Routing Protocols

Routing Protocols

1. OSPF

   • Metric: Cost Description: The sum of all interface cost settings for all links in a route, with the cost defaulting to be based on interface bandwith.
   • Classless/sends mask in updates/supports VLSM: Yes Algorithm (DV, advanced DV, LS): LS Supports manual summarization: Yes Cisco-proprietary: No Routing updates are sent to a multicast IP address: Yes Convergence: Fast Sends periodic full updates: No Sends periodic Hello messages: Yes Uses route poisoning for failed routes: Yes Uses split horizon to limit updates about working routes: No Address to which mesages are sent: 224.0.0.5, 224.0.0.6 Metric considered to be infinite: 2^24-1
   • The main phases of how OSPF routers accomplish the work of exchanging LSAs and calculating routes are: -Becoming neighbors: A relationship between two routers that connect to the same data link, created so that the neighboring routers have a means to exchange their LSDBs. -Exchanging databases: The process of sendin LSAs to neighbors so that all routers learn the same LSAs. -Adding the best routes: The process of each router independently running SPF, on their local copy of the LSDB, calculating the best routes, and adding those to the IPv4 routing table.
   • show ip ospf interface brief lists all interfaces on which OSPF is enabled including passive interfaces. show ip ospf interface type number command lists a single line that mentions that the interface is passive. show ip ospf neighbors show ip ospf database clear ip ospf process show ip protocols
   • Once SPF has identified a route, OSPF calculates the metric for a route as follows: The sum of the OSPF interface costs for all outgoing interfaces in the route.
   • OSPF Area design rules: -Put all interfaces connected to the same subnet inside the same area. -An area should be contiguous. -Some routers may be internal to an area, with all interfaces assigned to that single area. -Some routers may be Area Border Routers (ABR), because some interfaces connect to the backbone area, and some connect to nonbackbone areas. -All nonbackbone areas must connect to the backbone area (area 0) by having at least one ABR connected to both the backbone area and the nonbackbone area.
   • Configuring OSPF: 1-Use the router ospf process-id global command to enter OSPF configuration mode for a particular OSPF process. 2-(Optional) Configure the OSPF router ID by doing the following: A- Use the router-id id-value router subcommand to define the router ID. B-Use the interface loopback number global command, along with an IP address address mask command, to configure an IP address on a loopback interface (chooses the highest IP address of all working loopbacks). C-Rely on an interface IP address (chooses the highest IP address of all working nonloopbacks)(whose interface status code (first status code) is up) (In other words, an interface in up/down state will be included by OSPF when choosing its router ID). 3-Use one or more network ip-address wildcard-mask area area-id router subcommands to enable OSPFv2 on any interfaces matched by the configured address and mask, enabling OSPF on the interface for the listed area. ORAdd one ip ospf process-id area area-id command in interface configuration mode under each interface on which OSPF should operate, with the correct OSPF process (process-id) and the correct OSPF area number.4-(Optional) Use the passive-interface type number router subcommand to configure any OSPF interfaces as passive if no neighbors can or should be discovered on the interface.
   • Advertising default routes: default-information originate router subcommand default-information originate always router subcommand tells the router to always advertise the default route, no matter whether ther router's default route is working or not.
   • LSDB
     • The LSDB (link-state database) is simply the collection of all the LSAs known to a router.
     • show ip ospf database
   • LSA
     • Each LSA (link-state advertisements) is a data structure with some specific information about the network topology.
     • Link-State Update (LSU) packet holds data structures called link-state advertisements (LSA). The LSAs are not packets, but rather data structures that sit inside the LSDB and describe the topology.
     • Even when the network is completely stable, each router that creates an LSA also has the responsibility to reflood the LSA every 30 minutes (the default).
   • Hello messages
     • OSPF discovers neighboring routers using Hello messages that contain each router's router ID (RID), that is a 32-bit number. Command output lists these as dotted-decimal numbers (DDN). IOS chooses its OSPF RID based on an active interface IPv4 address, but can be configured as well.
     • The Hello messages themselves have the following features: -The Hello message follows the IP packet header, with IP protocol type 89. -Hello packets are sent to multicast IP address 224.0.0.5, a multicast IP address intended for all OSPF-speaking routers. -OSPF routers listen for packets sent to IP multicast address 224.0.0.5, in part hoping to receive Hello packets and learn about new neighbors.
     • Routers monitor each neighbor relationship using Hello messages and two related timers: Hello Interval and Dead Interval.
     • 2 way-state
       • 2-way state: -The router received a Hello from the neighbor, with that router's own RID listed as being seen by the neighbor. -The router has checked all the parameters in the Hello received from the neighbor, with no problems. The router is willing to become a neighbor. -If both routers reach a 2-way state with each other, it means that both routers meet all OSPF configuration requirements to become neighbors. Effectively, at that point, they are neighbors, and ready to exchange their LSDB with each other.
       • Adjacency: Adjacent
     • full state
       • Full state: When database exchange has completed.
       • Adjacency: Fully adjacent
   • DR
   • BDR
   • DROthers
   • passive interfaces
     • When an interface is configured as a passive interface, you are telling the router to do the following: -Quit sending OSPF Hellos on the interface. -Ignore received Hellos on the interface. -Do not form neighbor relationships over the interface.
     • To configure an interface as passive, two options exist: 1- passive-interface type number router subcommand 2- passive-interface default no passive-interface type number
   • cost
     • Formula = Reference_bandwith / Interface_bandwith All in kbps
     • To change the OSPF cost of an interface: 1-Set the cost explicitly, using the ip ospf cost x interface subcommand, to a value between 1 and 65535, inclusive.2- Change the interface bandwith with the bandwith speed command, with speed being a number in Kbps. 3-Change the reference bandwith, using router OSPF subcommand auto-cost reference-bandwith ref-bw with a unit of Mbps.
   • Load balancing
     • maximum-paths number router subcommand

2. EIGRP

   • Metric: Composite of bandwith and delay. Description: Calculated based on the route's slowest link and the cumulative delay associated with each interface in the route.
   • Classless/sends mask in updates/supports VLSM: Yes Algorithm (DV, advanced DV, LS): advanced DV Supports manual summarization: Yes Cisco-proprietary: Yes Routing updates are sent to a multicast IP address: Yes Convergence: Fast Sends periodic full updates: No Sends periodic Hello messages: Yes Uses route poisoning for failed routes: Yes Uses split horizon to limit updates about working routes: Yes Address to which messages are sent: 224.0.0.10 Metric considered to be infinite: 2^32-1
   • 1-Neighbor discovery: EIGRP routers send Hello messages to discover potential neighboring EIGRP routers and perform basic parameter checks to determine which routers should become neighbors. Neighbors that pass all parameter checks are added to the EIGRP neighbor table. 2-Topology exchange: Neighbors exchange full topology updates when the neighbor relationship comes up, and then only partial updates as needed based on changes to the network topology. The data learned in these updates is added to the router's EIGRP topology table. 3-Choosing routes: Each router analyzes its respective EIGRP topology tables, choosing the lowest-metric route to reach each subnet. EIGRP places the route with the best metric for each destination into the IPv4 routing table.
   • Metric=((10^7/least-bandwith)+cummulative-delay)*256 least-bandwith: kbps cummulative-delay: tens of microseconds
   • show ip eigrp topology show ip eigrp topology all-links (it show nonsuccessor routes as well)
   • When a route fails, and the route has no feasible successor, EIGRP uses a distributed algorithm called Diffusing Update Algorithm (DUAL) to choose a replacement route. EIGRP sends query messages to its neighbors and the neighbor with a working route to subnet sends a reply.
   • EIGRP Configuration: 1-Use the router eigrp as-number global command to enter EIGRP configuration mode and define the EIGRP autonomous system number (ASN). 2- Configure one or more network ip-address [wildcard-mask] routere subcommands. This enables EIGRP on any matched interface and causes EIGRP to advertise the connected subnet. 3-(Optional) Use the eigrp router-id value router subcommand to set the EIGRP router ID (RID) explicitly. 4-(Optional) Use the ip hello-interval eigrp asn time and ip hold-time eigrp asn time interface subcommands to change the interface Hello and hold timers. 5-(Optional) Use the bandwith value and delay value interface subcommands to impact metric calculations by tunning bandwith and delay. 6-(Optional) Use the maximum-paths number and variance multiplier router subcommands to configure support for multiple equal-cost routes. 7-(Optional) Use the auto-summary router subcommand to enable automatic summarization of routes at the boundaries of classful IPv4 networks.
   • update message
     • It is sent to the multicast address if  the sending router needs to update multiple routers on the same subnet. It is sent to the unicast IP address of the particular neighbor.
     • RTP
       • EIGRP sends update messages without UDP or TCP, but it does use a protocol called Reliable Transport Protocol (RTP). RTP provides a mechanism to resend any EIGRP messages that are not received by a neighbor.
   • Hello message
     • Hello Interval Hold Interval: 3 times the Hello Interval
     • EIGRP does not require two neighboring routes to use the same Hello and Hold intervals.
     • It is sent to 224.0.0.10 multicast address
   • neighbors
     • The router checks the following settings to determine whether the router should be allowed to be a neighbor: -It must pass the authentication process if used. -It must use the same configured autonomous system number (which is a configuration setting). -The source IP address used by the neighbor's Hello must be in the same subnet as the local router's interface IP address/mask. -The routers'EIGRP K-values must match. (However, Cisco recommends to not change these values.)
   • Feasible distance(FD)
     • The local router's composite metric of the best route to reach a subnet, as calculated on the local router.
   • Reported distance(RD)
     • The next-hop router's best composite metric for that same subnet.
   • Successor
     • For a particular subnet, the route with the best metric is called the successor. (The successor route's metric is called the feasible distance)
   • Feasible Successor
     • The alternative, immediately usable routes are called feasible successor routes because they can feasibly be used as the new successor route when the previous successor route fails. If a nonsuccessor route's RD is less than the FD, the route is a feasible successor route.
   • variance
     • Configuring the variance permits unequal-cost load-balancing. The variance multiplier EIGRP router subcommand defines an interger between 1 and 128. The router then multiplies the variance times a router's FD. Any FS routes whose metric is less than the product of the variance times the FD are considered to be equal routes and may be placed in the routing table, depending on the setting of the maximum-paths command. Routes that are neither successor nor FS can never be added to the IP routing table, regardless of the variance setting, because doing so may cause packets to loop.

3. BGP

   • BGP does form neighbor relationships, but BGP has no concept of being enabled on an interface, or of dynamically discovering neighbors. Instead, with BGP: -Predefine neighbors with the neighbor ip-address remote-as asn BGP subcommand. -Advertise about prefixes that have been added to the BGP table using    *The BGP network command    *Route redistribution    *By learning prefixes from a neighbor
   • BGP uses TCP to transport its messages between two BGP peers, using well-known port 179.
   • router bgp as-number network subnet mask network-mask neighbor ip-address remote-as asn-number show ip bgp show ip bgp summary show ip bgp neighbors
   • AS_Path
     • AS_Path path attribute (PA) is one step of the best path selection process. AS_Path is a path attribute sent with routes in BGP, and this attribute lists the ASNs in the route. The best path selection process considers a shorter AS_Path as better; you can think of it a little like a hop count, but the hops are entire ASNs rather than single routers.
   • single homed
     • A particular Internet edge design with a single link to one ISP.
   • dual homed
     • Two (or more) links to the same router (of the ISP)
   • single multihomed
     • two ISPs with only one connection between client and ISP
   • dual multihomed
     • Two ISPs with two or more connections between client and the ISP.
   • discarding route
     • A discarding route is used when you have your public address range subnetted, but you want to advertise only one route for the whole range. You create a static route using the command: ip route ip address mask null0 and then using under router configuration mode the network command refering the entire address range. 

4. RIP

   • Metric: Hop Counts Description: The number of routers (hops) between a router and the destination subnet
   • RIPv1: Classless/sends mask in update/supports VLSM: No Algorithm (DV, advanced DV, LS): DV Supports manual summarization: No Cisco-proprietary: No Routing updates are sent to a multicast IP address: No Convergence: Slow Sends periodic full updates: Yes Sends periodic Hello messages: No Use route poissoning for failed routes: Yes Use split horizon to limit updates about working routes: Yes Address to which messages are sent: 224.0.0.9 Metric considered to be infinite: 16
   • RIPv2: Classless/sends mask in updates/supports VLSM: Yes Algorithm(DV, advanced DV, LS): DV Supports manual summarization: Yes Cisco-proprietary: No Routing updates are sent to a multicast IP address: No Convergence: Slow

5. IS-IS

   • Classless/sends mask in updates/supports VLSM: Yes Algorithm (DV, advanced DV, LS): LS Supports manual summarization: Yes Cisco-proprietary: No Routing updates are sent to a multicast IP address: --- Convergence: Fast