Routes, Equal Cost Multipath Routing, Policy Routing

Document revision:	2.3 (July 20, 2007, 13:21 GMT)
Applies to:	V2.9

General Information

Summary

The following manual surveys the IP routes management, equal-cost multi-path (ECMP) routing technique, and policy-based routing.

Specifications

Packages required: system
License required: Level1
Submenu level: /ip route
Standards and Technologies: IP (RFC 791)
Hardware usage: Not significant

Related Documents

Description

MikroTik RouterOS has following types of routes:

ECMP (Equal Cost Multi-Path) Routing

This routing mechanism enables packet routing along multiple paths with equal cost and ensures load balancing. With ECMP routing, you can use more than one gateway for one destination network (Note! This approach does not provide failover). With ECMP, a router potentially has several available next hops towards a given destination. A new gateway is chosen for each new source/destination IP pair. It means that, for example, one FTP connection will use only one link, but new connection to a different server will use another link. ECMP routing has another good feature - single connection packets do not get reordered and therefore do not kill TCP performance.

The ECMP routes can be created by routing protocols (RIP or OSPF), or by adding a static route with multiple gateways, separated by a comma (e.g., /ip route add gateway=192.168.0.1,192.168.1.1). The routing protocols may create multipath dynamic routes with equal cost automatically, if the cost of the interfaces is adjusted properly. For more information on using routing protocols, please read the corresponding Manual.

Policy-Based Routing

It is a routing approach where the next hop (gateway) for a packet is chosen, based on a policy, which is configured by the network administrator. In RouterOS the procedure the follwing:

Note! In routing process, the router decides which route it will use to send out the packet. Afterwards, when the packet is masqueraded, its source address is taken from the prefsrc field.

Routes

Submenu level: /ip route

Description

In this submenu you can configure Static, Equal Cost Multi-Path and Policy-Based Routing and see the routes.

Property Description

as-path (text) - manual value of BGP's as-path for outgoing route

atomic-aggregate (yes | no) - BGP attribute. An indication to receiver that it cannot "deaggregate" the prefix

check-gateway (arp | ping; default: ping) - which protocol to use for gateway reachability

distance (integer: 0..255) - administrative distance of the route. When forwarding a packet, the router will use the route with the lowest administrative distance and reachable gateway

dst-address (IP address/netmask; default: 0.0.0.0/0) - destination address and network mask, where netmask is number of bits which indicate network number. Used in static routing to specify the destination which can be reached, using a gateway

gateway (IP address) - gateway host, that can be reached directly through some of the interfaces. You can specify multiple gateways separated by a comma "," for ECMP routes

local-pref (integer) - local preference value for a route

med (integer) - a BGP attribute, which provides a mechanism for BGP speakers to convey to an adjacent AS the optimal entry point into the local AS

origin (incomplete | igp | egp) - the origin of the route prefix

prefsrc (IP address) - source IP address of packets, leaving router via this route

prepend (integer: 0..16) - number which indicates how many times to prepend AS_NAME to AS_PATH

routing-mark (name) - a mark for packets, defined under /ip firewall mangle. Only those packets which have the according routing-mark, will be routed, using this gateway. With this parameter we provide policy based routing

scope (integer: 0..255) - a value which is used to recursively lookup the nexthop addresses. Nexthop is looked up only through routes that have scope <= target-scope of the nexthop

target-scope (integer: 0..255) - a value which is used to recursively lookup the next-hop addresses. Each nexthop address selects smallest value of target-scope from all routes that use this nexthop address. Nexthop is looked up only through routes that have scope <= target-scope of the nexthop

Notes

You can specify more than one or two gateways in the route. Moreover, you can repeat some routes in the list several times to do a kind of cost setting for gateways.

Example

To add two static routes to networks 10.1.12.0/24 and 0.0.0.0/0 (the default destination address) on a router with two interfaces and two IP addresses:

[admin@MikroTik] ip route> add dst-address=10.1.12.0/24 gateway=192.168.0.253
[admin@MikroTik] ip route> add gateway=10.5.8.1
[admin@MikroTik] ip route> print
Flags: X - disabled, A - active, D - dynamic,
C - connect, S - static, r - rip, b - bgp, o - ospf
 #     DST-ADDRESS        G GATEWAY         DISTANCE INTERFACE
 0 A S 10.1.12.0/24       r 192.168.0.253            Local
 1 ADC 10.5.8.0/24                                   Public
 2 ADC 192.168.0.0/24                                Local
 3 A S 0.0.0.0/0          r 10.5.8.1                 Public
[admin@MikroTik] ip route>

Policy Rules

Submenu level: /ip route rule

Property Description

action (drop | unreachable | lookup; default: unreachable) - action to be processed on packets matched by this rule:

dst-address (IP address mask) - destination IP address/mask

interface (name; default: "") - interface through which the gateway can be reached

routing-mark (name; default: "") - mark of the packet to be mached by this rule. To add a routing mark, use '/ip firewall mangle' commands

src-address (IP address mask) - source IP address/mask

table (name; default: "") - routing table, created by user

Notes

You can use policy routing even if you use masquerading on your private networks. The source address will be the same as it is in the local network. In previous versions of RouterOS the source address changed to 0.0.0.0

It is impossible to recognize peer-to-peer traffic from the first packet. Only already established connections can be matched. That also means that in case source NAT is treating Peer-to-Peer traffic differently from the regular traffic, Peer-to-Peer programs will not work (general application is policy-routing redirecting regular traffic through one interface and Peer-to-Peer traffic - through another). A known workaround for this problem is to solve it from the other side: making not Peer-to-Peer traffic to go through another gateway, but all other useful traffic go through another gateway. In other words, to specify what protocols (HTTP, DNS, POP3, etc.) will go through the gateway A, leaving all the rest (so Peer-to-Peer traffic also) to use the gateway B (it is not important, which gateway is which; it is only important to keep Peer-to-Peer together with all traffic except the specified protocols)

Example

To add the rule specifying that all the packets from the 10.0.0.144 host should lookup the mt routing table:

[admin@MikroTik] ip firewall mangle add action=mark-routing new-routing-mark=mt \
\... chain=prerouting
[admin@MikroTik] ip route> add gateway=10.0.0.254 routing-mark=mt
[admin@MikroTik] ip route rule> add src-address=10.0.0.144/32 \
\... table=mt action=lookup
[admin@MikroTik] ip route rule> print
Flags: X - disabled, I - invalid
 0   src-address=192.168.0.144/32 action=lookup table=mt
[admin@MikroTik] ip route rule>

Application Examples

Static Equal Cost Multi-Path routing

Consider the following situation where we have to route packets from the network 192.168.0.0/24 to 2 gateways - 10.1.0.1 and 10.1.1.1:

Note that the ISP1 gives us 2Mbps and ISP2 - 4Mbps so we want a traffic ratio 1:2 (1/3 of the source/destination IP pairs from 192.168.0.0/24 goes through ISP1, and 2/3 through ISP2).

IP addresses of the router:

[admin@ECMP-Router] ip address> print
Flags: X - disabled, I - invalid, D - dynamic
 #   ADDRESS            NETWORK         BROADCAST       INTERFACE
 0   192.168.0.254/24   192.168.0.0     192.168.0.255   Local
 1   10.1.0.2/28        10.1.0.0        10.1.0.15       Public1
 2   10.1.1.2/28        10.1.1.0        10.1.1.15       Public2
[admin@ECMP-Router] ip address>

Add the default routes - one for ISP1 and 2 for ISP2 so we can get the ratio 1:3:

[admin@ECMP-Router] ip route> add gateway=10.1.0.1,10.1.1.1,10.1.1.1
[admin@ECMP-Router] ip route> print
Flags: X - disabled, A - active, D - dynamic,
C - connect, S - static, r - rip, b - bgp, o - ospf
 #     DST-ADDRESS        G GATEWAY         DISTANCE INTERFACE
 0 ADC 10.1.0.0/28                                   Public1
 1 ADC 10.1.1.0/28                                   Public2
 2 ADC 192.168.0.0/24                                Local
 3 A S 0.0.0.0/0          r 10.1.0.1                 Public1
                          r 10.1.1.1                 Public2
                          r 10.1.1.1                 Public2
[admin@ECMP-Router] ip route>

Standard Policy-Based Routing with Failover

This example will show how to route packets, using an administrator defined policy. The policy for this setup is the following: route packets from the network 192.168.0.0/24, using gateway 10.0.0.1, and packets from network 192.168.1.0/24, using gateway 10.0.0.2. If GW_1 does not respond to pings, use GW_Backup for network 192.168.0.0/24, if GW_2 does not respond to pings, use GW_Backup also for network 192.168.1.0/24 instead of GW_2.

The setup:

Configuration of the IP addresses:

[admin@PB-Router] ip address> print
Flags: X - disabled, I - invalid, D - dynamic
 #   ADDRESS            NETWORK         BROADCAST       INTERFACE
 0   192.168.0.1/24     192.168.0.0     192.168.0.255   Local1
 1   192.168.1.1/24     192.168.1.0     192.168.1.255   Local2
 2   10.0.0.7/24        10.0.0.0        10.0.0.255      Public
[admin@PB-Router] ip address>

To achieve the described result, follow these configuration steps: