Cisco Routing 1 - General Features of Routing Protocols

This entire series on routing was created when I was preparing for the Cisco test 642-901 BSCI, as my notes. Subsequently, I made only a simple edit and published the text. If you think something important is missing, something is not described correctly or is not completely clear, I welcome your information in the comments.

The basics of routing are described in the article TCP/IP - Routing.

Classification of Routing Protocols

Several types of route records are created in the routing table, depending on how they were created. Packets are routed according to one of the basic routing methods:

• static routing - manually entered routes (records in the routing table), safe and good, but does not reflect changes in network topology
• dynamic routing - the network automatically adapts to changes in topology and traffic, routes are automatically calculated using a routing protocol
• default routing - if no other route exists, the default route is used

Dynamic routing protocols are of two basic types:

• Distance-Vector Routing Protocol
• Link-State Routing Protocol

Note: There is also a Path-Vector Routing Protocol, which falls under Distance-Vector Routing Protocol.

We further divide dynamic protocols based on whether they are intended for deployment within a local network (more precisely within an autonomous system (AS), which may contain several LANs) or operate across networks (connecting AS together).

• Interior Gateway Protocol - IGP - routes within Autonomous System (AS)
• Exterior Gateway Protocol - EGP - routes between AS

Distance-Vector Routing Protocol

• RIP, RIP2, IGRP, EIGRP, BGP
• routers maintain a routing table with information about the (vector) distance to a given network
• periodically send the routing table to neighbors, who adjust their table and send it on
• one (number of hops for RIP) or more (link bandwidth and delay for IGRP) metrics are used to calculate the best path
• a modified type of distance-vector protocol is the path-vector protocol.
• simple DVRPs (RIP, IGRP) do not create relationships with neighbors
• the problem is routing loops - solved by defining maximum distance (TTL), Split Horizon (doesn't send route to the interface it came from), Route Poisoning, hold-down timer (waiting interval until the network is stable, extends convergence)

Link-State Routing Protocol

• OSPF, IS-IS
• routers maintain a comprehensive database of network topology (created using LSA)
• exchange Link State Advertisements - LSA, LSAs are triggered by some event in the network, also Link State Packet - LSP or Link State PDU
• also sends Hello packets to its surroundings, where it sends information about itself
• quickly responds to changes in topology, but consumes more bandwidth (especially initially sends a lot of LSPs) and resources on the router
• the metric is complex, the best path is calculated using Dijkstra's algorithm shortest path first - SPF
• to improve properties, it is divided into smaller areas, border routers send summary routes, uses multicast, LSA numbering

General Terms

Autonomous System - AS

• is a group of IP networks and routers that are under the same technical administration
• numbers 1 to 65535
• private range 65512 - 65535
• AS uses EGP to communicate with another AS
• within AS, routes learned from BGP can be redistributed to IGP

Administrative Distance - AD

• determines the trustworthiness of the protocol - defines the reliability of the protocol and prioritizes better with a lower number
• is a property used on routers to determine the best path between multiple routing protocols
• In other words, multiple routing protocols can run on a router and AD decides which one to use.
• On Cisco routers, we can change the default values.

Setting the administrative distance AD (lower is better) is done by the following command. Weight is the AD value.

ROUTER(config-router)#distance weight [source-ip-address source-mask (access-list-number | name)]

Convergence

• processes and time needed for routing protocol conversion
• convergence is achieved when all routers have complete up-to-date information about the topology

Network Information

• serial interface has default bandwidth equal to T1 = 1.544Mbps
• point to multipoint Frame Relay has default speed = channel_throughput (T1) / number_of_subinterfaces
• Protocol Data Unit - PDU - depends on the protocol / OSI model layer, so L2 PDU is a frame, L3 PDU is a packet

General Configuration

Static Routing

SWITCH(config)#ip route [destination_network] [mask] [next_hop or exit_interface] [administrative_distance] [permanent]
SWITCH(config)#ip route 192.168.50.0 255.255.255.0 192.168.1.1 

Default Routing - Default gateway

SWITCH(config)#ip default-gateway 10.0.1.250       // GW definition, can be used only if routing is not enabled
SWITCH(config)#ip default-network 10.0.1.0         // sets the default network, propagates it using routing protocol
SWITCH(config)#ip route 0.0.0.0 0.0.0.0 10.0.1.250 // creates an entry in the routing table for GW

Dynamic Routing

SWITCH(config)#ip routing                      // enables routing on L3 switch 
SWITCH(config)#router [protocol] [keyword]     // general choice of routing protocol
SWITCH(config-router)#network [network-number] // definition of network to be propagated, for classless wildcard mask is used 

Loopback Interface Setup

Loopback is used for a number of purposes, its main advantage is that it's always up (never goes down). So where, for example, the router ID is calculated from the router's IP address, a loopback is set (otherwise, if the interface went down, everything would be recalculated).

SWITCH(config)#interface loopback 0
SWITCH(config-if)#ip address 215.10.7.1 255.255.255.255 

Information - show commands

ROUTER#show ip protocols  // which networks are routed by which protocol and parameters (timers, filters, metrics, networks.)
ROUTER#show ip route      // displays the routing table
ROUTER#show ip interface  // displays information about interfaces from IP perspective

Policy Based Routing - PBR - route-map

Similar to ACL or better class-map, a numbered list of rules is processed. Rules contain two parts:

• match - what it applies to, we can use ip address according to ACL or prefix-list, length min, max packet length, tag, route type, if we don't specify the match part, it applies to all packets
• set - what is done, we can use setting precedence, tos, ip next-hop - next router to send to, interface - which interface to route through, metric - change the metric

ROUTER(config)#route-map map-tag [permit | deny ] <Sequence Number>

ROUTER(config)#route-map ospf-to-eigrp deny 10
ROUTER(config-map-router)#match tag 6
ROUTER(config-map-router)#match route type external type-2  // this must be true, as well as the previous one
ROUTER(config)#route-map ospf-to-eigrp permit 20
ROUTER(config-map-router)#match ip address prefix-list pfx  // IP address is found using prefix-list, otherwise ACL can be used
ROUTER(config-map-router)#set metric 40000 1000 255 1 1500
ROUTER(config-map-router)#set ip next-hop 142.5.9.1         // which router to send to

Then we apply the route-map to the interface. We can apply only one route-map per interface, but a route-map can consist of multiple numbered parts.

ROUTER(config-if)#ip policy route-map ospf-to-eigrp

Aggregation - summarization

Often we don't need to know individual routes (especially when connecting multiple networks/protocols), but it's enough to know how to get to the main network. So we can replace groups of subnets accessible through one router with one summarized network. Some protocols support automatic summarization across classful network boundaries (like EIGRP).

When a summary route is created (in EIGRP or BGP), a Null0 interface is created (or needs to be created) where this summarized route points. During actual routing, a more specific address (from which summarization was performed) is used, so records for individual subnets must exist, otherwise data would route to Null0. In sent routing updates, only the summary route may then figure.

If the last specific route that was used inside the summarized one is deleted in EIGRP, the summary route is also deleted.

ROUTER(config)#ip route 198.10.0.0 255.255.0.0 null0        // only for summarization 
ROUTER(config-router)#redistribute static                   // one option for summarization, we manually enter routes, along with Null0
ROUTER(config-router)#network 198.10.0.0 mask 255.255.255.0 // another option for aggregation, also together with Null0
ROUTER(config-router)#no auto-summary                       // another option would be auto-summarization to classful (EIGRP), but this often needs to be turned off
ROUTER(config-if)#ip summary-address eigrp 100 192.1.0.0 255.255.0.0         // manual definition of summarization in EIGRP, 100 = AS number
ROUTER(config-router)#aggregate-address 200.52.0.0 255.255.0.0 summary-only  // in BGP, inserts summary route into routing table, if summary-only is not used, the subnets from which summarization was created are also published

Route Filtering - filtering routes in routing updates

We often want to control which routes should be in received (certain records are filtered out when learning from a neighbor) and sent (certain records are not even sent to a neighbor) neighbor updates. This is for basic security, hiding certain routes from the rest of the organization, controlling traffic overload, preventing routing loops. We have three basic methods for management:

• passive interface
• distribute list
• prefix list

Passive-interface

The simplest filtering method, which doesn't go into much detail. A passive interface doesn't send routing updates on the given interface, so its neighbors don't learn about routes. For RIP, RIP2 and IGRP, updates are not sent to neighbors on the given interface, but updates from neighbors are still received and used. For EIGRP and OSPF, Hello packets stop being sent, so the neighbor relationship is terminated, meaning it not only stops sending updates, but also stops receiving them.

ROUTER(config-router)#passive-interface Serial0/0 // the given interface doesn't send updates
ROUTER(config-router)#passive-interface default   // every interface, where not specified otherwise (no passive-interface s0/0), is passive

Distribute-list

Uses standard IP ACL (or even extended), with which we define which routes we don't want to send (deny) and which we want to (permit). For example, when we want to exclude network 192.168.1.0/24 from updates, which we have in the routing table, we can use ACL.

ROUTER(config)#access-list 10 deny 192.168.1.0 0.0.0.255
ROUTER(config)#access-list 10 permit any

We apply the distribute list within the routing process and specify whether it's used for outgoing (out) or incoming (in) updates.

ROUTER(config-router)#distribute-list 10 out   // filtering of outgoing updates, 10 is the ACL number

We can specify even more precisely which updates the restriction will apply to. For both incoming and outgoing, we can specify the interface to which the application applies. For outgoing updates, we can additionally specify routing process or AS number. This applies to redistribution and then applies only to updates from the specified routing process or AS

ROUTER(config-router)#distribute-list 10 in ethernet 0 // updates coming through interface e0
ROUTER(config-router)#distribute-list 10 out igrp 5    // outgoing updates that are redistributed from IGRP 5
ROUTER(config-router)#distribute-list 10 out 100       // outgoing updates that are redistributed from AS 100

In BGP, we can use distribute list on a neighbor.

ROUTER(config-router)#neighbor 10.0.10.1 distribute-list 10 out

Note: We can specify a range of distribute lists for a routing process, but there can be only one per interface and direction.

Prefix-list

A prefix list itself is not a filtering method. It's just a means for filtering. The actual prefix-list is something like an ACL, but works in a slightly different way. It's used to block certain networks in updates from/to neighbors. It can be used for BGP with neighbor settings or instead of ACL in a distribute list. Similar to ACL, a prefix list is a named numbered list of rules that are either permit or deny. By default, it starts with number 5 and increases by 5. Unlike ACL, it filters using prefixes, either exact prefix length or range. The rules include IP address and bit mask (CIDR).

Note: We can use prefix list not only for filtering, but also for Policy Based Routing to select an address.

ROUTER(config)#ip prefix-list test deny 0.0.0.0/0        // prohibits default route 0.0.0.0 
ROUTER(config)#ip prefix-list abc permit 192.0.0.0/8 ge 8 le 24 // takes all prefixes >= 8 and <= 24
ROUTER(config-router)#neighbor 170.10.20.1 prefix-list test in  // application of filtering to neighbor 
ROUTER(config-router)#distribute-list prefix abc in             // another application option is using distribute list
ROUTER#show ip prefix-list                                      // displays prefix lists 

Connecting Routing Protocols - Route Redistribution

To effectively support multiple routing protocols in one internetwork, information must be shared between protocols. This is called route redistribution. Redistribution can be one-way or two-way. Routers that perform redistribution are called boundary routers.

Redistribution is when a routing protocol publishes routes that it learned in another way, these are static routes, directly connected interfaces or another routing protocol. Different routing protocols have different characteristics (such as metrics, classless support).

The recommended solution for redistribution between two protocols where multiple paths exist (when they exchange routes with each other) is to use, due to loops:

• one-way redistribution (default route to the other side)
• route-maps or distribution-list, to prevent the transfer of information originating from a given process back to it
• static routes
• manually set Administrative Distance

When configuring redistribution, I configure the protocol that I want to publish routes from another source. The redistribute command is used. During redistribution, we must set the metric (setting depends on the protocol) or the default is used. OSPF has a default metric for other protocols of 20 except for BGP, for which it's 1. RIP, IGRP, EIGRP, IS-IS have a default metric of 0 (infinity, unreachable, not advertise).

When redistributing from a classful protocol (IGRP, RIP) to classless (OSPF), subneted routes are not distributed. When redistributing to OSPF, it's necessary to use the keyword subnets to transfer subnetted routes. By default, routes are redistributed to OSPF as External Type2 (E2).

We redistribute IGP protocols (like RIP, OSPF) to BGP to get them to other EBGPs. But we must be careful because there's a danger of loops, as redistributed routes may not originate from the given AS, but could have arrived here via BGP. For redistributing IGP route to BGP, it's recommended to use the aggregate-address command.

ROUTER(config)#router rip
ROUTER(config-router)#redistribute static metric 1  // redistribution of static routes
ROUTER(config-router)#redistribute ospf 1 metric 1  // redistribution of OSPF to RIP with metric setting
ROUTER(config-router)#default-metric  1   // setting default metric, which is used where I don't specify it
ROUTER(config)#router ospf 1
ROUTER(config-router)#redistribute static metric 200 subnets
ROUTER(config-router)#redistribute eigrp 1 metric 20 metric-type 1 subnet
ROUTER(config)#router eigrp 1                        
ROUTER(config-router)#redistribute rip metric 10000 100 255 1 1500
ROUTER(config-router)#redistribute ospf 1 match external 1 external 2  // we redistribute only type E1 and E2
ROUTER(config-router)#redistribute isis level-1-2 metric 100 100 200 1 1500 // we redistribute ISIS L1 and L2
ROUTER(config-router)#redistribute ospf 1 route-map ospf-to-eigrp      // using route-map for filtering what's transferred

Two commands can be used for troubleshooting.

ROUTER#show ip route    // on router inside and on ASBR, whether redistributed routes are here
ROUTER#show ip protocol // on ASBR, verifies configuration of redistributed routes, also displays filters