Routing: Dynamic Protocols

2 days until Christmas 🎅, and the fam went to see Star Wars: Rise of Skywalker....and it was OK. However, there were many protocol droids!

Dynamic Routing Protocols

• Routers automatically advertise their best paths to known networks
• Routes use this info to determine their own best path to known networks
• Routes update each other of network changes
• And automatically calculate new best paths

         RIP: Routing Information Protocol

         EIGRP: Enhanced Interior Gateway Routing Protocol

         OSPF: Open Shortest Path Protocol

         IS-IS: Intermediate System- Intermediate System

         BGP: Border Gateway Protocol
    IGPs are for routing within an organization

• Distance Vector only form adjacencies with directly connected routers; they only share information with their directly connected neighbor; distance can be the HOP count and the vector is the direction to send the patch; simple configuration; low CPU untilizaiton; slow convergence
• #sh ip rip database   shows all routes it has learned form the neigbors POV
• #sh ip rip    show the best routes that will be used
• Link State also form adjacencies.... and gather infomations from routers to form a complete network topology; based on the "cost" of the links, complexer configs; higher CPU unilization; faster convergence

    EGPs are for routing between organizations over the Internet

Metric: each possible path will be assigned a 'metric' value by the routing protocol which indicates how preferred the path is - '0' being best. Then the routing protocol independently calculated its own best path to each destination.

   If the best path is lost (like a link with down) it will be removed from the routing table and replaced with the next best route.

• RIP uses Hop count with default max metric of 15; it has scalability limitations.
• EIGRP is simple to maintain, calculated changes quickly and its metrical calculation will normally choose the best path by default, was Cisco proprietary. 
• OSPF metric calculation is by default chosen as best path. It is an open standard but more complicated to maintain than EIGRP.
• IS-IS links need to be manually configured or it will us hop count to determine the best path. It typically is only used in Service Provider networks or large organizations with their own MPLS network. 

ECMP (Equal Cost Multi Path)

• if multiple paths to a destination have an equal metric, the router will enter all of the paths into the routing table
• ECMP will load balance the outbound traffic to the destination over the different paths
• All IGP routing protocols will ECMP by default, however, EIGRP can also do UnEqual Cost Multi Path, if manually configured do to so.

Administrative Distance

• If paths to the same destination are received by different routing protocols, their metrics can not be compared. The router uses a different method to choose the best path, and that is the Administrative Distance (AD).
• The AD is a measure of how trusted the routing protocol is and the lower the value wins.
• AD is considered first to narrow the choice down to the single best routing protocol
• Then the Metric is considered to choose the best path or paths which make it into the routing table.

Floating Static Routes - changing the AD value for static routes

• If routes populated by a routing protocol are preferred, and you wanted a backup static route in the table, you would need to override the static default of "1"
• #ip route 10.0.1.0 255.255.255.0 10.1.3.2 115

Loopback Interface - logical interface↛ not physical

• Loopback interfaces are commonly used for traffic that terminates on the router itself, like management traffic, BGP peering, etc.
• The provides redundancy if there are multiple paths to a router, an example is if you are using an IP address on a router interface for management....and that interface goes down ⇏ you would not be able to manage that router.
• The loopback is also used to identify the router (Router ID) in OSPF.
• You must create the logical interface, add an address, and advertise it in the routing protocol.
• (config) interface lookback 0
• (config-if) IP address 192.168.1.1 255.255.255.255 (/32- not waisting IP address')
• (config-if) router eigrp 100   (assign Lookback to existing routing protocol)
• (config-router) network 192.168.1.1 0.0.0.0 (wildcard mask that is the inverse of the subnet mask)

Adjacencies - when an IGP routing protocol is configured globally on a router and then enabled on individual interfaces, the interfaces will look for directly connected devices that are also running this protocol to then form an adjacency and then exchange routing information. Modern routing protocols use multicast to send hello packets.

Passive Interfaces - they do not send out or listen for routing updates: allows you to include an IP subnet in the routing protocol without sending updates out of the interface, so if you need connectivity to another network, but don't want to send internal network information to this network that might belong to another organization. 
Also, loopback interfaces should be configured as passive interfaces, no need wasting resources sending out and listening for hello packets to a dead end (you can not connect other routes to the loopback interface), however, this interface will be advertised by the routing protocol.