-IS level 1 internal:15
IS-IS level 2 internal:18
RIP, RIPng:100
OSPF AS external:150
IS-IS external level 1:160
IS-IS external level 2:165

RIP: RIP refers to Routing Information Protocol, which is one of the dynamic routing. It receives the response message to trigger an update response sent by a neighbor. Response messages are also received in response to a request message generated by the router and for an unsolicited response message sent by the neighbor.

Important points on RIP and RIPv2:

  • RIP(Routing Information Protocol) uses Bellman-Ford algorithm.
  • Diffusing Update Algorithm is used by EIGRP.
  • Classless Inter Domain Routing(CIDR) is not supported in RIPV1. For example, if I have a network using /24 subnet mask then, RIPV1 changes /24 subnet mask into /8 mask i.e. it doesn't support CIDR. But CIDR is supported in RIP version 2.
  • The hop count in case of RIP is limited. It doesn't support Classless routing. The convergence time is slow and the security is weak.
  • RIPV2 supports Variable Length Subnet Mask(VLSM) also with the support of authentication. Unlike RIPV1, RIPV2 support classless routing.
  • RIP version1 is less preferred over RIP version2. However the hop count value is same on both versions of RIP
  • It supports simple authentication and md5 authentication. A simple authentication uses a plain-text password that is included in the transmitted packet. MD5 authentication uses the hash algorithm in the transmitted packet.
  • The largest usable metric that is available in case of RIP is 15. Any destination with 16 hop-count is considered unreachable
  • Request and Response are the two packet types used in the RIP network.
  • Request packet is sent to a neighbor to request neighbor's routing table.
  • Response packet is sent as a reply to a neighbor's request packet.
  • Split Horizon, Triggered update, and Hold-down timers avoid routing loops in RIP.
  • The maximum size of an IP RIP packet is limited to 512 bytes. This means that an individual Request or Response message can contain no more than 25 entries.
  • Authentication between RIP neighbors is disabled in JUNOS software. We can configure the authentication if we wish to provide authentication on our network environment.
  • A minimum RIP configuration must include at least the rip, group, and neighbor statements. All other RIP configuration statements are optional. We include one neighbor statement for each logical interface on which you want to receive routes.
  • The receive-options values in RIP are version-1, none and both. The 'none' receive-options value do not receive RIP packets, the version-1 value only accepts RIPv1 packets and 'both' receive-options accept RIPv1 and v2 packets.
  • Broadcast and Multicast are the two send-options values in RIP. Along with this there are also other two send-options values. They are version-1 and none.
  • Broadcast send-options value broadcast the RIPv2 packets and multicast send-options value multicast RIPv2 packets.
  • The maximum number of route entries that can be advertised in a single message of RIP is 255. We can also change this default value by using 'message size' command.
  • In RIP network, 15 is the largest usable metric allowed. We cannot use RIP network if the hop count in our network is more than 15. A metric of 16 is considered unreachable.
  • The entire routing table of RIP is advertised to its neighbor on a regular interval. This regular update is controlled by the update timer which runs every 30 seconds.
  • Route tagging is only supported in RIPv2. There are various features in RIP network supported in RIPv2 but not in RIPv1. These features include authentication, VLSM, etc.
  • RIP routers use UDP (User Datagram Protocol) port 520 to send messages to their neighbors. In addition, the RIP specification does not provide its own mechanism for reliable delivery.
  • The default operation of RIP within the JUNOS software is to receive routes but not to advertise routes. After configuring a routing policy, we can advertise the routes on our RIP network.
  • 20-byte trailer is added to the length of the RIP message with the use of MD5 authentication. Also, an additional route entry is used by the algorithm.

RIP configuration hierarchy as shown below

rip {
	group group-name {
	export [ policy-names ]; 
	neighbor neighbor-name {
	import [ policy-names ];

The preference value of the aggregated route is 130. RIP has the preference of 100 and Border Gateway Protocol (BGP) has the preference value of 170.

OSPF: Open Shortest Path First (OSPF) is an Open-Standard Interior Gateway Protocol (IGP) routing protocol. Unlike other Routing Protocols such as Routing Information Protocol (RIP), Enhanced Interior Gateway Routing Protocol (EIGRP) or Border Gateway Protocol (BGP), OSPF uses the Link State Algorithm in conjunction with Edsger W. Dijkstra Shortest Path First (SPF) algorithm to send out OSPF advertisements, known as Link-State Advertisements (LSAs), to share its Local Link-State Database (LSDB) with OSPF enabled devices to create an overall topology of every router, link state and link metric within a network. OSPF is defined in RFC2328:

OSPF is a link-state routing protocol. It is designed to be run internal to a single Autonomous System. Each OSPF router maintains an identical database describing the Autonomous System's topology. From this database, a routing table is calculated by constructing a shortest-path tree.

OSPF recalculates routes quickly in the face of topological changes, utilizing a minimum of routing protocol traffic. OSPF provides support for equal-cost multipath. An area routing capability is provided, enabling an additional level of routing protection and a reduction in routing protocol traffic. In addition, all OSPF routing protocol exchanges are authenticated.

OSPF advertises and receives LSAs to/from neighbouring routers; these LSAs are stored with the router's local LSDB. Whenever there is a change in the network new LSA's will be flooded across the routing domain and all the routers will have to update their LSDB. This is due to the nature of the Link State and SPF Algorithms; essentially all OSPF routers have to same synchronized identical copy of the Link State Database to have a complete loop-free map of the network topology.

Down is the first starting state for all OSPF routers. After this state Init state is seen. Also, 2-way is one of the state for all OSPF routers. It indicates that the local router has received hello packets with its own router ID in the neighbor field. OSPF uses Dijkstra Algorithm. hello packets and link-state update packets are the two packets of OSPF. Along with this, there are other OSPF packets like Database Descriptor packets, link-state request packets and link-state acknowledge packets. In an OSPF network, DR and BDR election takes place. DR is known as the Designated Router and BDR is known as Backup Designated Router.

Backbone router is a router that has at least one interface in area 0.

Internal Router maintains all operational interfaces.

Area border router connects one or more OSPF areas to the backbone.

We use "show ospf route" command to verify the route of OSPF network. It displays the result of the SPF algorithm.

The two criteria used to elect a designated router are the router priority and the router ID. Designated Router (DR) is responsible for sending the routing updates to the other routers on our network.

There are six different LSA type which are router, network, network summary, ASBR summary, AS external, and NSSA external LSAs.

By default, internal OSPF routes have a preference value of 10, and external OSPF routes have a preference value of 150

Multi-area OSPF is supported in Juniper devices. It means that we can create two different OSPF areas on the same Autonomous System (AS). But all OSPF network must be a member of Area 0.

The inet protocol supports an IPv4 packet. The Intermediate System to Intermediate System (IS-IS) routing protocol uses a data link encapsulation defined by the International Standards Organization (ISO).

DD packets are exchanged by the local router and the neighbor router, that describe their local database. If the OSPF state is stuck at exchange state, OSPF neighborship is not established. It is because of OSPF MTU mismatch.

An internal router, a backbone router and an area border router are the types of OSPF router. Other than this, there is still one other type of OSPF router and that is an Autonomous System Border Router.

To establish and maintain a neighbor relationship, an OSPF-speaking router determines whether any directly connected routers also speak OSPF. The OSPF router sends hello packets out all configured interfaces.

The full-form of DR in OSPF network is Designated Router. Designated Router is responsible for sending out updates to all other routers in OSPF network. Backup Designated Router (BDR) is the backup router in OSPF network. If DR fails, BDR becomes DR.

OSPF has five different packet types they are:

1. hello
2. link-state acknowledgement
3. link-state request packet
4. database description
5. link-state updates

OSPF Configuration Hierarchy

ospf {    
      area area-id {                             
         interface interface-name {
            hello-interval seconds;
            dead-interval seconds;
            neighbor neighbor_address;
         stub <(no-summaries | summaries)>;
         virtual-link neighbor-id router-id transit-area area-id {
      export [ policy-names ];            

BGP: The Border Gateway Protocol (BGP) is the routing protocol that is extensively used in the Internet to connect ISP networks. BGP is a path vector routing protocol. BGP uses route mechanism that is comparable to OSPF or IS-IS. The two of the BGP states used when establishing relationships are Idle and Active. Idle is the initial neighbor state, in which it rejects all incoming session requests. In the Active state, the local router is trying to initiate a TCP session with its peer.

In BGP networks, 4 message types are exchanged between two peers. Those 4 message types are Open, Update, Notification, and keepalive.

Routing information is sent and withdrawn in BGP using the Update message. If needed, each message contains information previously advertised by the local router that is no longer valid.

A BGP peer sends a Notification message to the remote router when a BGP peer detects an error. This is the error that is detected within the session and immediately both BGP and TCP sessions are closed.

A BGP Keepalive message contains only the 19-octet message header and no other data. These messages are exchanged at one-third the negotiated hold-time value for the session, if necessary.

The full-form of RIB is Routing Information Base. Each BGP router establishes memory locations in which to store routing knowledge. These are collectively known as Routing Information Base (RIB).

BGP Configuration hierarchy:

     bgp {
        group <group_name> {
                type <type-name>
                peer-as <asnum of the peer>
                neighbor <neg_ipaddress>
                neighbor <neg_ipaddress> {
                        peer-as <asnum of the peer> 
                hold-time <seconds>

Local loopback and the Remote loopback are configured as a physical interface property and affect the operations of PIC and ports.


BGP is not the junos protocol family.

The inet is the protocol family that supports IPv4 packtets.

The mpls protocol family provides support for MPLS packets.

The iso protocol family allows the processing of the IS-IS protocol.

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