Configuring Dynamic Routes
When a dynamic routing protocol is enabled, the corresponding routing process monitors route updates and advertises routes. Routing protocols enable an upstream router to use the equal cost multipath (ECMP) technique to load balance traffic to identical virtual servers hosted on two standalone Citrix ADC appliances. Dynamic routing on a Citrix ADC appliance uses three routing tables. In a high-availability setup, the routing tables on the secondary appliance mirror those on the primary.
For command reference guides and unsupported commands on dynamic routing protocol, see Dynamic Routing Protocol Command Reference Guides and Unsupported Commands.
The Citrix ADC supports the following protocols:
- Routing Information Protocol (RIP) version 2
- Open Shortest Path First (OSPF) version 2
- Border Gateway Protocol (BGP)
- Routing Information Protocol next generation (RIPng) for IPv6
- Open Shortest Path First (OSPF) version 3 for IPv6
- ISIS Protocol
You can enable more than one protocol simultaneously.
Routing Tables in Citrix ADC
In a Citrix ADC appliance, the Citrix ADC kernel routing table, the FreeBSD kernel routing table, and the NSM FIB routing table each hold a different set of routes and serve a different purpose. They communicate with each other by using UNIX routing sockets. Route updates are not automatically propagated from one routing table to another. You must configure propagation of route updates for each routing table.
NS Kernel Routing Table
The NS kernel routing table holds subnet routes corresponding to the NSIP and to each SNIP and MIP. Usually, no routes corresponding to VIPs are present in the NS kernel routing table. The exception is a VIP added by using the add ns ip command and configured with a subnet mask other than 255.255.255.255. If there are multiple IP addresses belonging to the same subnet, they are abstracted as a single subnet route. In addition, this table holds a route to the loopback network (127.0.0.0) and any static routes added through the CLI (CLI). The entries in this table are used by the Citrix ADC in packet forwarding. From the CLI, they can be inspected with the show route command.
FreeBSD Routing Table
The sole purpose of the FreeBSD routing table is to facilitate initiation and termination of management traffic (telnet, ssh, etc.). In a Citrix ADC appliance, these applications are tightly coupled to FreeBSD, and it is imperative for FreeBSD to have the necessary information to handle traffic to and from these applications. This routing table contains a route to the NSIP subnet and a default route. In addition, FreeBSD adds routes of type WasCloned(W) when the Citrix ADC establishes connections to hosts on local networks. Because of the highly specialized utility of the entries in this routing table, all other route updates from the NS kernel and NSM FIB routing tables bypass the FreeBSD routing table. Do not modify it with the route command. The FreeBSD routing table can be inspected by using the netstat command from any UNIX shell.
Network Services Module (NSM) FIB
The NSM FIB routing table contains the advertisable routes that are distributed by the dynamic routing protocols to their peers in the network. It may contain:
- Connected routes. IP subnets that are directly reachable from the Citrix ADC. Typically, routes corresponding to the NSIP subnet and subnets over which routing protocols are enabled are present in NSM FIB as connected routes.
- Kernel routes. All the VIP addresses on which the -hostRoute option is enabled are present in NSM FIB as kernel routes if they satisfy the required RHI Levels. In addition, NSM FIB contains any static routes configured on the CLI that have the - advertise option enabled. Alternatively, if the Citrix ADC is operating in Static Route Advertisement (SRADV) mode, all static routes configured on the CLI are present in NSM FIB. These static routes are marked as kernel routes in NSM FIB, because they actually belong to the NS kernel routing table.
- Static routes. Normally, any static route configured in VTYSH is present in NSM FIB. If administrative distances of protocols are modified, this may not always be the case. An important point to note is that these routes can never get into the NS kernel routing table.
- Learned routes. If the Citrix ADC is configured to learn routes dynamically, the NSM FIB contains routes learned by the various dynamic routing protocols. Routes learned by OSPF, however, need special processing. They are downloaded to FIB only if the fib-install option is enabled for the OSPF process. This can be done from the router-config view in VTYSH.
Dynamic Routing in a High Availability Setup
In a high availability setup, the primary node runs the routing process and propagates routing table updates to the secondary node. The routing table of the secondary node mirrors the routing table on the primary node.
After failover, the secondary node takes some time to start the protocol, learn the routes, and update its routing table. But this does not affect routing, because the routing table on the secondary node is identical to the routing table on the primary node. This mode of operation is known as non-stop forwarding.
Black Hole Avoidance Mechanism
After failover, the new primary node injects all its VIP routes into the upstream router. However, that router retains the old primary node’s routes for 180 seconds. Because the router is not aware of the failover, it attempts to load balance traffic between the two nodes. During the 180 seconds before the old routes expire, the router sends half the traffic to the old, inactive primary node, which is, in effect, a black hole.
To prevent this, the new primary node, when injecting a route, assigns it a metric that is slightly lower than the one specified by the old primary node.
Interfaces for Configuring Dynamic Routing
To configure dynamic routing, you can use either the GUI or a command-line interface. The Citrix ADC supports two independent command-line interfaces: the CLI and the Virtual Teletype Shell (VTYSH). The CLI is the appliance’s native shell. VTYSH is exposed by ZebOS. The Citrix ADC routing suite is based on ZebOS, the commercial version of GNU Zebra.
Citrix recommends that you use VTYSH for all commands except those that can be configured only on the CLI. Use of the CLI should generally be limited to commands for enabling the routing protocols, configuring host route advertisement, and adding static routes for packet forwarding.
Dynamic Routing Protocol Command Reference Guides and Unsupported Commands
The following table lists command reference guide links, for various dynamic routing protocols, and unsupported commands on the Citrix ADC appliance: Dynamic routing protocol reference guides and unsupported commands.